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.
GANIL, Caen
INFN/LNS, Catania
ALTEK, San Gregorio (CATANIA)
The SPIRAL2 LEBT line uses a single chopper situated in the line section common to protons, deuterons and A/Q=3 ions. The paper describes the design and the test of the power circuits, based on standard components and working up to 10 kV, at a 1 kHz repetition rate.
GANIL, Caen
The SPIRAL2 project uses a RFQ, normal conducting rebunchers and a superconducting linac to accelerate high intensity beams of protons, deuterons and heavier ions. All cavities work at 88 MHz, the beta after of the RFQ is 0.04 and 3 rebunchers are located in the MEBT line, which accepts ions with A/q up to 6. The paper describes the RF design and the technological solutions proposed for an original 3-gap cavity, characterised by very large beam holes (60 mm) and providing up to 120 kV of effective voltage.
CERN, Geneva
MPI-K, Heidelberg
GSI, Darmstadt
Rebunching cavities are today routinely used for matching a beam of charged particles between different accelerator structures, and thus optimizing the overall transmission and beam quality. At low resonance frequencies, unnecessary large dimensions of these cavities can be avoided by using spiral-loaded cavities. The optimization of these structures is a complicated process in which a wide range of different parameters have to be modified essentially in parallel. In this contribution, we investigate in detail the characteristics of a model structure with the 3D code OPERA/SOPRANO. This includes the optimization of the structure in terms of the spiral geometry for a given resonance frequency, the investigation of power losses on the inner surfaces, and the possibility of cavity tuning by means of a tuning cylinder.
JASRI/SPring-8, Hyogo-ken
The acceleration gradient is limited by breakdown in an accelerating rf structure, including its surface condition of the inner wall. The surface treatment is an important technique to achieve the maximal acceleration gradient of an accelerating structure. We chose chemical etching as a method of surface treatment for accelerating rf structures made of copper. To study rf breakdown and effect of surface treatments, we used a pillbox-type single cell rf gun cavity. The highest cathode surface field (190 MV/m) of rf gun cavity was accomplished with this surface treatment under rf-conditioning elapsed time (21 days) in 2004. SPring-8 rf gun has been operating with the highest gradient in the world. This indicates that our treatment is considerably effective to improve the inner cavity surface made of copper. Further, we developed the high-pressure chemical etching for more complicated inner structures in 2006. Using a cartridge-type photocathode rf gun, high-field experiments were performed with cathode plugs chemical etching treated under deferent pressure condition. We report these results on highest gradient, using test copper samples treated with high-pressure chemical etching.
KEK, Ibaraki
Nextef is a new X-band (11.4GHz) test facility in KEK. All of the key devices of this facility are from our old X-band Test Facility(XTF). By combining the power from two klystrons, 100 MW maximum X-band rf power is produced and 75MW is available in the bunker where the high power test of the high gradient accelerator structures will be done. The commissioning of the facility for the structure testing has almost done. The status of the facilityis is reported.
KEK, Ibaraki
High-gradient experiments have been performed using a narrow waveguide that has a field of approximately 200 MV/m at an rf power of 100 MW. The study investigates the characteristics of different materials at high-gradient rf breakdown. This paper reports the results of high-gradient experiments and observations of the surface of stainless-steel waveguides.
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.
ISIR, Osaka
Funding: This research is partly supported by the accelerator support program to universities conducted by the High Energy Accelerator Research Organization in Japan.
The 40 MeV L-band electron linac at the Institute of Scientific and Industrial Research, Osaka University is operated for joint-use in Osaka University. It is equipped with a three-stage sub-harmonic buncher (SHB) system consisting of two 108 MHz and a 216 MHz rf cavities to produce a high-intensity single-bunch beam. They were quarter-wavelength coaxial cavities made of a clad plate of copper on stainless steel and were inefficiently cooled with water flowing through a pipe wound on their outer surfaces made of stainless steel. We have renewed the cavities with new ones made only of oxygen-free copper to solve the problem. We made physical design and basic mechanical design of the new rf cavities by ourselves by taking a mechanical design of the SHB cavity of the electron-positron linac at KEK, Japan as a model. Special care was devoted in the mechanical design to cool the most part of the cavities directly with water instead of relying on heat conductivity in copper so that they are stable in regard to temperature. They have been installed in the linac and have been working well in expected performance. We will report details of design and fabrication of the new SHB cavities.
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.
RAS/INR, Moscow
TRIUMF, Vancouver
In this report we consider room temperature DTL structure for heavy ions acceleration in energy range 150 keV/u - 400 keV/u. The structure design is based on known and proven solutions. Due to design idea, the structure has no end wall problem. It allows flexible segmentation in cavities and transverse focusing elements placing outside cavity. As compared to well known IH DTL, considered structure has smaller transverse dimensions and is designated for lower operating frequency. The structure promises high rf efficiency - with careful elements optimization calculated effective shunt impedance value is higher than 1.0 GOhm/m for operating frequency ~ 70 MHz, E~150 keV/u.
CERN, Geneva
SLAC, Menlo Park, California
KEK, Ibaraki
In recent years evidence has been found that the maximum sustainable gradient in an accelerating structure depends on the rf power flow through the structure. The CLIC study group consequently designed a new prototype structure for CLIC with a very low group velocity, input power and average aperture (a/λ = 0.12). The 18 cell structure has a group velocity of 2.4% at the entrance and 1% at the last cell. Several of these structures have been made in collaboration between KEK, SLAC and CERN. A total of five brazed-disk structures and two quadrant structures have been made. The high power results of some of these structures are presented. The first KEK/SLAC built structure reached an unloaded gradient in excess of 100 MV/m at a pulse length of 230 ns with a breakdown rate below 10-6. The high-power testing was done using the NLCTA facility at SLAC.
CERN, Geneva
The rf design of an accelerating structure for the CLIC main linac is presented. The structure is designed to provide 100 MV/m averaged accelerating gradient at 12 GHz with an rf-to-beam efficiency as high as 27.7%. The design takes into account both aperture and HOM damping requirements coming from beam dynamics as well as the limitations related to rf breakdown and pulsed surface heating.
CERN, Geneva
A new local field quantity which gives the high gradient performance limit of accelerating structures in the presence of vacuum rf breakdown is presented. A model of the breakdown trigger based on the pulsed heating of a potential breakdown site by the field emission currents and driven by a new field quantity, a modified Poynting vector, has been derived. The field quantity Sc takes into account both active and reactive power flow on the surface. This new quantity has been evaluated for many X-band and 30 GHz rf tests, both travelling wave and standing wave, and the value of Sc achieved in the experiments agrees well with analytical estimates.
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.
STFC/RAL/ASTeC, Chilton, Didcot, Oxon
STFC/RAL/ISIS, Chilton, Didcot, Oxon
The ISIS linac consists of four DTL tanks that accelerate a 50 pps, 20 mA H- beam up to 70 MeV before injecting it into an 800 MeV synchrotron. Over the last decades, the linac has proved to be a strong and reliable injector for ISIS, which is a significant achievement considering that two of the tanks are more than 50 years old. At the time the machine was designed, the limited computing power available and the absence of 3D electromagnetic (EM) simulation codes, made the creation of a linac optimized for power efficiency almost impossible, so from this point of view, the ISIS linac is quite simple by today's standards. In this paper, we make a shunt impedance comparison study using the power consumption data collected from ISIS and the results obtained when simulating each of the four DTL tanks with 2D and 3D EM codes. The comparison will allow us to check the accuracy of our simulation codes and models and to assess their relative performance. It is particularly important to benchmark these codes against real data, in preparation for their use in the design of a proposed new linac, which will replace the currently aging ISIS injector.
Muons, Inc, Batavia
Old Dominion University, Norfolk, Virginia
LBNL, Berkeley, California
Voss Scientific, Albuquerque, New Mexico
ODU, Norfolk, Virginia
The performance of many particle accelerators is limited by the maximum electric gradient that can be realized in rf cavities. Recent studies have shown that high gradients can be achieved quickly in 805 MHz cavities pressurized with dense hydrogen gas, because the gas can suppress, or essentially eliminate, dark currents and multipacting. In this project, two new test cells operating at 500 MHz and 1.3 GHz will be built and tested, and the high pressure technique will be used to suppress the vacuum effects of evacuated rf cavities, so that the role of metallic surfaces in rf cavity breakdown can be isolated and studied as a function of external magnetic field, frequency, and surface preparation. Previous studies have indicated that the breakdown probability is proportional to a high power of the surface electromagnetic field, in accordance with the Fowler-Nordheim description of electron emission from a cold cathode. The experiments will be compared with computer simulations of the rf breakdown process.
NSCL, East Lansing, Michigan
To meet the requirement of a small output longitudinal beam emittance from the reaccelerator, a triple-harmonic buncher operating at the fundamental frequency of 80 MHz upstream the Radio Frequency Quadrupole (RFQ) linac has been designed, manufactured and tested at the National Superconducting Cyclotron Laboratory (NSCL). The buncher consists of two coaxial resonators with a single gridded gap. One cavity provides both the fundamental and the third harmonic simultaneously with l/4 and 3l/4 modes respectively, while the other for the second harmonic with a l/4 mode. This buncher combines the advantages of using high quality factor resonator and only a pair of grids. Details on design considerations, electromagnetic simulations, and test results are presented.
LANL, Los Alamos, New Mexico
The surface losses in the drift-tube linac (DTL) tanks 3 and 4 of the LANSCE linear accelerator are calculated using 3-D electromagnetic modeling with the CST MicroWave Studio (MWS). The results are used to provide more realistic power estimates for the 201.25 MHz rf upgrade design within the LANSCE-R project. We compared 3-D MWS results with those from traditional 2-D Superfish computations for DTL cells and their simplified models and found differences on the level of a few percent. The differences are traced to a 3-D effect consisting in a redistribution of the surface currents on the drift tubes (DT) produced by the DT stem. The dependence of MWS results on the mesh size used in computations is also discussed.
LANL, Los Alamos, New Mexico
We are developing high-efficiency room-temperature rf accelerating structures for beam velocities in the range of a few percent of the speed of light by merging two well-known ideas: H-mode cavities and the transverse beam focusing with permanent-magnet quadrupoles (PMQ). Combining electromagnetic 3-D modeling with beam dynamics simulations and thermal-stress analysis, we have found that the H-mode structures with PMQ focusing provide a very efficient and practical accelerator for light-ion beams of considerable currents. Such accelerating structures following a short RFQ can be used in the front end of ion linacs or in stand-alone applications such as a compact deuteron-beam accelerator up to the energy of a few MeV.
SLAC, Menlo Park, California
Funding: Work supported by Department of Energy contract DE-AC03-76SF00515.
A 5-cell, normal-conducting, 1.3 GHz, standing-wave cavity was built as a prototype capture accelerator for the ILC positron source. Although the ILC uses predominately super-conducting cavities, the capture cavity location in both a high radiation environment and in a solenoidal magnetic field requires it to be normal conducting. With the ILC requirements of relatively long beam pulse on-time (1 msec at 5 Hz) and high gradient for efficient positron capture (15 MV/m), achieving adequate cavity cooling to prevent detuning was challenging. This paper presents the operational performance of this cavity including its breakdown characteristics as a function of gradient, pulse length and solenoidal magnetic field strength. In addition, these results are compared with those from other normal-conducting cavities at various frequencies
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.
SLAC, Menlo Park, California
Funding: Work supported by U.S. Department of Energy, contract DE-AC02-76SF00515 (SLAC)
Design studies on the X-Band transverse rf deflectors operating at HEM11 mode have been made for two different applications. One is for beam measurements of time-sliced emittance and slice energy spread for the upgraded LCLS project, its optimization in rf efficiency and system design are carefully considered. Another is to design an ultra-fast rf kicker in order to pick up single bunches from the bunch-train of the B-factory storage ring. The challenges are to obtain very short structure filling time with high rf group velocity and good rf efficiency with reasonable transverse shunt impedance. Its rf system will be discussed.