IAP, Frankfurt am Main
GSI, Darmstadt
Funding: Work supported by BMBF
The Frankfurt Neutron Source at the Stern-Gerlach-Zentrum will comprise a short 175 MHz linac sequence consisting of a 1.4 m long 700 keV 4-rod type RFQ followed by a 50 cm IH-DTL for proton acceleration up to 2 MeV. The beam current is 200 mA at pulsed and 30 mA at cw operation. The aim is to have a very compact device driven by only one rf amplifier to reduce costs and required installation space. A coupling between the RFQ and the IH resonators by means of a galvanic connection is foreseen, which is realized by two brackets going right away through a common partitioning end flange lid. The accelerators could also be driven separately by just removing the brackets. The distance between the end of the RFQ electrodes and the middle of the first DTL gap is only 5 cm, there is no additional beam optics in between. Preliminary beam dynamics and rf simulations have been carried out together with accompanying measurements on rf models.
RAS/INR, Moscow
Instead of proved application of superconducting structures for high energy part of intense linear proton accelerators, normal conducting structures are still considered for medium and low energy parts below 200 MeV. Operation with accelerating rate ~4 MeV/m and duty factor ~5% results for standing wave normal conducting structure in an average heat loading ~30 kW/m. Due to the high heat loading an operating mode frequency shift is significant during operation. In this paper conditions for field distribution stability against small deviations in time of individual cell frequencies are considered. For pi/2 structures these conditions were formulated by Y. Yamazaki and L. Young. General case of 0, pi/2 and pi operating modes is considered with common approach.
CERN, Geneva
BINP SB RAS, Novosibirsk
RFNC-VNIITF, Snezhinsk, Chelyabinsk region
The 352 MHz CCDTL will accelerate the Linac4 beam from 50 to 102 MeV. It will be the first CCDTL used in a proton linac. Three short DTL tanks, each having two drift tubes, are connected by coupling cavities and form a chain of 5 resonators operating in the stable π/2 mode. The CCDTL section is made of 7 such chains, each fed by a 1.3 MW klystron. Focusing quadrupoles are placed between tanks, easing their alignment with respect to a conventional DTL thus making the structure less sensitive to manufacturing errors. In order to validate the design and to develop the production technology, two prototypes have been constructed and successfully tested. The first prototype, built at CERN, consists of two half-cavities and one coupling cell, whereas the second, with two full cavities and one coupling cell, was built at VNIITF and BINP in Russia in the frame of an R&D contract funded by the ISTC Organisation. Both prototypes have been tested at CERN slightly beyond their nominal power level, at the design duty cycle of 10%. In this paper we present the results of high-power tests, the results of the technological developments prior to production, and the final design of the CCDTL.
INFN/LNL, Legnaro, Padova
Università degli Studi di Milano, Milano
The RFQ of IFMIF-EVEDA project is characterized by very challenging specifications, with 125 mA of deuteron current accelerated up to 5 MeV. Upon beam dynamics studies, it has been chosen a law for the variation of R0 and voltage along the structure; this law provides a significant reduction in terms of structure length, beam losses and rf power consumption. Starting from these outcomes, the rf study of the RFQ, aimed at determining the optimum design of the cavity shape, was performed. The stabilization issues were also addressed, through the analysis of the RFQ sensitivity to geometrical errors, by means of perturbative theory-based algorithms developed for this purpose . Moreover the determination of the main 3D details of the structure was also carried out. In this article the results of the rf studies concerning the above-mentioned topics are outlined.
Consorzio RFX, Associazione Euratom-ENEA sulla Fusione, Padova
INFN/LNL, Legnaro, Padova
CERN, Geneva
The Legnaro National Laboratory (LNL) is building the 30 mA, 5 MeV front end injector for the production of intense neutron fluxes for interdisciplinary application. This injector comprises a proton source, a low energy beam transport line (LEBT), a radio frequency quadrupole (RFQ) and a beam transport line designed to provide a 150 kW beam to the berillium target used as neutron converter. The RFQ, developed within TRASCO project for ADS application, is designed to operate cw at 352.2 MHz. The structure is made of OFE copper and is fully brazed. The RFQ is built in 6 modules, each approximately 1.2 meter long. This paper covers the mechanical fabrication, the brazing results and acceptance tests for the various modules.
GANIL, Caen
CEA, Gif-sur-Yvette
LPSC, Grenoble
IPN, Orsay
The SPIRAL2 superconducting linac is composed of 2 cryomodule families, basically one of low beta, called Cryomodule A, and one of high beta, called Cryomodule B. The low beta family is composed of 12 single cavity cryomodule. The high energy section is composed of 7 cryomodules hosting 2 cavities each. According to beam dynamics calculations all the cavities will operate at 88 MHz: one family at beta=0.07, and one at beta=0.12. The design goal for the accelerating field Eacc of the SPIRAL2 QWRs is : 6.5 MV/m. The configuration, cavities and cryomodule tests and status and the foreseen linac tuning will be described.
IIT, Chicago, Illinois
Illinois Institute of Technology, Chicago, Illinois
LBNL, Berkeley, California
Funding: This work was partially supported by the Office of Science, U. S. Department of Energy, under Contract No. DE-AC02-05CH11231.
A key unanswered question in particle physics is why the universe consists only of matter. It is believed that CP violation in the lepton sector is the answer. The best tool to find this is a muon-based Neutrino Factory. Muons can also be used for an energy-frontier collider that would fit on an existing laboratory site. Since muons are produced as a tertiary beam, their phase space and energy spread are large and must be reduced (cooled) to create a usable beam. Ionization cooling, comprising momentum loss in material followed by rf reacceleration, is the only suitable technique. A cooling channel is merely a linac with absorbing material in the beam path. To demonstrate an understanding of the physics and technology issues, MICE will test a section of cooling channel exposed to a muon beam derived from the ISIS synchrotron at RAL. The muon beam line is now installed and commissioning is under way. Fabrication of cooling channel components and the required detector systems has begun and will be described. A successful demonstration will go a long way toward proving the value of muon beams for future accelerator-based particle physics experiments.
UMAN, Manchester
The globalised scattering matrix (GSM) method provides an efficient means of obtaining the electromagnetic field in interconnected multi-cavity structures. In the proposed XFEL at DESY and the ILC facilities, energetic electron beams can readily excite higher order modes which if left unchecked can dilute the emittance of the beams. The GSM in conjunction with finite element modelling of the scattering matrices of the linac cavities is used to enable the characteristic eigenmodes to be rapidly obtained and the potential for trapped modes is investigated. This characteristic eigensystem allows the wakefield experienced by the beam to be analysed and the consequences on beam quality ascertained. The impact of fabrication errors on the transverse electromagnetic field and corresponding resonant frequencies of the modes is also explored in detailed simulations.
LANL, Los Alamos, New Mexico
Particle beams have exhibited a two-stream instability for many decades; this undesirable trait has been well-understood for many years. We propose creating a scheme that uses a beam of electrons with two distinct energies that will develop the two-stream instability as a bunching mechanism. By controlling the beam parameters and seeding them with a low-level rf signal, a gain as high as 2.5 dB per centimeter is predicted. We show the theory behind this concept and recent progress in a developing experiment.
Kyoto ICR, Uji, Kyoto
KEK, Ibaraki
A final focus quadrupole (FFQ) doublet of ILC should have excellent properties such as strong focusing, compactness and less vibrations. In a baseline design, superconducting magnet is supposed to be used, which may have some vibrations traveling through liquid helium. It may not be suitable for FFQ of ILC unless the vibration effect is proven to be negligible. Since the five-disc-singlet proposed by Gluckstern satisfies these properties including continuous adjustability, we are developing a FFQ aiming at a beam test at ATF2. Although the x-y coupling effect is carefully cancelled in the design, fabrication errors or rotation errors may break the cancellation. We are estimating the effect of these errors on the beam size at the interaction point. Two methods are currently carried out. The first one is transfer matrix calculations, which neglects fringing field and higher multipole components. The second one is beam-tracking calculation in measured or calculated magnetic field. The fabricated magnet is under adjustment measuring the magnetic field. The recent results will be presented.
LANL, Los Alamos, New Mexico
AES, Medford, NY
Funding: This work is supported by ONR and HEL-JTO.
The LANL/AES 2.5-cell, normal-conducting radio-frequency (NCRF) injector has been fabricated. This room-temperature injector can be used to generate cw electron beams with average current greater than 100 mA and beam energy up to 2.5 MeV prior to injection into an energy-recovery linac. PARMELA simulations show the effectiveness of emittance compensation in generating high-brightness electron beams at relatively low accelerating gradients. We present the initial measurement results of the rf, accelerator and vacuum properties of the NCRF injector and the associated ridge-loaded waveguides. The impact of these rf measurement results on the planned thermal and electron beam tests will also be discussed.
Cockcroft Institute, Lancaster University, Lancaster
UMAN, Manchester
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
The CLIC linear collider will require a crab cavity to align bunches prior to collision. Consideration of the bunch structure leads us to favour the use of X-band copper cavities. Due to the large variation of train to train beam loading, it is necessary to minimise the consequences of beam loading. One solution is to use a travelling wave structure with a large group velocity allowing rapid propagation of amplitude errors from the system. Such a design makes this structure significantly different from previous travelling wave deflecting structures. This paper will look at the implications of this on other cavity parameters and the optimization of the cavity geometry.
ANL, Argonne
A set of six new 109 MHz β=0.15 superconducting quarter-wave resonators (QWR) has been built at ANL as part of an upgrade to the ATLAS superconducting heavy-ion linac. The final cavity string assembly will also use many of the techniques needed for the next generation of large high-performance ion linacs such as the U.S. Department of Energy's FRIB project. Single-cavity cold tests at T=4.5 K have been performed for three cavities with moveable coupler, rf pickup, and VCX fast tuner as required for the full 6-meter cryomodule assembly. The average maximum accelerating gradient of 4 cavities (3 new + 1 prototype), is EACC=11.2 MV/m (BPEAK=65 mT). Clean cavity string assembly techniques, required here and for most future SRF ion linacs, are fairly well developed. Details on cavity performance including high-field cw operation, microphonics and fast tuning are presented.
SLAC, Menlo Park, California
Funding: Work supported by DOE contract DE-AC02-76SF00515.
The TESLA-shape cavity has been chosen as the baseline design for the 1.3 GHz SCRF linacs of the International Linear Collider. However, there is ongoing research to develop new cavity shapes that will support higher gradients and hence lower the machine cost. The critical magnetic flux (Bc) of the niobium, which is approximately 180 mT, ultimately limits the gradient achievable in a superconducting cavity. Thus far, the new designs have focused on minimizing the peak surface magnetic field (Bs) for a given on-axis gradient, while relaxing the requirement on the peak surface electric field (Es). For example, the Low Loss (LL) design reduces Bs by more than 10% relative to the baseline design, which should allow a gradient of up to 50 MV/m with a 20% reduction in cryogenics loss. However, Es is about 15% higher in this case, which enhances field emission that in practice is one of the main impediments to achieving the Bc-limited gradient. In this paper, we will present an optimized cavity shape that reduces both Bs and Es, and thus should have a better chance of reaching higher gradients. The design of HOM couplers for wakefield damping in this cavity will also be presented.
JLAB, Newport News, Virginia
ODU, Norfolk, Virginia
Funding: Supported by US DOE Contract No. DE-AC05-06OR23177
In low current applications superconducting cavities have a high susceptibility to microphonics induced by external vibrations and pressure fluctuations. Due to the narrow bandwidth of the cavities, the amount of rf power required to stabilize the phase and amplitude of the cavity field is dictated by the amount of microphonics that need to be compensated. Electronic damping of microphonics is investigated as a method to reduce the level of microphonics and of the amount of rf power required. The current work presents a detailed analysis of electronic damping and of the residual cavity field amplitude and phase errors due to the fluctuations of cavity frequency and beam current.
JLAB, Newport News, Virginia
SLAC, Menlo Park, California
Funding: This manuscript has been authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
Higher Order Modes generated by a particle beam passing through a superconducting accelerating cavity have to be damped to avoid beam instabilities. A coaxial coupler located in the beam pipes of the cavities provides for better propagation of HOMs and strong damping in appropriate HOM dampers. The whole damping device can be designed as a detachable system. If appropriately dimensioned, the rf currents can be minimized at the flange position. Additionally, the coaxial system also provides efficient coupling of fundamental mode rf power into the superconducting cavity. Compared to presently available solutions for HOM damping, this scheme provides for several advantages: stronger HOM damping, flangeable solution, exchangeability of the HOM damping device on a cavity, less complexity of the superconducting cavity, possible cost advantages. This contribution will describe the results of room temperature measurement and discuss modeling, which resulted in an optimized layout of a cavity-coupler system.
JAEA/LINAC, Ibaraki-ken
JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken
KEK, Ibaraki
RAS/INR, Moscow
J-PARC, KEK & JAEA, Ibaraki-ken
The ACS (Annular-ring Coupled Structure) cavities were under development for the J-PARC Linac from 190 MeV to 400 MeV. We have fixed the cavity specification, taking into account the results of the high-power conditioning and the fabrication experience. The mass production of the ACS with a tight time schedule is now an issue, since the user community strongly requests the beam power upgrade as early as possible. Therefore, the design and the fabrication process of the ACS cavity have been reexamined on the basis of the experience, stored during the course of the fabrication and the tuning of the prototype ACS tanks. Here, we also discussed about the key issues on the mass production with a manufacturer. The cavity shape, that required complicated machining, was simplified to some extent, while the frequency tuning strategy was reconsidered to reduce the production period. The paper describes these recent activities on the ACS development.
MEPhI, Moscow
ScanTech, Atlanta, Georgia
The results of analysis and comparison of different linear accelerator designs for 10 MeV facility powered by 4.5 MW klystron on 5712 MHz operation frequencies presented. Several concepts of accelerator including standing wave and traveling wave ones with either rf or magnetic focusing were considered. Cells geometry and beam dynamics parameters in these types of accelerators featuring high capture factor were obtained using numeric simulation methods. The computer simulation code for traveling wave linac optimization based on beam dynamics with space charge consideration was developed. Accelerating structures and input coupler for traveling wave linac along with standing wave one were designed. The task of energy variation was solved.
RAS/INR, Moscow
For intense proton beam acceleration the structure aperture diameter should be ~30 mm. With such aperture room temperature coupled cell accelerating structures have the maximal effective shunt impedance Ze at operating frequency ~650 MHz. For this frequency well known Side Coupled Stricture (SCS), Disk and Washer Structure (DAW), Annular Coupled Structure (ACS) have large transversal dimension, leading to essential technological problems. The Cut Disk Structure (CDS) has been proposed to join high Ze and coupling coefficient kc values, but preferably for high energy linacs. In this report parameters of the four windows CDS option are considered at operating frequency ~700 MHz for proton energy range from 80 MeV to 200 MeV. The cells diameter ~30 cm and kc ~0.12 result naturally, but Ze value is of (0.7-0.9) from Ze value for SCS (kc=0.03). Small cells diameter opens possibility of CDS applications for twice lower frequency and structure parameters at operating frequency ~ 350 MHz are estimated too. Cooling conditions for heavy duty cycle operation are considered.
CERN, Geneva
The high-energy section of Linac4, between 100 and 160 MeV, will be made of a sequence of 12 seven-cell accelerating cavities of the Pi-Mode Structure (PIMS) type, resonating at 352 MHz. Compared to other structures used in this energy range, cavities operating in pi-mode with a low number of cells have the advantage of simplified construction and tuning, compensating for the fact that the shunt impedance is about 10% lower because of the lower frequency. Field stability in steady state and in presence of transients is assured by the low number of cells and by the relatively high coupling factor of 5%. Standardising the linac rf ystem to a single frequency is considered as an additional economical and operational advantage. The mechanical design of the PIMS will be very similar to that of the 352 MHz normal conducting 5-cell LEP accelerating cavities, which have been successfully operated at CERN for 15 years. After reviewing the basic design principles, the paper will focus on the tuning strategy, on the field stability calculations and on the mechanical design. It will also report the results of measurement on a cold model and the design of a full-scale prototype.
SLAC, Menlo Park, California
Funding: Work supported by the U.S. Department of Energy under contract DE-AC02-76SF00515.
We report on the L-band rf power distribution system (PDS) developed at SLAC for Fermilab's NML superconducting test accelerator facility. The makeup of the system, which allows tailoring of the power distribution to cavities by pairs, is briefly described. Cold test measurements of the system and the results of high power processing are presented. We also investigate the feasibility of eliminating the expensive, lossy circulators from the PDS in the ILC linacs by taking advantage of our scheme of pair-feeding through 3-dB hybrids. A computational model is used to simulate the impact on field stability of inter-cavity coupling due to reduced isolation. Measurements of typically achievable hybrid port isolations provide the likely magnitude for such coupling.
SLAC, Menlo Park, California
Funding: Work supported by the U.S. Department of Energy under contract DE-AC02-76SF00515.
I introduce a novel normal-conducting accelerator structure combining standing wave and traveling wave characteristics, with relatively open cells. I describe the concept and geometry, optimize parameters, and discuss the advantages and limitations this new structure presents.
Linac Systems, LLC, Albuquerque, New Mexico
Most small proton and other ion linacs involve two different linac structures, namely an RFQ linac section and some other, more efficient, linac structure, such as the Drift Tube Linac (DTL), the interdigital (Wideroe) linac, or the Rf Focused Interdigital (RFI) linac. Such linacs can benefit a lot by being resonantly coupled into a single resonant unit. The resonantly coupled structures can be driven by a single rf power system, through single rf drive loop, at a single rf frequency. The relative phase and relative amplitude of the fields in the two structures are locked by the resonant coupler. Such systems require no control of phase of the rf power. By designing the rf power system to track the resonant frequency of the combined structures, the control of the resonant frequencies of the two structures is greatly simplified. A simple, compact, resonant coupler, based on a quarter-wave-stub, will be described. Models of this resonant couple have been tuned and adjusted, and are scheduled to be tested at operating powers in the early fall (2008).
Fermilab, Batavia
Following development and testing a prototype waveguide-based high power phase shifter, a design concept of a high power fast phase shifter has been developed. The shifter uses ferrite blocks positioned in a rectangular waveguide. The waveguide cross-section is chosen to suppress most of resonances that could otherwise be a limiting factor for the phase shifter high power performance. Base bias field is created with the use of permanent magnets. Low inductance coils in the same magnetic circuit excite fast (pulsed) bias field component. The waveguide is designed in a way to ensure that the pulsed magnetic field penetrates inside the waveguide with minimum delay while allowing effective heat extraction from the ferrite blocks. This report provides details of the system design, including expected rf behavior and frequency range.
ORNL, Oak Ridge, Tennessee
Funding: This work was supported by SNS through UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the U.S. DOE.
An algorithm for direct high power rf vector control of fan-out rf distribution using reactive circuit elements is presented. In this approach, rf control is performed for the entire fan-out system with many cavities as one system to maximize the rf power efficiency. Control parameters for a set of required rf voltage vectors in the accelerating cavities are determined and maintained for the whole system. Maximizing rf power efficiency with fan-out power distribution can be valuable for large scale SRF accelerators since construction and operation costs can be saved significantly. If a fan-out system employs a fixed power splitter with high power vector modulators in cavity inputs, the optimum power efficiency especially for a SRF system can not be provided since certain rf power headroom is needed for the vector control at each cavity. In the new fan-out vector control approach, a set of required cavity rf voltages is delivered by adjusting the phase delays between the cavities and the reactive loadings at the cavity inputs. The phase shifts and the reactive loadings are realized with high power rf phase shifters.
TRIUMF, Vancouver
A major focus of the linac community is to develop technology in support of the ILC project. The science motivation for the ILC will be presented with reference to the particle physics programs at Fermilab and the LHC.