KEK, Ibaraki
A few issues on the path of the luminosity upgrade of KEKB B-Factory is described, including coherent synchrotron radiation, design of the interaction region, crab crossing, and high current operation. These issues will raise more obstacles on the upgrade with the High-Current Scheme. As an alternative, {¥it Nano-Beam Scheme} should be considered as a possible option for the upgrade.
BNL, Upton, Long Island, New York
CERN, Geneva
Lancaster University, Lancaster
KEK, Ibaraki
SLAC, Menlo Park, California
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
LBNL, Berkeley, California
Fermilab, Batavia
Funding: This work was partially performed under the auspices of the US DOE and the European Community-Research Infrastructure, FP6 programme (CARE, contract number RII3-CT-2003-506395)}
The LHC crab cavity program is advancing rapidly towards a first prototype which is anticipated to be tested during the early stages of the LHC phase I upgrade and commissioning. Some aspects related to crab optics, collimation, aperture constraints, impedances, noise effects, beam transparency and machine protection critical for a safe and robust operation of LHC beams with crab cavities are addressed here.
NSRRC, Hsinchu
National Tsing Hua University, Hsinchu
The Taiwan Photon Source (TPS) vacuum system has been designed for a 3 GeV electron storage ring of 24DBA lattice, 518.4 m circumference, 24 unit cells and 24 long straight sections of 6 in 12 m and 18 in 7 m. The vacuum ducts for each cell made from thick aluminum plates and extruded aluminum beam pipes will be precisely machined and welded for obtaining a low impedance with small quantity of flanges and bellows. The beam ducts in long straight sections will be flat extruded aluminum pipes of 10 mm vertical height inside which will be ready for installation of the undulators without breaking the vacuum. The BPMs, 2 in each straight sections and 5 in each cells, will be fixed on the ground or on the girder rigidly through the strong supports maintaining a displacement of < 0.1 micron against the stress force of 10 kg from the beam ducts. The small aperture of 10 mm inside the aluminum bending chamber rejects the PSD outgas from the crotch absorbers backfilled to the beam channel, while the surface of bending chamber will be cleaned with ozonated water to reach lower thermal outgassing rate that maintains a much lower averaged pressure below 100 nPa inside the beam ducts.
Cockcroft Institute, Warrington, Cheshire
The University of Liverpool, Liverpool
UNM, Albuquerque, New Mexico
SLAC, Menlo Park, California
Microbunching can cause an instability which degrades beam quality. This is a major concern for free electron lasers where very bright electron beams are required. A basic theoretical framework for understanding this instability is the 3D Vlasov-Maxwell system. However, the numerical integration of this system is computationally intensive. Investigations to date have used simplified analytical models or numerical solvers based on simple 1D models. We have developed an accurate and reliable 2D Vlasov-Maxwell solver which we believe improves existing codes. This solver has been successfully tested against the Zeuthen benchmark bunch compressors. Here we apply our self-consistent, parallel solver to study the microbunching instability in the first bunch compressor system of FERMI@Elettra.
GANIL, Caen
Solid state rf amplifiers are being considered for an increasing number of accelerator applications, both circular and linear. Their capabilities extend from a few kW to several hundred kW, and from less than 100 MHz to above 1 GHz, for operation both in the linear and saturated regime. This talk will review the state of the art and future prospects of rf power amplifiers for accelerator applications.
KEK, Ibaraki
KEK/JAEA, Ibaraki-Ken
J-PARC is a unique accelerator, because magnetic alloy (MA) loaded cavities are employed for the first time in the rf systems of high intensity proton synchrotrons. High field gradients of more than 20 kV/m are achieved covering the frequency range from 0.9 MHz to 3.4 MHz. The peak voltage of 45 kV per cavity is obtained by driving with two 600 kW tetrodes in push-pull. The first high intensity beam acceleration was successfully initiated at J-PARC RCS. Although RCS beam commissioning started with 10 rf systems, instead of 11 as designed, RCS succeeded in the acceleration of an intense proton beam, which is equivalent to 300 kW when operated at 25 Hz. The longitudinal painting based on the simulation with superimposed second harmonics and with phase and momentum manipulations was the key of success. In December 2008, the J-PARC MR beam is scheduled for its first acceleration up to 30 GeV, and the Material and Life Science facilities start the user operations. During the development stage of the MA cavities, some serious problems such as electrical breakdown on core surfaces occurred. The problems were solved in a short term, and all rf systems were completed on schedule.
CELLS-ALBA Synchrotron, Cerdanyola del Vallès
The Dampy cavity, operating at 500 MHz and up to 160 kW, normal conducting HOM damped, will be used in the ALBA storage ring. The pre-series has shown two problems. First, the HOM damping is very efficient but for one mode. The longitudinal impedance of the E011 mode was found to be around 11 kΩ, slightly above ALBA stability threshold. Second, overheating close to the dampers flanges induces a vacuum leak after several thermal cyclings. The maximum achieved operational dissipated power is 40 kW, if power is further increased a leak opens at one of the dampers flanges. In order to alleviate this latter drawback, two modifications have been implemented in two different cavities. The pre-series has been provided with short-circuits bridging waveguide ridges and cavity body. These are supposed to decrease the current in the area of the flanges. The first production cavity features a reduced thermal impedance between the water cooling channels and the area of overheating by replacing stainless steel by copper in the critical area. Both these cavities are currently under test and this paper will show the results.
GSI, Darmstadt
The 'Facility of Antiproton and Ion Research' (FAIR) project will be realized at the 'GSI Helmholtzzentrum für Schwerionenforschung GmbH' (Darmstadt, Germany) in the scope of a large international collaboration. One of the FAIR storage rings is the collector ring (CR) whose main purpose is to allow a fast cooling of secondary beams (rare isotopes and antiprotons). The RF system of the collector ring has to allow pulsed operation (40kV, duty cycle 5e-4) as well as continuous operation (2kV) in the frequency range of 1.2 to 1.4MHz. The detailed conceptual design of this RF system is introduced here. It will be based (similar to the existing RF system 'SIS18 bunch compressor' which will also be presented at PAC09) on two inductively loaded quarter wavelength coaxial resonators operating on a common ceramic gap. The resonator will be loaded with twelve ring cores (rout=313mm, rin=145mm, h=25mm) of a cobalt based amorphous magnetic alloy (VitroVac6030F); it will be cooled by forced air. The cavity will be driven by a push-pull amplifier operated in class A consisting of two tetrodes (TH555A) that will be coupled inductively to the cavity.
IAP, Frankfurt am Main
A short RFQ prototype was built for tests of high power RFQ structures. We will study thermal effects and determine critical points of the design. Simulations with CST Microwave Studio and first measurements were done. First results and the status of the project will be presented.
KEK, Ibaraki
JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken
GUAS, Kanagawa
J-PARC RCS accelerates a high intensity beam using 11 sets of Magnetic Alloy loaded cavities. It supplies the proton beam to the MLF (Material Life Science Facility) for the neutron and muon experiments. For very high resolution muon experiments, a short proton beam bunch of few ten ns is necessary. To reduce the bunch width to several ten ns, a bunch rotation scheme before extraction will be useful. For the bunch width of few ten ns, a much higher RF voltage is also required. Based on a new magnetic alloy core technology, a design of a new RF cavity to increase the maximum RF voltage by a factor of two will be described in this paper.
HUST, Wuhan
Funding: Work supported by National Nature Science Foundation of China, 10435030
In order to reduce the cost of the signal generator comprising a high performance direct digital synthesizer (DDS), the method of picking up a desired aliased signal of DDS output is adopted in the study. The chip AD9850 is used to synthesize RF signal in the system, and the amplitude modulation of the system is achieved by altering the external connection resistance of the chip. The output frequency is tunable from 99.5 to 101MHz. The principle and the test results of the signal synthesizer will be presented. The amplifier based on tetrode technology can deliver the 10kW RF power in a continuous wave (CW) mode of operation. The driver amplifier consists of two solid-state modules, and it can provide the tetrode with up to 300W input power. The tetrode operates in the grounded cathode configuration. The conceptual design of the final stage amplifier will also be demonstrated in this paper.
BNL, Upton, Long Island, New York
Omega-P, Inc., New Haven, Connecticut
Funding: Work supported by Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy.
A novel tuner has been developed by the Omega-P company to achieve fast control of the accelerator RF cavity frequency. The tuner is based on the ferroelectric property which has a variable dielectric constant as function of applied voltage. Tests using a Brookhaven National Laboratory (BNL) 1.3 GHz RF cavity have been carried out for a proof-of-principle experiment of the ferroelectric tuner. Two different methods were used to determine the frequency change achieved with the ferroelectric tuner. The first method is based on a S11 measurement at the tuner port to find the reactive impedance change when the voltage is applied. The reactive impedance change then is used to estimate the cavity frequency shift. The second method is a direct S21 measurement of the frequency shift in the cavity with the tuner connected. The estimated frequency change from the reactive impedance measurement due to 5 kV is in the range between 3.2 kHz and 14 kHz, while 9 kHz is the result from the direct measurement. The two methods are in reasonable agreement. The detail description of the experiment and the analysis will be discussed in the paper.
Fermilab, Batavia
In the paper the Crab Cavity is described for local Crab schemes for LHC that demand reduced transverse cavity dimensions small enough to fit limited space necessary for the beams separation. The results of the configuration cavity optimization are presented that include (a) the surface field minimization; (b) parasitic monopole and dipole spectrum optimization and dumping, (c) the input and the parasitic mode damping couplers design. The results of multipacting simulations, which were performed in order to understand the possible gradient limitations, are discussed also.
ANL, Argonne
Funding: Work supported by U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
The redesign, construction, and high-power testing of a 95-kV DC, 2MW water-cooled resistive load has been completed. This load was built and installed to test and troubleshoot the Advanced Photon Source (APS) 352-MHz high voltage klystron power supplies. The original resistive load*,** was modified to enhance and improve the load performance .In this paper, we describe the redesign of the DC load, report on the recent test results, and discuss it’s performance improvements.
*D. Horan et al., “A 2-Megawatt Load for Testing High Voltage DC Power Supplies”.
** D. Horan et al., “Performance of a 2-Megawatt High Voltage test Load”.
ELETTRA, Basovizza
Funding: The work was supported in part by the Italian Ministry of University and Research under grant FIRB-RBAP045JF2
The former linac sections used in the injector system of the Elettra Laboratory storage ring will be upgraded for use on the FERMI@elettra project, a free-electron laser user facility operating down to 3 nm. These seven accelerating sections are 3π/4 mode backward-travelling wave (BTW) constant-impedance structures, powered by 45 MW TH2132A klystrons couple to what was called a PEN – power enhancement network, or more commonly referred to as a SLED system. Due to breakdown problems inside the sections, that was the result of high peak fields generated during conventional SLED operation, the sections experienced difficulties in reaching the design gradients. To lower the peak field and make the compressed pulse “flatter”, phase-modulation of the SLED drive power option is investigated. This paper presents the results of this investigations and includes a detailed mathematically analysis.
Thomson Broadcast & Multimedia AG, Turgi
DESY, Hamburg
DESY Zeuthen, Zeuthen
The European XFEL project at DESY in Germany requires 27 RF stations capable of 10 MW RF power each. Each RF station needs one high voltage modulator that generates pulses up to 12 kV and 2 kA with a duration of 1.7 ms and a nominal repetition rate of 10 Hz. DESY decided to investigate new modulator prototypes and Thomson has been awarded to design and build one of these prototype modulators. The Thomson modulator is based on the pulse step modulator (PSM) principle. This technology allows the regulation of the pulse voltage during the pulses and by this achieving a good flatness. In addition to the common PSM technology this modulator design includes additional features. The first one is a constant power regulation system in order to prevent a 10 Hz loading of the mains. The second one is the extension of a part of the system to allow 2-quadrant mode in order to demagnetise the core of the pulse transformer between the pulses. The modulator has been delivered to DESY in July 2008 and is under testing at the modulator test facility in Zeuthen. The paper will give a detailed overview on the system and shows the results of the factory testing and of the testing at DESY.
ANL, Argonne
JLAB, Newport News, Virginia
Funding: * Work supported by U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
A superconducting multi-cell cavity for the production of short x-ray pulses at the Advanced Photon Source (APS) has been explored as an alternative to a single-cell cavity design in order to improve the packing factor and potentially reduce the number of high-power RF systems and low-level RF controls required. The cavity will operate at 2815 MHz in the APS storage ring and will require heavy damping of parasitic modes to maintain stable beam operation. Novel on-cell dampers, attached directly to the cavity body, have been utilized by taking advantage of the magnetic field null on the equatorial plane in order to enhance damping. Design issues and simulation results will be discussed.
DESY, Hamburg
The European XFEL contains hundreds of sources of the coupled impedances. To have an overview of them an impedance budget database is developed. It contains wake functions of the point charge (Green functions) and allows to calculate the wake potentials for arbitrary bunch shapes.
LAL, Orsay
CNAO Foundation, Milan
SOLEIL, Gif-sur-Yvette
NSC/KIPT, Kharkov
The goal of X-ray sources based on Compton back scattering processes is to develop a compact device, which could produce an intense flux of monochromatic X-rays. Compton back-scattering resuls from collisions between laser pulses and relativistic electron bunches. Due to the relative low value of the Compton cross section, a high charge electron beam, a low emittance and a high focusing at the interaction point are required for the electron beam. In addition, the X-ray flux is related to the characteristics of the electron beam, which are themselves dynamically affected by the Compton interaction. One possible configuration is to inject frequently into a storage ring with a low emittance linear accelerator without waiting for the synchrotron equilibrium. As a consequence, the optics should be designed taking into account the characteristics of the electron beam from the linear accelerator. The accelerator ring design for a 50 MeV electron beam, aiming at producing a flux higher than 1013 ph/s, will be presented.
CERN, Geneva
The CERN Proton Synchrotron Booster (PSB) has been running for more than 30 years. Originally designed to accelerate particles from 50 to 800 MeV, later upgraded to an energy of 1 GeV and finally 1.4 GeV, it is steadily being pushed to its operational limits. One challenge is the permanent demand for intensity increase, in particular for CNGS and ISOLDE, but also in view of LINAC4. As it is an accelerator working with very high space charge during the low energy part of its cycle, its operational conditions have to be precisely tuned. Amongst other things resonances must be avoided, stop band crossings optimized and the machine impedance minimized. Recently, an operational intensity record was achieved with >4.25·1013 protons accelerated. An orbit correction campaign performed during the 2007/2008 shutdown was a major contributing factor to achieving this intensity. As the PSB presently has very few orbit correctors available, the orbit correction has to be achieved by displacing and/or tilting some of the defocusing quadrupoles common to all 4 PSB rings. The contributing factors used to optimize performance will be reviewed.
Fermilab, Batavia
Fermilab is in the process of upgrading its Booster Correction Element System to include full field correction element magnets to correct position and chromaticity throughout the booster cycle. From a reliability standpoint, it is important to limit both the maximum temperature and the repetitive temperature cycling of the silicon junctions of the switching elements. We will describe how we measured these parameters and the results of our measurements.
CERN, Geneva
The conceptual considerations of a fast injection system for protons and ions in the proposed PS2 accelerator are presented. Initial design parameters of the injection septum and kicker systems are derived, taking into account rise and fall times, apertures and machine optics. The requirements for an injection dump used for failures are described. Possible limitations and technical issues are outlined.
HUST, Wuhan
Funding: Nation Nature Science Foundation of China,10435030
The design study of Cyclotron CYCHU 10MeV has been developed at Huazhong University of Science and Technology (HUST). Because of the low center frequency (100MHz) of it’s RF system, we should choose suitable directional couplers for the RF system which is supposed to be high-directivity and tight-structure. This paper analyses and synthesizes kinds of directional couplers, espacially microstrip structure, for it’s tinier volume at the low center frequency compared with stripline and branch structures. The achievement of the high-directivity with microstrip configuration is carried out by the distributed capacitor to decrease the even and odd mode phase difference. Capacitive compensation is performed by the interdigital capacitors. The proposed structure is easy to fabricate and incorporate another microwave device due to planner microstrip.
STFC/RAL/ISIS, Chilton, Didcot, Oxon
Funding: Supported by STFC/RAL/ASTeC and by EC Research Infrastructure Activity (FP6) "Structuring the European Research Area" programme (CARE, contract number RII3-CT-2003-506395).
A description is given of the development of a slow-wave chopper structure for the 3.0 MeV, 60 mA, H‾ MEBT on the RAL Front-End Test Stand (FETS). Two candidate structures, the so called RAL ‘Helical’ and ‘Planar’ designs have been previously identified, and are being developed to the prototype stage. Three test assemblies have been designed by modelling their high frequency electromagnetic properties in the time domain, using a commercial 3D code, and their subsequent manufacture, using standard NC machining practice, has helped to validate the selection of machine-able ceramics and copper alloys. In addition, an electro-polishing technique has been developed that enables the ‘fine tuning’ of strip-line characteristic impedance, and edge radius. Measurements of the transmission line properties of the ‘Helical’ and ‘Planar’ test assemblies are presented.
CIAE, Beijing
Department of Physics, Central China Normal University, Wuhan
Light II-A pulsed power generator was used as a power driver of pumping KrF laser at CIAE. The redesign of Light II-A pulsed power generator is based on the consideration that the machine will consist of one single Marx generator with two different experimental lines，which is presented in this paper. The original experimental line with characteristic impedance of 5Ω is remained, and a new line of low impedance (about 1.5Ω ) is added to the Marx generator. The structure design and the electric insulation design are introduced. It is also outlined here the manipulation of modeling the dynamic behavior of gas discharge arc as well as the circuit simulation results of the two experimental lines. Meanwhile a brief introduction is given to the potential application of the low impedance line.
DULY Research Inc., Rancho Palos Verdes, California
Marx modulators, operated by the solid-state switches of Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) or Insulated Gate Bipolar Transistors (IGBTs), offer an alternative to conventional high voltage modulators for rf power sources. They have the advantages of compact size, high-energy efficiency, high reliability, pulse width control and cost reduction. However, Marx modulators need a complex voltage compensation circuit if they are employed in long pulse applications such as the ILC project. We describe novel schemes to compensate the voltage droop of the Marx modulator and minimize the flattop fluctuation of the voltage pulse output through the utilization of inductances and the fast switching properties of solid-state switches. The feasibility of the schemes has been analyzed and relevant data will be presented.
Diversified Technologies, Inc., Bedford, Massachusetts
Funding: Funding by U.S. Department of Energy SBIR program.
Diversified Technologies, Inc. (DTI) has developed high power, solid-state Marx Bank modulators for a range of accelerators and colliders. We estimate the Marx topology can deliver equivalent performance to conventional designs, while reducing system costs by 25-50%. In this paper DTI will describe the application of Marx based technology to a long-pulse (140 kV, 160 A, 1.5 ms) modulator design focused on the International Linear Collider. The primary engineering challenge is minimizing the overall size and cost of the storage capacitors in the modulator. Unique choices in components and controls are needed, including the use of electrolytic capacitors. This paper will review recent progress in the development and testing of this long pulse Marx modulator built under a U.S. Department of Energy Phase II SBIR grant.
CERN, Geneva
The goal of the present CLIC Test Facility (CTF3) is to demonstrate the technical feasibility of specific key issues in the CLIC scheme. The extracted drive beam from the combiner ring (CR), of 35 A in magnitude and 140 ns duration, is sent to the new CLic EXperimental area (CLEX) facility. A Tail Clipper (TC) is required, in the CR to CLEX transfer line, to allow the duration of the extracted beam pulse to be adjusted. Fours sets of striplines are used for the tail clipper, each consisting of a pair of deflector plates driven to equal but opposite potential. The tail clipper kick must have a fast field rise-time, of not more than 5 ns, in order to minimize uncontrolled beam loss. High voltage MOSFET switches have been chosen to meet the demanding specifications for the semiconductor switches for the modulator of the tail clipper. This paper discusses the design of the modulator; measurement data obtained during testing and operation of the tail clipper is presented and analyzed.
CERN, Geneva
Fast kicker magnets are used to inject beam into and eject beam out of the CERN SPS accelerator ring. These kickers are generally ferrite loaded transmission line type magnets with a rectangular shaped aperture through which the beam passes. Unless special precautions are taken the impedance of the ferrite yoke can provoke significant beam induced heating, even above the Curie temperature of ferrite. In addition the impedance can contribute to beam instabilities. In this paper different variants of the coaxial wire method, both for measuring longitudinal and transverse impedance, are briefly discussed in a tutorial manner and do's and don'ts are shown on practical examples. In addition we present the results of several impedance measurements for SPS kickers using the wire method and compare those results with theoretical models.
Fermilab, Batavia
Funding: Fermilab is operated by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the United States Department of Energy.
A fast kicker magnet has been designed for use in Main Injector at Fermilab. The magnet will be used for controlled removal of unbunched beam created in the slip stacking process. The strength of each of the six magnets is 75 G·m at 500 A. The aperture is 11.4 cm wide x 5.3 cm high x 64 cm long. The field rise time from 3% to 97% of less than 57 ns has been achieved along with a flattop variation of less than ±3% variation. Results of simulation and measurements will be presented. The pulser is described in a companion paper.
INFN/LNF, Frascati (Roma)
New injection stripline kickers are operating since December 2007 at the DAΦNE collider. They are designed to operate with very short pulse generators to perturb only the injected bunch and the two stored adjacent ones at 2.7 ns and are a test for the design of the fast kickers of the damping ring of the International Linear Collider (ILC). Stripline frequency response and impedance measurements have been performed to characterize the structure and are compared to the simulation results. Operational performances are also described, pointing out the problems occured and the flexibility of the stripline structure that worked with both the short and the old pulse generators and has been used as an additional damping kicker to improve the efficiency of the horizontal multibunch feedback system.
Tsukuba University, Ibaraki
A new concept of a fast magnetic kicker system - the bridged-T network lumped kicker - is proposed. The rise time is as fast as that of a transmission line kicker, while the input-impedance can be matched with a characteristic impedance of the pulse power supply. The proposed scheme is compared with several conventional schemes. The demonstration of this proposed scheme is also performed. The results show expected performances.
NSRRC, Hsinchu
The correction power supplies are working in the storage ring of NSRRC. They are required to output high quality and high performance current that is long-term stability and output current ripple are required to be under 100ppm. The storage ring consists of more than one hundred units of independence power supplies working together when beam current in 1.5GeV status. The power supplies also are all working under current mode. We just build a new conduction EMI (Electromagnetic Interference) and EMC (Electromagnetic Compatibility) measurement laboratory to measure and test the switching power supplies. That is AC to DC voltage bus source to supply for the switching correction power supply. Using the LISN to obtain conduction noise, it is high frequency voltage noise generated by the switching mode of power supply conduction noise. The current signal pass AC source impendence stabilize network LISN and spectrum analyzer will obtain the conduction noise. We can use a noise separator to separate common EMI noise and difference-mode EMI noise for EMI filtering design. The measurement result will be illustrated in the paper.
JAEA/J-PARC, Tokai-mura
KEK, Ibaraki
KEK/JAEA, Ibaraki-Ken
J-PARC consists of a 181 MeV linac, a 3GeV Rapid Cycling Synchrotron (RCS) and a 50 GeV Synchrotron (MR). The RCS is designed to accelerate a high intensity proton beam. One of the key issues of the RCS RF system is how to achieve the very high accelerating field gradient of more than 20kV/m. This is impossible with conventional ferrite-loaded cavities. We reach this goal by the development of Magnetic Alloy (MA) core loaded RF cavities. We installed 10 RF cavities in the RCS tunnel on May 2007. The RCS beam commissioning was started on September 2007 and we successfully accelerated a proton beam up to 3GeV on October 2007. We also employed MA cores for MR RF cavities and use a cut core configuration to adjust the Q-value. The MR beam commissioning was started on May 2008. We didn't have any trouble caused by the MA cores during operation. We measured the impedance of the RF cavities several times at the shutdown periods. We show the results of impedance measurements. From these results, we can make an assumption about the core condition.
BINP SB RAS, Novosibirsk
RRC, Moscow
The project of upgraded RF System of Siberia-2 Electron Storage Ring / SR Source, Moscow, Russia, is presented. The upgraded RF system will allow to increase the total accelerating voltage up to 1.8MV and ensure operation of the storage ring with new superconducting wiggler at beam currents up to 0.3A. RF system operates at 181MHz. It consists of 3 single bi-metal cavities, 2 power amplifiers based on GU-101A tetrodes with output power of 200kW, power transmission lines and control system. Parameters of the upgraded RF system are given, the design of its main elements is shown.
SAMEER, Mumbai
Shunt impedance of an accelerating structure is an important parameter. It gives an idea of the power coupled to the beam. A 6 MeV to 15 MeV ‘S’ band standing wave side coupled linac structure is developed in SAMEER*. The measurement of the shunt impedance of the cavity is done using bead pull method. The shunt impedance is calculated after plotting the electric field profile. The calculation is done using a C code which first calculates the area of the plot and then uses appropriate variables to give the final value of the shunt impedance. The automation of the bead pull setup is planned and then the integration of calculation and automated setup. This paper describes the method used in the code and outlines the results of the measurement.
*R.Krishnan et.al. submitted in this conference.
JLAB, Newport News, Virginia
Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
For the planned CEBAF upgrade ten new cryomodules are required to increase the beam energy to the envisaged 12 GeV. Extensive cavity and cryomodule R&D has been done previously, including the installation of a new cryomodule dubbed “Renascence” in CEBAFs north linac in 2007. It houses both seven-cell low loss and high gradient type of cavities thereby serving as a testbed to address and cope with crucial technological challenges. Based on this experience a final iteration on the upgrade cavity has been performed to improve various aspects of HOM-damping and thermal stability. Two such cavities have been produced and qualified. A thorough cavity HOM-survey has been performed to verify the integrity of the cavities and to guarantee the impedance requirements of each crucial HOM. This paper details the results of HOM-surveys performed for the first two upgrade style low loss cavities tested both individually in a vertical Dewar and horizontally in a dedicated cavity pair cryomodule. The safety margin to the worst beam break-up scenario at 12 GeV has been concluded.
JLAB, Newport News, Virginia
Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
Recent global interest in high duty factor or CW superconducting linacs with high average beam power highlights the need for robust and reliable SRF structures capable of delivering high average RF power to the beam with moderate HOM damping, low interception of halo and good efficiency. Potential applications include proton or H- drivers for spallation neutron sources, neutrino physics, waste transmutation, subcritical reactors, and high-intensity high-energy physics experiments. We describe a family of SRF cavities with a range of Betas capable of transporting beam currents in excess of 10 mA CW with large irises for minimal interception of halo and HOM and power couplers capable of supporting high average power operation. Goals include an efficient cell shape, high packing factor for efficient real-estate gradient and strong HOM damping to ensure stable beam operation. Designs are being developed for low-frequency (e.g. 650-975 MHz), but can easily be scaled to high-frequency (e.g. 1.3-1.5 GHz), depending on the application. We present the results of conceptual design studies, simulations and prototype measurements.
JLAB, Newport News, Virginia
The College of William and Mary, Williamsburg
Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
Future accelerators require unprecedented cavity performance, which is strongly influenced by interior surface nanosmoothness. Electropolishing is the technique of choice to be developed for high-field superconducting radiofrequency cavities. Electrochemical impedance spectroscopy (EIS) and related techniques point to the electropolishing mechanism of Nb in a sulfuric and hydrofluoric acid electrolyte of controlled by a compact surface salt film under F- diffusion-limited mass transport control. These and other findings are currently guiding a systematic characterization to form the basis for cavity process optimization, such as flowrate, electrolyte composition and temperature. This integrated analysis is expected to provide optimum EP parameter sets for a controlled, reproducible and uniform surface leveling for Nb SRF cavities.
JLAB, Newport News, Virginia
ANL, Argonne
Funding: This manuscript has been authored by Jefferson Science Associates, LLC and by UChicago Argonne, LLC under U.S. DOE Contract numbers DE-AC05-06OR23177 and DE-AC02-06CH11357.
After design optimization of a squashed elliptical single-cell crab cavity at 2.8 GHz, a copper prototype has been bench measured in order to determine its rf properties and the effectiveness of waveguide damping of parasitic modes, especially the low-order mode (LOM)*. We also present detailed results of the RF cold test at 2K on niobium single-cell and two-cell prototype cavities operating either in the zero or pi mode. Further progress will be discussed on the design of high-order mode (HOM) waveguide damping, the analysis of the Lorenz force detuning simulations by ANSYS, and the prototype of on-cell damping in which a waveguide port is attached directly on the cavity’s long equator. Details of LOM/HOM impedance calculations and experimental bench measurements will be reported and compared to strict requirements for satisfying the APS impedance budget.
*J. Shi et. al., “Superconducting RF Deflecting Cavity Design and Prototype for Short X-ray Pulse Generation”, EPAC 2008, paper MOPP155.
JLAB, Newport News, Virginia
The College of William and Mary, Williamsburg
Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
Much remains to be learned regarding the details of SRF performance effects with material variation, including niobium treated in different ways, and different bulk/thin film materials that are fabricated under different conditions. A facility that can measure small samples’ RF properties in a range of 0~180mT magnetic field and 2~20k temperature is necessary in order to answer this question. The Jefferson Lab surface impedance characterization (SIC) system has been designed to attempt to meet this requirement. The SIC system uses a sapphire-loaded cylindrical Nb cavity at 7.5GHz with 50mm diameter flat sample placed on a non-contacting end plate and a calorimetric technique to directly measure the rf dissipation in the sample in response to known rf fields over ~1 cm2. We report on the commissioning of this system and its first uses for characterizing materials. Preliminary tests with Nb thin film sample sputtered on Cu substrate, and bulk Nb sample have been done at low field. The presently available hardware is expected to enable tests up to 20 mT peak magnetic field on the sample CW. Paths to higher field tests have been identified.
ASCo, Clayton, Victoria
RF cavities are routinely detuned slightly from resonance to maintain Robinson stability of the beam as beam loading increases. Detuning the cavities results in a reduction of the overall energy efficiency of the RF and can waste many MW hours of energy per year. It is therefore desirable to only detune as much as required by the beam loading to maintain stability. A new system for monitoring the load impedance of the Storage Ring RF cavities has been developed at the Australian Synchrotron. The system utilises the Analogue devices AD8302 chip to monitor the load impedance of the Cavities and allow for more efficient detuning of the system. An overview and commissioning results of this system will be presented.
UPC, Barcelona
CELLS-ALBA Synchrotron, Cerdanyola del Vallès
The conventional electrical model analogy of a RF cavity is a shunt RLC circuit supplied by two current sources representing the RF amplifier and the beam. In the literature, the impedance of the cavity is often calculated in the Fourier domain. This type of cavity modelling has two drawbacks: First, it assumes a perfect matching between the cavity and the amplifier therefore it neglects the reflected voltage. And, second, it does not provide any information about the cavity transient response, for example at startup or upon beam arrival, while this information can be very important for the design of the regulation loops. In this work we will remove these drawbacks by calculating the cavity impedance in Laplace domain taking the reflected voltage into account. We will then modify our model so that it also includes the influence of the beam on the cavity. For transient RF simulations, though, a typical problem is the long simulation time due to the relatively slow transient response compared to the RF period. To overcome this problem, finally, we will use a mathematical method to map the cavity frequency response from RF to baseband to reduce the simulation time significantly.
CERN, Geneva
In the framework of the LHC project the concept has been developed of a global digital signal processing unit (DSPU) that implements in numerical form the architecture of low-level RF systems [1]. The approach, using an FPGA as core for the low-level system, is very flexible and allows the upgrade of the signal processing by modification of the original firmware [2]. The achieved performances of the LHC 1-Turn delay Feedback are compared with project requirements. The PS Transverse Damper DSPU, with automatic loop delay compensation adapting to the beam’s time of flight and Hilbert Filter for single pick-up betatron phase adjustment, is presented. A modified DSPU with digital inputs for the LHC Transverse Damper is also presented.
[1] V. Rossi, CERN SL-2002-047-HRF, CERN, Geneva, July 2002.
[2] V. Rossi, CERN BE-2009-009, CERN, Geneva, January 2009.
NSRRC, Hsinchu
The direct RF feedback has been adopted in storage ring to reduce the beam loading effect for maximizing the stored beam current. Its performance in reducing beam loading is determined by the operational parameters, including the feedback gain, RF phase shift and the loop delay time. This paper presents a mathematical method, based on the Pedersen model, to study the effects of the direct RF feedback on beam loading. Through an example, the influences of different operational parameters on the performance of the direct RF feedback is analyzed by examining the characteristic equation of the feedback loop. The Nyquist criterion is applied for the determination of system stability.
SLAC, Menlo Park, California
Funding: Work supported by the DOE under contract DE-AC02-76SF00515.
The Multi-TeV colliders should have the capability to accelerate low emittance beam with high rf efficiency, X-band normal conducting high gradient accelerating structure is one of the promising candidate. However, the long range transverse wake field which can cause beam emittance dilution is one of the critical issues. We examined effectiveness of dipole mode damping in three kinds of X-band, π-mode standing wave structures at 11.424GHz with no detuning considered. They represent three damping schemes: damping with cylindrical iris slot, damping with choke cavity and damping with waveguide coupler. We try to reduce external Q factor below 20 in the first two dipole bands, which usually have very high (RT/Q)T. The effect of damping on the acceleration mode is also discussed.
UW-Madison/SRC, Madison, Wisconsin
SLAC, Menlo Park, California
The microbunching gain of a free-electron laser (FEL) driver is affected by the beam spreader that distributes bunches to the FEL beam lines. For the Wisconsin FEL (WiFEL), analytic formulas and tracking simulations indicate that a beam spreader design with a low value of R56 has little effect upon the gain.
SLAC, Menlo Park, California
INFN/LNF, Frascati (Roma)
Funding: Work supported by the U.S. Department of Energy under contract number DE-AC03-76SF00515.
The use of normal conducting cavities and an ion-clearing gap will cause a significant RF accelerating voltage gap transient and longitudinal phase shift of the individual bunches along the bunch train in both rings of the SuperB accelerator. Small relative centroid position shifts between bunches of the colliding beams will have a large adverse impact on the luminosity due to the small beta y* at the interaction point (IP). We investigate the possibility of minimizing the relative longitudinal position shift between bunches by reducing the gap transient in each ring and matching the longitudinal bunch positions of the two rings at the IP using feedback/feedforward techniques in the LLRF. The analysis is conducted assuming maximum use of the klystron power installed in the system.
EPFL, Lausanne
CERN, Geneva
KIT, Karlsruhe
Due to the known limitations of Phase I LHC collimators in stable physics conditions, the LHC collimation system will be complemented by additional 30 Phase II collimators. The Phase II collimation system is designed to improve cleaning efficiency and to minimize the collimator-induced impedance with the main function of protecting the Super Conducting (SC) magnets from quenching due to beam particle losses. To fulfil these requirements, different possible innovative collimation designs were taken in consideration. Advanced jaw materials, including new composite materials (e.g. CuDiamond), jaw SiC insertions, coating foil, in-jaw instrumentation (e.g. BPM) and improved mechanical robustness of the jaw are the main features of these new promising Phase II collimator designs developed at CERN. The FLUKA Monte Carlo code is extensively used to evaluate the behavior of these collimators in the most radioactive areas of LHC, supporting the mechanical integration. These studies aim to identify the possible critical points along the IR7 line.
CERN, Geneva
Phase I collimators, equipped with Carbon-Carbon jaws, effectively met specifications for the early phase of LHC operation. However, the choice of carbon-based materials is expected to limit the nominal beam intensity mainly because of the high RF impedance and limited efficiency of the collimators. Moreover, C/C may be degraded by high radiation doses. To overcome these limitations, new Phase II secondary collimators will complement the existing system. Their extremely challenging requirements impose a thorough material investigation effort aiming at identifying novel materials combining very diverse properties. Relevant figures of merit have been identified to classify materials: Metal-diamonds composites look a promising choice as they combine good thermal, structural and stability properties. Molybdenum is interesting for its good thermal stability. Ceramics with non-conventional RF performances are also being evaluated. The challenges posed by the development and industrialization of these materials are addressed in a collaboration program, involving academic and industrial partners and complementing material research with an innovative design.
SLAC, Menlo Park, California
EPFL, Lausanne
Funding: Work supported in part by the U.S. Department of Energy contract DE-AC02-76SF00515
The Phase II upgrade to the LHC collimation system calls for complementing the 30 high robust Phase I graphite collimators with 30 high Z Phase II collimators. One option is to use metallic rotatable collimators and this design will be discussed here. The Phase II collimators must be robust in various operating conditions and accident scenarios. Design issues include: 1) Collimator jaw deflection due to heating and sagita must be small when operated in the steady state condition, 2) Collimator jaws must withstand transitory periods of high beam impaction with no permanent damage, 3) Jaws must recover from accident scenario where up to 7 full intensity beam pulses impact on the jaw surface and 4) The beam impedance contribution due to the collimators must be small to minimize coherent beam instabilities. The current design will be presented.
MIT/PSFC, Cambridge, Massachusetts
The University of Texas at Austin, Austin, Texas
Funding: US Department of Energy, Office of High Energy Physics
Negative refraction metamaterials (NR MTL) have been developed at microwave, THz, and optical frequencies. At present, microwave MTL's are studied for applications such as microwave filters and patch antennas. Accelerator-relevant applications, such as measuring electron bunch length using its inverse Cherenkov radiation in a NR MTL, have also been proposed. Here we propose a MTL based linear accelerator structure. The MTL is built as an array of complimentary split-ring resonators cut in two metallic plates. The accelerating electron bunch traverses between the plates. The operating mode's properties and standard accelerator parameters (R/Q, accelerating gradient, etc.) of the proposed structure will be reported.
USTC/NSRL, Hefei, Anhui
Funding: the National Nature Science Foundation of China, Grant No. 10375060, 10375061 and 10675116
One of the major concerns in the development of hybrid dielectric-iris-loaded structure is the performance of the used dielectric. The previous dielectric is machinable but the loss tangent is slightly high. So we adopt the new dielectric (Mg-Ca-Ti-O) with loss tangent of about 2·10-4. Because of its high hardness and brittleness, the machining technology and methods are attempted. In this paper, we present a new design of the structure. The model cavities and the coupler for this structure with the new dielectric are investigated experimentally. The experiment results are accorded with the simulated results. In the end, the amplitude and phase shift of the electric field and R/Q of this structure at the operation frequency are even got by a bead-pull experiment.
LLNL, Livermore, California
TomoTherapy, Madison
TPL, Albuquerque, NM
CPAC, Madison
Funding: This work performed under the auspices of the U.S. Department of Energy by Lawrence Livvermore National Laboratory under Contract DE-AC52-07NA27344.
The dielectric wall accelerator* (DWA) system being developed at the Lawrence Livermore National Laboratory (LLNL) uses fast switched high voltage transmission lines to generate pulsed electric fields on the inside of a high gradient insulating (HGI) acceleration tube. High electric field gradients are achieved by the use of alternating insulators and conductors and short pulse times. The system is capable of accelerating any charge to mass ratio particle. Applications of high gradient proton and electron versions of this accelerator will be discussed. The status of the developmental new technologies that make the compact system possible will be reviewed. These include high gradient vacuum insulators, solid dielectric materials, photoconductive switches and compact proton sources.
*Patents pending.
CERN, Geneva
SLAC, Menlo Park, California
The LHC collimation system is implemented in phases, in view of the required extrapolation by 2-3 orders of magnitude beyond Tevatron and HERA experience in stored energy. All available simulations predict that the LHC proton beam intensity with the "phase 1" collimation system may be limited by the impedance of the collimators or cleaning efficiency. Maximum efficiency requires collimator materials very close to the beam, generating the dominant resistive impedance in the LHC. Above a certain intensity the beam is unstable. On the other hand, even if collimators are set very close to the beam, the achievable cleaning efficiency is predicted to be inadequate, requiring either beam stability beyond specifications or reduced intensity. The accelerator physics concept for upgrading cleaning efficiency, for both proton and heavy ion beams, and reducing collimator-related impedance is described. Besides the "phase 2" secondary collimators, new collimators are required in a few super-conducting regions.
CERN, Geneva
The usual approach and treatment for the interaction of a particle beam with wake fields start from the assumption of ultrarelativistic beams. This is not the case, for example, for the Proton Synchrotron Booster (PSB) whose particles have a kinetic energy up to 1.4 GeV, with a relativistic gamma close to 2.5. There are some examples in literature which derive non ultrarelativistic formulas for the resistive wall impedance. In this paper we have extended the Broad-Band resonator model, allowing the impedance to have poles even in the half upper complex plane, in order to obtain a wake function different from zero for z greater than zero. The Haissinski equation has been numerically solved showing longitudinal bunch shape changes with the energy. In addition some longitudinal bunch profile measurements, taken for different energies and bunch intensities at the PSB, are shown.
CERN, Geneva
To understand one contribution to the intensity limitations of the CERN Proton Synchrotron Booster (PSB) in view of its operation with beams from Linac 4, the impedance of the machine has been characterized. Measurements of tune shift as a function of the intensity have been carried out in order to estimate the low frequency imaginary part of the impedance. Since the PSB is a low energy machine, these measurements have been done at two different energies,so as to enable us to disentangle the effect of the indirect space charge and resistive wall from the contribution of the machine impedance. An estimation of the possible resonant peaks in the impedance spectrum has been made by measuring a fast instability in Ring 4.
CERN, Geneva
The usual approach for the resistive pipe wall assumes the beam moves with the speed of light. For many low energy rings, such as the Proton Synchrotron Booster (PBS), possible performance limitations may arise from non relativistic resistive wall wake fields. In this regime not only the head of the bunch can interact with the tail but also the vice versa holds. In this paper we analyze numerical results showing the resistive wake field calculated from non relativistic impedance models. In addition we analyze the well known two particles model assuming that even the trailing particle can affect the leading one. We observe significant changes in the stability domain.
CERN, Geneva
The HEADTAIL code models the evolution of a single bunch interacting with a localized impedance source or an electron cloud, optionally including space charge. The newest version of HEADTAIL relies on a more detailed optical model of the machine taken from MAD-X and is more flexible in handling and distributing the interaction and observation points along the simulated machine. In addition, the option of the interaction with the wake field of specific accelerator components has been added, such that the user can choose to load dipolar and quadrupolar components of the wake from the impedance database Z-BASE. The case of a single LHC-type bunch interacting with the realistic distribution of the kicker wake fields inside the SPS has been successfully compared with a single integrated beta-weighted kick per turn. The current version of the code also contains a new module for the longitudinal dynamics to calculate the evolution of a bunch inside an accelerating bucket.
Fermilab, Batavia
The Synergia framework has been enhanced to include new Poisson solvers and new collective physics effects. Synergia now includes Sphyraena, a solver suite that provides the ability to handle elliptical beam pipes. Resistive wall effects, including intra- and inter-bunch effects in the presence of multiple bunches are also available. We present an overview of the updates in Synergia, focusing on these developments.
BNL, Upton, Long Island, New York
Three-dimensional electromagnetic simulations have suggested a variety of measures to mitigate the problem of button BPM trapped mode heating. A test fixture, using a combination of commercial-off-the-shelf and custom machined components, was assembled to validate the simulations. We present details of the fixture design, measurement results, and a comparison of the results with the simulations.
BNL, Upton, Long Island, New York
The outer circumference of a BPM button and the inner circumference of the button housing comprise a transmission line. This transmission line typically presents an impedance of a few tens of ohms to the beam, and couples very weakly to the 50 Ω coaxial transmission line that comprises the signal path out of the button. The modes which are consequently excited and trapped often have quality factors of several hundred, permitting resonant excitation by the beam. The combination of short bunches and high currents found in modern light sources and colliders can result in the deposition of tens of watts of power in the buttons. The resulting thermal distortion is potentially problematic for maintaining high precision beam position stability, and in the extreme case can result in mechanical damage. We present here a simple algorithm that uses the input parameters of beam current, bunch length, button diameter, beampipe aperture, and fill pattern to calculate a figure-of-merit for button heating. Data for many of the world’s light sources and colliders is compiled in a table.
BNL, Upton, Long Island, New York
Funding: This work was supported by Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy.
Continuing interest in computing the coupling impedance of cylindrical multi-layer beam tubes led to several recent publications. A novel matrix method is here presented in which radial wave propagation is treated in analogy to longitudinal transmission lines. Starting from the Maxwell equations the solutions for monopole and dipole electromagnetic fields are in each layer described respectively by a 2×2 and 4×4 matrix. Assuming isotropic material properties within one layer, the radially transverse field components at the inner boundary of a layer are uniquely determined by matrix transfer of the field components at its outer boundary. By imposing power flow constraints on the matrix, field matching between layers is enforced and replaced by matrix multiplication. The wall impedance is found as eigen solution to the scalar Helmholtz equation with the additional boundary condition that the longitudinal magnetic field vanishes at the inner beam tube wall. The matrix method is demonstrated via the example of the longitudinal impedance of a multi-layer HOM absorber, involving a ceramic tube with metal coating and an external ferrite layer.
UMAN, Manchester
A technique has been developed which enables the calculation of resistive particle wake effects. The technique can simply be calculated to any order, and is easy and quick to evaluate. No assumptions are made about the range of the interaction, but this is especially useful for short range effects. We show how the exact evaluation compares with various common approximations for some simple cases, and implement the technique in the Merlin and PLACET simulation programs. The extension from cylindrical to rectangular apertures is highlighted.
SLAC, Menlo Park, California
BINP SB RAS, Novosibirsk
Funding: Work supported by US DOE contracts DE-AC03-76SF00515
We study the impedance due to coherent synchrotron radiation (CSR) generated by a short bunch of charged particles passing through a bend magnet of finite length in a vacuum chamber of a given cross section. Our method represents a further development of the previous papers*. In this method we decompose the electromagnetic field of the beam into the eigenmodes of the toroidal chamber. We derive a system of equations for the expansion coefficients in the series, and develop a numerical algorithm for practical calculations. We illustrate our general method by calculating the CSR impedance of a beam moving in a vacuum chamber of rectangular cross section.
*G. V. Stupakov and I. A. Kotelnikov, PRST-AB 6, 034401 (2003); T. Agoh, K.Yokoya, PRST-AB, 7, 054403 (2004)
TEMF, TU Darmstadt, Darmstadt
GSI, Darmstadt
Funding: This work is supported by the GSI.
Beam coupling impedances have been identified as an appropriate quantity to describe collective instabilities caused through beam-induced fields in heavy ion synchrotron accelerators such as the SIS-18 at the planned SIS-100 at the GSI facility. The impedance contributions caused by the multiple types of beamline components need to be determined to serve as input condition for later stability studies. This paper will present an approach exploiting the abilities of commercial FDTD wake codes such as CST PARTICLE STUDIO® for a benchmark problem with cylindrical geometry. Since exact analytical formulae are available, the obtained numerical results will be compared. Special attention is paid towards the representation of the particle beam as the source of the EM fields and conductive losses.
BNL, Upton, Long Island, New York
The thermal distortion resulting from BPM button trapped mode heating is potentially problematic for achieving the high precision beam position measurement needed to provide the sub-micron beam position stability required by light source users. We present a button design that has been thermo-mechanically optimized via material selection and component geometry to minimize this thermal distortion. Detailed electromagnetic analysis of the button geometry is presented elsewhere in these proceedings.
BNL, Upton, Long Island, New York
Funding: Work supported by U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-98CH10886.
With growth of circulating current in the storage rings the heating of the beam position monitor (BPM) buttons due to the induced trapped modes is drastically increasing. Excessive heating can lead to the errors in the measuring of beam position or even catastrophic failures of pick-up assembly. In this paper we present calculations of heat generated in the button for different geometries and materials. The obtained results are used for the optimization of the BPM design for the NSLS-II project.
CIAE, Beijing
The beam phase monitor was designed to address phase slide issue, which can lead to significant beam loss inside 100MeV cyclotron. The measured phase information can be used to direct cyclotron magnetic field fine tuning. The system describes in this paper consists of the following part: 10 sets of beam phase pickup, a phase detector, a set of RF multiplexer and a phase shifter to compensate different phase offset generated by cables, connectors etc. The last one is a computer interface consisting of two 16 bits AD converters, one ARM 7 processor was included in this module to support RS232 connection and perform necessary signal process. All parts except the probe were located in one 3U VME standard crate, 8 slots were occupied and one user defined backplane was developed to carry necessary power supply lines and inter-connections. Preliminary tests for the electronic system has been performed, and a good result was obtained in the procedure. Yet the leakage from RF cavity in the 100MeV cyclotron is still an undermined limitation for this application.
CERN, Geneva
BNL, Upton, Long Island, New York
Graz University of Technology (TUG), Signal Processing and Speech Communication Laboratory (SPSC), Graz
In particle accelerators coaxial cables are commonly used to transmit wideband beam signals covering many decades of frequencies over long distances. Those transmission lines often have a corrugated outer and/or inner conductor. This particular construction exhibits a significant amount of frequency dependent group delay variation. A comparison of simulations based on theoretical models and S11 and S21 network analyzer measurements up to 2.5 GHz is presented. It is shown how the non-linear phase response and varying group delay leads to ringing in the pulse response and subsequent distortion of signals transmitted through such coaxial transmission lines.
CERN, Geneva
CIEMAT, Madrid
In the framework of EUROTeV, a Precision Beam Position Monitor (PBPM) has been designed, manufactured and tested. The new PBPM, based on the inductive BPM presently used in the CERN Clic Test Facility (CTF3), aims to achieve a resolution of 100 nm and an accuracy of 10μm in a 6 mm aperture. A dedicated test bench has been designed and constructed to fully characterize and optimize the PBPM. This paper describes the final design, present the test bench results and reports on the beam tests carried out in the CERN CTF3 Linac.
SLAC, Menlo Park, California
ORNL, Oak Ridge, Tennessee
Funding: work supported by the Department of Energy under contract number DE-AC03-76SF00515 and DE-AC05
Fields induced by a beam and penetrated outside the beam pipe can be used for a beam diagnostic. Wires placed in longitudinal slots in the outside wall of the beam pipe can work as a beam pickup. This has a very small beam-coupling impedance and avoids complications of having a feed-through. The signal can be reasonably high at low frequencies. We calculate the beam-coupling impedance due to a long longitudinal slot in the resistive wall and the signal induced in a wire placed in such a slot and shielded by a thin screen from the beam. We present a field waveform at the outer side of a beam pipe, obtained as a result of calculations and measurements. Such kind of diagnostic can be used in storage rings, synchrotron light sources, and free electron lasers, like LINAC coherent light source.
JLAB, Newport News, Virginia
RUBYTRON, Rye Brook, New York
In-air test fixtures for beamline elements typically utilize an X-Y positioning stage, and a wire antenna excited by an RF source. In most cases, the antenna contains a standing wave, and is useful only for coarse alignment measurements in CW mode. A surface-wave (SW) based transmission line permits RF energy to be launched on the wire, travel through the beamline component, and then be absorbed in a load. Since SW transmission lines employ traveling waves, the RF energy can be made to resemble the electron beam, limited only by ohmic losses and dispersion. Although lossy coaxial systems are also a consideration, the diameter of the coax introduces large uncertainties in centroid location. A SW wire is easily constructed out of 200 micron magnet wire, which more accurately approximates the physical profile of the electron beam. Benefits of this test fixture include accurate field mapping, absolute calibration for given beam currents, Z-axis independence, and temporal response measurements of sub-nanosecond pulse structures. Descriptions of the surface wave launching technique, transmission line, and receiver electronics are presented, along with measurement data.
FEL/Duke University, Durham, North Carolina
PAL, Pohang, Kyungbuk
Dimtel, San Jose
Funding: work supported by US Air Force Office of Scientific Research medical FEL grant FA9550-04-01-0086
The coupled bunch mode instabilities (CBMIs) caused by vacuum chamber impedance limit and degrade the performance of the storage ring based light sources. A bunch-by-bunch longitudinal feedback (LFB) system has been developed to stabilize beams for the operation of a storage ring based Free Electron Laser (FEL) and the High Intensity Gamma-ray Source (HIGS) at the Duke storage ring. Employing a Giga-sample FPGA based processor (iGP), the LFB is capable of damping out the dipole mode oscillation for all 64 bunches. As a critical subsystem of the LFB system, kicker cavity is developed with a center frequency of 938 MHz, a wide bandwidth (> 90 MHz), and a high shunt impedance (> {10}00 Ω). First commissioned in summer 2008, the LFB has been operated to stabilize high current multi-bunch operation. More recently, the LFB system is demonstrated as a critical instrument to ensure stable operation of the HIGS with a high intensity gamma beam above 20 MeV with a frequent top-off injection to compensate for the substantial and continuous electron beam loss in the Compton scattering process. In the future, we will perform detailed studies of the impedance effects using the LFB system.
SOLEIL, Gif-sur-Yvette
Ever since the first user operation, the digital transverse bunch by bunch feedback system developed at SOLEIL has successfully been operated, achieving a stable multibunch beam at the highest intensity in the two planes at zero chromaticity with a single chain working in a diagonal mode. Since then a vertical stripline, optimised to generate large deflections to combat the strong single bunch headtail instability, was installed to construct another chain. The combined use of the two chains allowed enhancing the feedback performance. In particular, by differentiating the feedback gain between high and low intensity bunches, the system is capable of working in hybrid filling modes. In parallel, online applications were integrated into the control system to allow measuring the tunes by selectively exciting a single bunch, damping and growth rates, and analysing the bunch by bunch data in frequency or time domain for post-mortem purposes. Future plans including installation of a horizontal stripline and a noise reduction by avoiding the baseband conversion of the beam signal are also discussed.
USTC/NSRL, Hefei, Anhui
A bunch-by-bunch analogue transverse feedback system at the Hefei Light Source (HLS) is to cure the resistive wall instability and the transverse coupled bunch instabilities. The kicker of the feedback system has four 21-cm-long electrodes of stripline type mounted in a skew 45°. Calculation and Simulation of the transverse kicker are shown.
CERN, Geneva
Fast kicker magnets are used to inject beam into and eject beam out of the CERN SPS accelerator ring. These kickers are generally ferrite loaded transmission line type magnets with a rectangular shaped aperture through which the beam passes. Unless special precautions are taken the impedance of the ferrite yoke can provoke significant beam induced heating, over several hours, even above the Curie temperature of the ferrite. At present the nominal bunch spacing in the SPS is 25 ns, however for an early stage of LHC operation it is preferable to have 50 ns bunch spacing. Machine Development (MD) studies have been carried out with an inter-bunch spacing of 25 ns, 50 ns or 75 ns. For some of the SPS kicker magnets the 75 ns bunch spacing resulted in considerable beam induced heating. In addition the MDs showed that 50 ns bunch spacing could result in a very rapid pressure rise in the kicker magnet and thus cause an interlock. This paper discusses the MD observations of the SPS kickers and analyses the available data to provide explanations for the phenomena: possible remedies are also discussed.
SLAC, Menlo Park, California
Funding: Work supported by US DOE contracts DE-AC03-76SF00515.
A remarkable progress over the last decade in development of computer codes significantly advanced our capabilities in calculation of wakefields and impedances for accelerators. There are however a number of practical problems that, when approached numerically, require a huge mesh, and hence memory, or an extraordinary CPU power, or both. One class of such problems is related to wakes of ultra short bunches, typical for many next generation electron/positron accelerators and photon sources. Another class is represented by long shallow collimators and tapers, often with non round cross sections. The numerical difficulties with these problems can be traced to a small parameter in the system, such as, e.g., a ratio of the bunch length to the length of a taper. It is remarkably, however, that the same small parameter often allows developing approximate analytical methods that provide a simplified solution to the impedance problem. In this paper, we review recent results in the analytical theory of wakefields, which include calculation of the wakes of very short bunches, long transitions and some special cases of the resistive wall impedance.
SLAC, Menlo Park, California
Funding: This work was supported by Department of Energy contract DE-AC02-76SF00515.
Gravitational instability of a star distribution in a galaxy is a well-known phenomenon in astrophysics. This problem can be analyzed using the standard tools developed in accelerator physics for analyzing the onset of beam instability and loss of Landau damping. An attempt is made here for a nonrotating galaxy. Predictions for the maximum stable galaxy size are in remarkable agreement with observations.
ORNL, Oak Ridge, Tennessee
IUCF, Bloomington, Indiana
Funding: ORNL/SNS is managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725.
During one of the high beam intensity runs in SNS, a coasting beam instability was observed in the ring when the beam was stored for 10000 turns. This instability was observed at an intensity of about 12 microcoulombs and was characterized by a frequency spectrum peaking at about 6 MHz. A likely cause of the instability is the impedance of the ring extraction kickers. We carry out here a detailed benchmark of the observed instability, uniting an analysis of the experimental data, a precise ORBIT Code tracking simulation, and a theoretical estimate of the observed beam instability.
SLAC, Menlo Park, California
CERN, Geneva
Funding: This work was supported by DOE Contract No. DE-AC02-76SF00515 and used resources of NERSC supported by DOE Contract No. DE-AC02-05CH11231, and of NCCS supported by DOE Contract No. DE-AC05-00OR22725.
In recent years, SLAC's Advanced Computations Department (ACD) has developed the parallel 3D Finite Element electromagnetic time-domain code T3P. Higher-order Finite Element methods on conformal unstructured meshes and massively parallel processing allow unprecedented simulation accuracy for wakefield computations and simulations of transient effects in realistic accelerator structures. Applications include simulation of wakefield damping in the Compact Linear Collider (CLIC) Power Extraction and Transfer Structure (PETS).
JLAB, Newport News, Virginia
Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
With the proposal of medium to high average current accelerator facilities the demand for cavities with extremely low HOM impedances is increasing. Modern numerical tools are still under development to more thoroughly predict impedances that need to take into account complex absorbing boundaries and lossy materials. With the usually large problem size it is preferable to utilize massive parallel computing when applicable and available. Apart from such computational issues, we have developed methods using available computer resources to enhance the information that can be extracted from a cavities’ wake potential computed in time domain. In particular this is helpful for a careful assessment of the extracted RF power and the mitigation of potential beam breakup or emittance diluting effects, a figure of merit for the cavity performance. The methods are described as well as examples of their implementation.
BNL, Upton, Long Island, New York
Funding: Work supported by Brookhaven Science Associates, LLC under contract No. DE-AC02-98CH10886 with the U.S. Department of Energy
Couples bunch longitudinal instability in the presence of high frequency impedance is considered. A frequency domain technique is developed and compared with simulations. The frequency domain technique allows for absolute stability tests and is applied to the problem of longitudinal stability in RHIC with the proposed 56 MHz rf system.
BNL, Upton, Long Island, New York
Impedance of two vacuum chamber components, Bellows and BPM, is considered in some detail. In order to avoid generation of Higher-Order Modes (HOM’s) in the NSLS-II bellows, we designed a new low-impedance RF shielding consisting of 6 wide and 2 narrow metal plates without opening slots between them. The short-range wakepotential has been optimized taking into account vertical offset of RF fingers from their nominal position. The results were compared with data of bellows designed at other laboratories. Narrow-band impedance of the BPM Button has been studied. TE-modes in the BPM button were suppressed by a factor of 8 by modification of existing housings. Two new types of housings are shown. The total impedance of the NSLS-II storage ring is discussed in terms of the loss factor and the vertical kick factor for a 3mm-Gaussian bunch.
BNL, Upton, Long Island, New York
The short- and long-range wakepotentials have been studied for the design of the infrared (IR) extraction chamber with large full aperture: 67mm vertical and 134mm horizontal. The IR-chamber will be installed within a 2.6m long wide-gap bending magnet with 25m bend radius. Due to the large bend radius it is difficult to separate the light from the electron trajectory. The required parameters of the collected IR radiation in location of the extraction mirror are ~50mrad horizontal and ~25mrad vertical (full radiation opening angles). If the extraction mirror is seen by the beam, resonant modes are generated in the chamber. In this paper, we present the detailed calculated impedance for the design of the far-IR chamber, and show that placing the extraction mirror in the proper position eliminates the resonances. In this case, the impedance reduces to that of a simple tapered structure, which is acceptable in regard to its impact on the electron beam.
BNL, Upton, Long Island, New York
CERN, Geneva
Funding: This work has been partially performed under the auspices of US department of energy
Measurements of transverse impedance in the SPS to track the evolution over the last few years show discrepancies compared to the analytical estimates of the major contributors. Recent measurements to localize the major sources of the transverse impedance using intensity dependent optics are presented. Some simulations using HEADTAIL to understand the limitations of the reconstruction and related numerical aspects are also discussed.
BNL, Upton, Long Island, New York
NSCL, East Lansing, Michigan
CERN, Geneva
Studies of space charge effects in the Small Isochronous Ring (SIR) at Michigan State University revealed a fast longitudinal instability at and below the transition that could not be explained by the conventional negative mass instability. The observed beam behavior can be explained by the effect of the radial component of the coherent space charge force on the longitudinal motion. The transverse coherent space charge force effectively modifies the slip factor shifting the isochronous point and enhancing the negative mass instability. This paper presents results of numerical and experimental studies of the longitudinal beam dynamics in SIR and proposes an analytical model explaining the results.
CANDLE, Yerevan
PSI, Villigen
The longitudinal and transverse impedances of CPMU (cryogenic permanent magnet undulators) of the SLS storage ring are evaluated. The study takes into account the walls frequency dependent conductivity and the electrical and magnetic properties of the material at low temperature.
CANDLE, Yerevan
PSI, Villigen
YSU, Yerevan
The resistive longitudinal and transverse wakefields, longitudinal loss and transverse kick factors excited by the electron bunch in undulator section of the PSI-XFEL are given. The ordinary and in vacuum undulators are considered. For in vacuum undulator the modified technique for impedance calculation is developed.
Cockcroft Institute, Warrington, Cheshire
STFC/DL, Daresbury, Warrington, Cheshire
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
Vacuum chamber transitions of the ILC damping rings associated with BPM insertions, vacuum ports, antechamber tapers etc, may make a significant contribution to the overall machine impedance. Since most transitions are not azimuthally symmetric, commercial 3D codes based on the finite element method have been used to compute their wake fields. The results for selected vacuum chamber components are presented in this paper, together with some estimates of the impact of the wake fields on the beam dynamics in the damping rings.
CLASSE, Ithaca, New York
Wake fields arising from the discontinuities in the vacuum chamber produce energy spread. In an energy recovery linac (ERL), a spent beam is decelerated before it is dumped in order to use its energy for the acceleration of new beam. While the energy spread accumulated from wakes before deceleration does not increase during deceleration, it becomes more important relative to the beam's decreasing energy. Therefore, in an ERL, wakes can produce very significant energy spread in the beam as it is decelerated to the energy of the beam dump so that beam transport to the dump may become impractical. This effect can place a limit either on the maximum charge per bunch or on the wake field-budget for the ERL. As an example of these wake field effects, this paper discusses their impact for the present design of the Cornell ERL and estimates the effects for typical vacuum chamber components being considered.
SOLEIL, Gif-sur-Yvette
Diamond, Oxfordshire
Good understanding of instabilities is of great importance in light source rings that provide high current beams. The inherently large machine impedance, which often evolves with continuous changes of insertion devices, enhances collective effects that need to be well controlled to assure the machine performance. The problem is usually not straightforward, as one must quantify short and long range wakes that excite single and multi bunch instabilities, the coupling between instabilities and different planes, as well as Landau effects in arbitrary filling modes. The paper presents the study made on DIAMOND and SOLEIL using the multiparticle tracking codes sbtrack and mbtrack. While sbtrack performs a 6-dimensional single bunch tracking, mbtrack does its direct extension to multibunches. The most recent code development includes a MATLAB version and a high precision Fourier analysis of collective modes. The study emphasises the use of realistic impedance models, either empirically or numerically constructed, and aims to elucidate the relative importance of different physical effects by closely comparing with experimental observations.
EPFL, Lausanne
CERN, Geneva
BNL, Upton, Long Island, New York
The upgrade of the CERN Large Hadron Collider (LHC) would require a four- to fivefold increase of the single bunch intensity presently obtained in the Super Proton Synchrotron (SPS). Operating at such high single bunch intensities requires a detailed knowledge of the sources of SPS beam coupling impedance, so that longitudinal and transverse impedance reduction campaigns can be planned and performed effectively if needed. In this paper, the transverse impedance of the SPS is studied by injecting a single long bunch into the SPS, and observing its decay without RF. This particular setup enhances the resolution of the frequency analysis of the longitudinal and transverse bunch signals acquired with strip line couplers connected to a fast data acquisition. It also gives access to the frequency content of the transverse impedance. Results from measurements with short and long bunches in the SPS performed in 2008 are compared with simulations and theoretical predictions.
EPFL, Lausanne
CERN, Geneva
A detailed knowledge of the beam coupling impedance of the CERN synchrotrons is required in order to identify the impact on instability thresholds of potential changes of beam parameters, as well as additions, removal or modifications of hardware. To this end, an update of the impedance database was performed, so that impedance results from theoretical calculations using new multilayer models, impedance results from electromagnetic field simulations and impedance results from bench measurements can be compiled. In particular, the impedance database is now set to separately produce the dipolar and quadrupolar transverse impedance and wakes that the HEADTAIL simulation code needs to accurately simulate the effect of the impedance on the beam dynamics.
EPFL, Lausanne
INFN/LNF, Frascati (Roma)
CERN, Geneva
BNL, Upton, Long Island, New York
UMAN, Manchester
A detailed knowledge of the beam coupling impedance of the CERN Super Proton Synchrotron (SPS) is required in order to operate this machine with a higher intensity for the foreseen Large Hadron Collider (LHC) luminosity upgrade. A large number of Beam Position Monitors (BPM) is currently installed in the SPS, and this is why their contribution to the SPS impedance has to be assessed. This paper focuses on electromagnetic simulations and bench measurements of the longitudinal and transverse impedance generated by the horizontal and vertical BPMs installed in the SPS machine.
CERN, Geneva
EPFL, Lausanne
The CERN PS crosses transition energy at about 6 GeV by using a second order gamma jump performed with special quadrupoles. However, for high-intensity beams, and in particular the single bunch beam for the neutron Time-of-Flight facility, a controlled longitudinal emittance blow-up is still needed to prevent a fast single-bunch vertical instability from developing near transition. A series of studies have been done in the PS in 2008 to measure the beam behaviour near transition energy for different settings of the gamma transition jump. The purpose of this paper is to compare those measurements with simulations results from the HEADTAIL code, which should allow to understand better the different mechanisms involved and maybe improve the transition crossing.
CERN, Geneva
Wendel GmbH, Dillenburg
During the 2007 run with the nominal LHC proton beam, electron cloud has been clearly identified and characterized in the PS using a dedicated setup with shielded button-type pickups. Efficient electron cloud suppression could be achieved with a stainless steel stripline-type electrode biased to negative and positive voltages up to ± 1 kV. For the 2008 run, a second setup was installed in straight section 84 of the PS where the stainless steel was replaced by a stripline composed of an enamel insulator with a resistive coating. In contrast to ordinary stripline electrodes this setup presents a very low beam coupling impedance and could thus be envisaged for long sections of high-intensity machines. Here, we present first comparative measurements with this new type of enamel clearing electrode using the nominal LHC beam with 72 bunches and 25 ns bunch spacing.
CERN, Geneva
UMAN, Manchester
EPFL, Lausanne
Honorary CERN Staff Member, Grand-Saconnex
The LHC phase 2 collimation project aims at gaining a factor ten in cleaning efficiency, robustness and impedance reduction. From the impedance point of view, several ideas emerged during the last year, such as using dielectric collimators, slots or rods in copper plates, or Litz wires. The purpose of this paper is to discuss the possible choices, showing analytical estimates, electro-magnetic simulations performed using Maxwell, HFSS and GdFidL, and preliminary bench measurements. The corresponding complex tune shifts are computed for the different cases and compared on the stability diagram defined by the settings of the Landau octupoles available in the LHC at 7 TeV.
CERN, Geneva
The HEADTAIL code has been used for many years to study the interaction of a single bunch with a localized or lumped source of electromagnetic perturbation, usually self-induced (impedance, electron cloud or space charge). It models the bunch as macroparticles and at each turn slices up the bunch into several adjacent charged disks, which are made to subsequently interact with the perturbing agent. A first step toward the extension of HEADTAIL to multi-bunch simulations is presented in this paper. In this case, the bunches themselves are modeled as charged disks and are not sliced, which makes us lose information on the intra-bunch motion but can describe a zero mode interaction between different bunches in a train. The interaction of an SPS bunch train of 72 bunches with the resistive wall or a narrow-band impedance is studied as an example.
CERN, Geneva
First reference measurements of the longitudinal impedance were made with beam in the SPS machine in 1999 to quantify the results of the impedance reduction programme, completed in 2001. The 2001 data showed that the low-frequency inductive impedance had been reduced by a factor 2.5 and that bunch lengthening due to the microwave instability was absent up to the ultimate LHC bunch intensity. Measurements of the quadrupole frequency shift with intensity in the following years suggest a significant increase in impedance (which nevertheless remains below the 1999 level) due to the installation of eight extraction kickers for beam transfer to the LHC. Microwave instability is still not observed up to the maximum bunch intensities available from injector. The experimental results are compared with expectations based on the known longitudinal impedance of the different machine elements in the SPS.
Fermilab, Batavia
CERN, Geneva
Funding: Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy and CARE-HHH
Motivated by the discussions on scenarios for LHC upgrades, beam studies on the stability of flat bunches in a double-harmonic RF system have been conducted in the CERN Proton Synchrotron (PS). Injecting nearly nominal LHC beam intensity per cycle, 18 bunches are accelerated on harmonic h=21 to 26 GeV with the 10 MHz RF system. On the flat-top, all bunches are then transformed to flat bunches by adiabatically adding RF voltage at h=42 from a 20 MHz cavity in anti-phase to the h=21 system. The voltage ratio V(h42)/V(h21) of about 0.5 was set according to simulations. For the next 140 ms, longitudinal profiles show stable bunches in the double harmonic RF bucket until extraction. Without the second harmonic component, coupled-bunch oscillations are observed. The flatness of the bunches along the batch is analyzed as a measure of the relative phase error between the RF systems due to beam loading. Measurements of electron cloud effects induced by the beam are also discussed. The results of beam dynamics simulations and their comparison with the measured data are presented.
Fermilab, Batavia
Funding: work supported by the US Department of Energy
Stability issues of the mu2e proton beam are discussed. These include space-charge distortion of bunch shape, microwave instabilities, head-tail instabilities, as well as electron cloud effects.
HZB, Berlin
Bursts of coherent synchrotron radiation (CSR) in the far IR and down to the μ-wave region have been observed in many synchrotron light sources. At BESSY II the temporal structure of these pulses in the THz-region was observed as a function of the bunch length which was varied by changing the momentum compaction factor and as a function of the number of electrons in the single bunch. It was found, that for a bunch length between 3 and 15 ps the first signs of time dependent CSR occur at a frequency which is a multiple of the zero current synchrotron frequency. This frequency increases with the bunch length and indicates that higher azimuthal modes become unstable first. Slower bursts, with repetition rates on the time scale of mill seconds and much slower than the synchrotron period, show up slightly above this threshold. These bursts possess the much faster initial temporal structure and are probably the result of longitudinal mode mixing. The experimental observations are presented and compared to calculations.
Illinois Institute of Technology, Chicago, Illinois
IIT, Chicago, Illinois
Fermilab, Batavia
Funding: This work supported by NSF grant No. 0237162, and DOE SCIentific Discovery through Advanced Computing: Accelerator science and simulation DE-PS02-07ER07-09
The FNAL Booster is a combined-function proton synchrotron with a bunch intensity of ~6·1010 protons; significantly greater than expected in the original design. The injection energy is 400 MeV (gamma factor 1.4), low enough for space charge forces to play a role in beam dynamics. The magnets are used directly as vacuum tanks, so the laminated pole surfaces contribute significantly to impedance. A study of the transverse coupling dependence on beam intensity is presented here. Experimental results are being analyzed using Synergia, a high-fidelity, parallel, fully 3D modeling code that includes both space charge and impedance dynamics. Previously, Synergia has always shown good agreement with experimental data. Our initial studies show that the direct space charge contribution to beam dynamics is too small to account for the increase in the coupling seen experimentally, corroborating analytic results. Parametric studies of the impedance needed to match the measured coupling are being done. Agreement between simulation and experiment should provide an independent measure of the Booster impedance, which has been analytically modeled and calculated elsewhere.
JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken
KEK, Ibaraki
Kicker magnets are ones of dominant sources of impedances in the 3GeV Rapid Cycling Synchrotron (RCS) at Japan Proton Accelerator Research Complex (J-PARC). They may be limiting factors in achieving high intensity beams. Recently, the 300kW beam was accomplished at 3GeV RCS, while no instability was observed. In this paper, the space-charge effects are studied as beam stabilization effects.
SLAC, Menlo Park, California
KEK, Ibaraki
Using a streak camera, we measured the longitudinal profiles of a positron bunch in the Low Energy Ring (LER) of KEKB at various currents. The measured charge densities were used to construct a simple Q=1 broadband impedance model. The model with three parameters not only gave an excellent description of longitudinal dynamics for a positive momentum compaction factor but also for the negative ones, including bunch shortening bellow a threshold and bursting modes beyond the threshold. Furthermore, our study indicated that the threshold of microwave instability was about 0.5 mA in bunch current in the LER. At the nominal operating current 1.0 mA, there was a 20% increase of the energy spread. The results of measurement, analysis, and simulations will be presented in this paper.
SLAC, Menlo Park, California
INFN/LNF, Frascati (Roma)
Funding: work supported by the Department of Energy under contract number DE-AC03-76SF00515
We give an overview of wake fields and impedances in a proposed Super B project, which is based on extremely low emittance beams colliding at a large angle with a crab waist transformation. Understanding the effect wake fields have on the beam is critical for a successful machine operation. We use our combined experience from the operation of the SLAC B-factory and DAΦNE Phi-factory to eliminate strong HOM sources and minimize the chamber impedance in the Super B design. Based on a detailed study of the wake fields in this design we have developed a quasi-Green’s function for the entire ring that is used to study bunch lengthening and beam stability. In particular, we check the stability threshold using numerical solutions of the Fokker-Plank equation. We also make a comparison of numerical simulations with the bunch lengthening data in the B- factory.
SLAC, Menlo Park, California
A proposed high-brightness synchrotron light source (PEP-X) is under design at SLAC. The 4.5-GeV PEP-X storage ring has four theoretical minimum emittance (TME) cells to achieve the very low emittance and two double-bend achromat (DBA) cells to provide spaces for IDs. Damping wigglers will be installed in zero-dispersion straights to reduce the emittance below 0.1nm. In this paper, we present a preliminary estimation of the threshold of the transverse mode coupling instability(TMCI). Three approaches have been used in the estimation and they agree well with each other.
Tech-X, Boulder, Colorado
Fermilab, Batavia
Funding: This work was supported by the US DOE Office of Science, Office of Nuclear Physics, under Grant No.DE-FG02-08ER85183
Recent improvements to BBSIM permit detailed simulations of collective effects due to resistive wall wake fields. We compare results of beam transfer measurements (BTF) in the Tevatron and RHIC with and without the effects of resistive wall wake fields. These are then compared to actual BTF measurements made in both machines and the impact of intensity on our measurements. We also investigate the impact of resistive wall wake fields on various chromaticity measurement approaches.
CERN, Geneva
The PIMS will accelerate an H- beam from 100 MeV to 160 MeV, the output energy of Linac4. The cell length is constant within each of the 12 seven-cell cavities, but increases from cavity to cavity according to the increasing beam velocity. Its mechanical design is derived from the five-cell normal conducting LEP cavities, which were in operation at CERN for approximately 15 years. Even though the shunt impedance is around 10% lower than for a Side-Coupled Linac (SCL) operating at 704 MHz, the PIMS has the advantage of using the same RF frequency (352 MHz) as all the other accelerating structures in Linac4, thus simplifying and standardising the linac RF system. Furthermore, the simplified mechanical construction of the PIMS, which uses only 84 cells instead of over 400 for the SCL, also reduces construction costs and tuning effort. In this paper we present the electromagnetic design of the PIMS, including the arguments for the choice of a 5% cell-to-cell coupling factor, the shape of the coupling cells, the dimensioning of the wave-guide ports, and the expected field errors during operation.
Naples University Federico II and INFN, Napoli
Istituto Nazionale di Fisica Nucleare, Milano
Naples University Federico II, Napoli
The use of BBAC (Back-to-Back Accelerating Cavity) tiles in proton Side Coupled Linacs can be extended down to energies of the order of 20 MeV, keeping more than suitable shunt impedances and energy gradients. However, the considerable energy absorption from the cavity noses may induce a remarkable increase in their temperature. This may cause both a strong duty-cycle-dependent detuning of the modules, and dangerous thermo-mechanical stress due to the non-uniform temperature distribution. An innovative shape of the BBAC tile is proposed, which allows to limit the temperature rise within a safe range, without introducing detrimental effects neither on the shunt impedance nor on the working frequency. A protocol for the design of such a cavity will be presented.
PKU/IHIP, Beijing
Funding: supported by NSFC (19775009)
A trapezoidal IH-RFQ (T-IH-RFQ) is being built to accelerate 14C^+ from 40 keV to 500 keV, motivated by RFQ based 14C AMS application at Peking University. The last design of beam dynamics and the optimized results of RF structure will be presented in this paper. The length of the cavity is about 1.1m operating at {10}4MHz, with a designed transmission efficiency of more than 97%. A special feature is that the RFQ output beam energy spread is as low as 0.6% approached by the method of internal discrete bunching. On the other hand, the new RF cavity structure T-IH-RFQ was proposed for the beam dynamics design, which has higher resonant frequency than traditional four rods RFQ and IH-RFQ at the same transverse dimension. Microwave Studio (MWS) simulations have been performed to study the field distribution and power consumption characteristic of this T-IH-RFQ. The specific shunt impedance and the quality factor have been optimized. Those details will be given.
RLNR, Tokyo
An Interdigital-H (IH) linac has been used for ion acceleration in low beta range. It can realize a resonant cavity of a convenient size at low frequencies and higher shunt impedance at low energy range. These characteristics are advantageous especially for heavy ion acceleration. Since the shunt impedance of the IH linac reduces according to the increasing of beam energy, the linacs operated by the TM010 mode such as an Alvarez type and a coupling cavity type are adopted for medium and high energy range. However, we propose the new IH linac using the TE11n mode, the higher order mode IH (HOM-IH) linac. By using the higher order mode, the resonance frequency is higher than that of the IH linac. This property is suitable for middle and high beta linacs, and a proton linac as well. The design of the cavity structure and the possibility are presented.