KEK, Ibaraki
Hitachi Technologies and Services Co., Ltd., Kandatsu, Tsuchiura
Two superconducting crab cavities have been operated stably without any significant trouble for three years in KEKB since Feb/2007. At present (Dec/2009), maximum beam current with 'Crab ON' achieves 1200mA for HER (High Energy Ring, electron) and 1640mA for LER (Low Energy Ring, positron), respectively. RF trip rate per day due to crab cavity during 'physics run' was 2.8/day for HER and 0.4/day for LER at the beginning, and is 0.8/day for HER and 0.1/day for LER at present, respectively. Although Piezo actuator was frequently broken down at the beam abort with RF trip of the crab cavity, it was controlled stably by only LLRF (Low Level RF) feed-back system without Piezo actuator. Maximum HOM (Higher Order Mode) power, which is measured at HOM dampers made from ferrite, is 9.1kW for HER and 14.6kW for LER at the maximum beam current, respectively. LER crab voltage, which had suddenly dropped from 1.50MV to 1.10MV on March/2007, was gradually recovered from 1.14MV to 1.33MV in 2008.
SLAC, Menlo Park, California
Crab cavities are proposed for the LHC upgrade to improve the luminosity. There are two possible crab cavity installations for the LHC upgrade: the global scheme at Interaction Region (IR) 4 where the beam-beam separation is about 420-mm, and the local scheme at the IR5 where the beam-beam separation is only 194-mm. One of the design requirements as the result of a recent LHC-Crab cavity workshop is to develop a 400-MHz cavity design that can be utilized for either the global or local schemes at IR4 or IR5. Such a design would offer more flexibility for the final upgrade installation, as the final crabbing scheme is yet to be determined, and save R&D cost. The cavity size of such a design, however, is limited by the beam-beam separation at IR5 which can only accommodate a cavity with a horizontal size of about 145-mm, which is a design challenge for a 400-MHz cavity. To meet the new design requirements, we have developed a compact 400-MHz half-wave spoke resonator (HWSR) crab cavity that can fit into the tight spaces available at either IR4 or IR5. In this paper, we present the optimization of the HWSR cavity shape and the design of HOM, LOM, and SOM couplers for wakefield damping.
KEK, Ibaraki
STFC/RAL/ISIS, Chilton, Didcot, Oxon
ANL, Argonne
Tsukuba University, Ibaraki
A low-output-impedance RF system for the second harmonic cavity in the ISIS synchrotron has been developed by collaboration between Argonne National Laboratory (US), KEK (Japan) and Rutherford Appleton Laboratory (UK). The system has less than 30 Ω of output impedance over wide frequency range of 2.7-6.2 MHz. However, distortions of voltage waveform in the driver stage have been a long-standing issue. It was found such distortions were generated depending upon the higher-order-modes of the anode-choke impedance. In this report, method to realize the smooth sinusoidal waveform in the wideband system is presented.
Fermilab, Batavia
University of Delhi, Delhi
650 MHz option for the high energy part of the 2.6 GeV, CW Project X linac is discussed. It may give significant benefits compared to current 1.3 GHz option based on the utilization of ILC-type beta=1 cavities. Results of the break point optimization for linac stages, cavity optimization and beam dynamics optimization are presented. Possible reduction in the number of cryomodules and linac length compared to the current linac project version is discussed. Cryogenic losses are analyzed also.
Fermilab, Batavia
University of Delhi, Delhi
Results of analysis are presented for the longitudinal and transverse effects of High-Order Mode (HOM) excitation in the acceleration RF system of the CW proton linac of the Project X facility. Necessity of HOM dampers in the SC cavities of the linac is discussed.
KEK, Ibaraki
Eight superconducting accelerating cavities have been stably operated in the KEKB with sufficiently low trip rates. Two superconducting crab cavities were installed in 2007 and soon the crab crossing operation started. Recently the KEKB luminosity reached the world record of 2.1 x 1034 cm-1s-1. Stable operations of the accelerating cavities contributed for the luminosity increase. For the future Super-KEKB, we are developing a high power coupler for an input power of 600 kW and a HOM damper for RF power absorption more than 30 kW. The Super-KEKB requires RF operations with the high beam loading and the low RF voltage than the present KEKB operation. To suppress klystron output powers the external Q value has to be reduced. A new operation was proposed for superconducting cavities. In order to keep high RF voltages in each cavity, some cavities reverse its synchronous beam phase while the total RF voltage is kept as low as the required one. Beam studies were successfully carried out with one cavity reversed its synchronous beam phase.
SLAC, Menlo Park, California
We study the bunch field diffusion and energy dissipation in the beam pipe of the Super-B detector, which consists of two coaxial Be thin pipes (half a millimeter). Cooling water will run between these two pipes. Gold and nickel will be sputtered (several microns) onto the beryllium pipe. The Maxwell equations for the beam fields in these thin layers are solved numerically for the case of infinite pipes. We also calculate the amplitude of electromagnetic fields outside the beam pipe, which may be noticeable as the beam current can reach 4 A in each beam. Results of simulations are used for the design of this central part of the Super-B detector.
Tech-X, Boulder, Colorado
Imagine a virtual cylinder passing through an rf cavity. Given field data on the surface of this cylinder, one can compute accurate high-order transfer maps for particles traversing the cavity*. This technique is robust against errors or noise present in the surface data; moreover, it is not limited to accelerating modes. We describe this technique and present recent work that uses VORPAL** field data as a starting point for modeling crab cavities. In addition, we present realistic models, including fringes, for several standing-wave modes. These models, which include a simple accelerating mode and a TM-110 (crab) mode, are useful for the accurate computation of transfer maps as well as for constructing model fields that can be used for testing and comparing a variety of rf cavity codes.
* D.T. Abell, Phys. Rev. ST Accel. Beams 9, 052001, (2006).
** C. Nieter and J. R. Cary, J. Comput. Phys. 196, 448 (2004).
KEK, Ibaraki
As part of the design works of the interaction region (IR) of SuperKEKB (the upgrade of KEKB B-factory (KEKB)), the loss factor and impedance of beam ducts for the interaction point (IP duct) were calculated by GdfiedL. The IP duct is round and connected to beam ducts for electron and positron beams with a diameter of 20 mm via Y-shaped crotch ducts at both ends. The lengths of the straight section and crotch section are about 200 mm, respectively. The beam crossing angle is 83 mrad. Calculations for two types of IP duct were performed. Both ducts are almost same in design except for the diameter of the straight section (20 mm and 30 mm). The loss factors were about 0.001 V/pC in both cases when the bunch length was 6 mm. The longitudinal impedances showed that there were no modes trapped longitudinally in IP duct. However, from the results of the transverse impedance and eigenmode calculation, it was found that many TE modes can be trapped at the crotch section if the beam is off-center of the beam duct. For comparison, the loss factor and impedance of the IR beam duct of KEKB are also being calculated now. Full details of the calculation results will be provided in this report.
SLAC, Menlo Park, California
The geometry of storage ring collider interaction regions present an impedance to beam fields resulting in the generation of additional electromagnetic fields (higher order modes or wake fields) which affect the beam energy and trajectory. These affects are computed for the Super B interaction region by evaluating longitudinal loss factors and averaged transverse kicks for short range wake fields. Results indicate at least a factor of 2 lower wake field power generation in comparison with the interaction region geometry of the PEP-II B-factory collider. Wake field reduction is a consideration in the Super B design. Transverse kicks are consistent with an attractive potential from the crotch nearest the beam trajectory. The longitudinal loss factor scales as the -2.5 power of the bunch length. A factor of 60 loss factor reduction is possible with crotch geometry based on an intersecting tubes model.
JAEA/ERL, Ibaraki
KEK, Ibaraki
ISSP/SRL, Chiba
HOM absorbers are one of the key components to determine the ERL cavity performance to reduce the HOM problem for the high current operation. When a beam line HOM damper is installed inside the cryomodule, the HOM absorber is cooled down to liquid nitrogen temperature. The RF absorber used for the HOM absorber is required to have good frequency and temperature properties at low temperature. The RF absorber was selected by permittivity and permeability measurement of some ferrites and ceramics from room temperature to 40 K. The HOM absorber is designed by optimizing the parameters such as length, thickness and position with microwave simulation codes. The HOM absorber test model was designed and fabricated to test the RF, mechanical, cooling and temperature properties.
BNL, Upton, Long Island, New York
A prototype ampere-class superconducting energy recovery linac (ERL) is under advanced construction at BNL. The ERL facility is comprised of a five-cell SC Linac plus a half-cell SC photo-injector RF electron gun, both operating at 703.75 MHz. The facility is designed for either a high-current mode of operation up to 0.5 A at 703.75 MHz or a high-bunch-charge mode of 5 nC at 10 MHz bunch frequency. The R&D facility serves a test bed for an envisioned electron-hadron collider, eRHIC. The high-current, high-charge operating parameters make effective higher-order-mode (HOM) damping mandatory, and requires to determination of HOM tolerances for a cavity upgrade. The niobium cavity has been tested at superconducting temperatures and has provided measured dipole shunt impedances for the estimate of a beam breakup instability. The facility will be assembled with a highly flexible lattice covering a vast operational parameter space for verification of the estimates and to serve as a test bed for the concepts directed at future projects.
SSRF, Shanghai
SINAP, Shanghai
To investigate the higher order mode(HOM) damping in the higher harmonic cavity for Shanghai Synchrotron Radiation Facility(SSRF) when using HOM absorbers,simulations have been done for changing the position and the length as well as the thickness of ferrite of HOM damper. The best values under which the Q value of HOMs can be greatly lowered and the impedance of harmonic cavity will be trapped in the impedance threshold have been found.
BNL, Upton, Long Island, New York
The booster synchrotron for NSLS-II accepts beam with 200 MeV from a linac and raises its energy up to 3 GeV. In order to raise beam energy up to 3 GeV, a 7-cell PETRA cavity is installed. Beam instabilities related with the cavity are discussed. In particular, in order to avoid coupled-bunch instability, we consider that cooling water temperature for the cavity should be changed to shift frequencies of higher order modes (HOM) to avoid beam revolution lines. To obtain the relation between the temperature dependence of amount of frequency shift in each HOM and cavity body temperature, we carried out the measurement by changing cavity body temperature. From the measurement data, we calculate the required temperature variation. We summarize the results and describe the system design.
ESS-Bilbao, Zamudio
ESS Bilbao, Bilbao
University of the Basque Country, Faculty of Science and Technology, Bilbao
The use of distributed control systems for improving control system's performance is a hot research topic. Thus, the importance of developing control systems across networked environment is rising, a lot of research is focused on developing middleware based solutions. On the other hand, EPICS is an extended control system middleware, which is based on TCP/UDP protocol. This protocol has non-deterministic characteristics, limiting its use for networked control systems. Despite of these characteristics, the interest on TCP based networks in industrial field has been increasing due to its advantages in cost and easy integration. In this work, EPICS as a networked control system is analyzed in order to develop strategies to improve its performance. For this purpose, an EPICS based networked control scheme is presented, where control loop is closed over the net. As opposed to usual way of working with EPICS, two IOCs are used located in different hosts. The first one performs data acquisition, while the second one calculates the control signal. The analysis and control performance study of such scheme is presented by using periodic sampling, as well as event based sampling approach.
Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock
Beam energy deposited in Higher-Order-Modes may affect both beam stability and cryo power requirements of the planned CERN Superconducting Proton Linac SPL. We report on numerical studies of the high-beta cavity type, analyzing it's HOM spectrum. The most dangerous modes are identified and different possibilities of appropriate damping are discussed.
University of Delhi, Delhi
Fermilab, Batavia
Eleven cell elliptical cavity is designed for acceleration of particles traveling at 81 % of the speed of light. It will operate at 1.3 GHz and will be used to accelerate the particles from 0.4 GeV to 1.2 GeV. The cavity is studied for higher order mode (HOM) and trapped modes. The shapes of end cells of cavity is optimized to increase the field amplitude in end cells so that coupling of trapped modes may increase with HOM coupler and they can be extracted easily but keeping the field flatness & operating frequency undisturbed.
KEK, Ibaraki
JAEA/ERL, Ibaraki
ISSP/SRL, Chiba
A prototype module including a couple of 1.3 GHz superconducting 9-cell cavities has been designed as the main linac of cERL which is the test facility to establish the basic ERL technology at KEK. The shape of 9-cell Nb structure has been optimized to accelerate a CW beam of 100 mA with sufficiently damped higher order modes (HOM) which is achieved by adopting an eccentric fluted beam pipe and a cylindrical beam pipe of a large diameter of 123 mm. Extracted HOMs are absorbed by the ferrite cylinders bonded on the copper beam pipes by HIP process. A power coupler with double disk-ceramics has been developed to transfer an RF of 20 kW CW to the cavity in full reflection. The test results of fabrication, cooling and RF performance for these components are integrated as the prototype module of the main linac for cERL facility.
KEK, Ibaraki
A superconducting cavity system has been developing for cERL injector Linac at KEK. Two prototype 2-cell niobium cavities and two prototype input couplers were fabricated. The vertical tests of the cavities at 2 K were carried out to qualify their performance. The rf conditioning of the input couplers were carried out at a high power test stand with a cw-300kW klystron. The results of the cavity performances at high gradients and the conditioning of the input couplers will be presented in this paper.
KEK, Ibaraki
JAEA/ERL, Ibaraki
ISSP/SRL, Chiba
We are developing the superconducting (SC) cavity for Energy Recovery Linac (ERL) in Japan. In order to survey the electron emission and the heating spot of the cavity inner surface in detail, cavity diagnostics with the rotating mapping system was applied. Two types of sensors, one of which is the carbon resistor and the other is the Si PIN photo diode, were set to detect the temperature rise and electron emission. By rotating the sensor arrays around the cavity axis, a lot of information is obtained all over the cavity surface in detail. This paper reports the results of vertical tests by using this rotating mapping system with Nb 9-cell ERL cavity.
KEK, Ibaraki
JAEA/ERL, Ibaraki
ISSP/SRL, Chiba
In order to verify the technology needed for ERL main linac cavities, we fabricated a prototype of L-band 9-cell KEK-ERL superconducting cavity. For the ERL, along with high gradient and high Q-value, strong HOM damping is required. Its cell shape is optimized for the HOM damping. The cavity has large irises of 80 mm diameter, large beampipes of 120 mm and 100 mm diameter and the eccentric fluted beampipe. After a series of surface treatment, such as annealing, electro-polishing, high-pressure-rinsing and baking, several vertical tests have been performed. As for cavity diagnostics, a rotating X-ray and temperature mapping system was constructed. The cavity performance was limited to less than 20 MV/m by the field emissions. The X-ray distributions caused by field emission were clearly observed by X-ray mapping system. In this report, we summarize the recent results of the vertical tests.
PAL, Pohang, Kyungbuk
In the PLS-II storage ring, the available length of a long straight section for RF system is 6.28 m, which is from quadrupole magnet to quadrupole magnet beam-pipe valves with an elliptical transverse cross section. In this room, two beam-pipe transitions from elliptical to circle cross section, two commercial cryomodules with a circle transverse cross section, three bellows for adjusting cryomodule length and four vacuum valves could need to be installed. Two commercial cryomodules are too long to be installed into this section. In order to install two cryomodules into this section, we need to modify the tapers for reducing the total length of these parts. In this paper, the HOM damping effects for different taper shapes has been studied. The beam loss factor influence and broad-band impedance change due to taper shape changes have been estimated.
PAL, Pohang, Kyungbuk
The RF system for PLS-II upgrade, of which beam current and emittance are 400 mA and 5.6 nmrad at 3 GeV, becomes much more important compared to PLS. To reduce the HOM intensity in RF cavities for stable beam, a superconducting RF cavity is selected for the PLS-II. The RF system has to compensate beam loss power of 663 kW from 24 bending magnets, 20 insertion devices and other losses by RF HOM and broadband losses along vacuum chambers. For sufficient energy acceptance and lifetime the design RF voltage is 4.5 MV. Three 500 MHz superconducting cavities will be operated from October 2012, following successful commissioning with PLS NC cavities from July 2011. For the 3 SRF cryomodules, a 700 W class He cryogenic system will be prepared in 2011. The design of PLS-II SRF system including cryogenic system will be reported in the paper.
Cockcroft Institute, Lancaster University, Lancaster
JLAB, Newport News, Virginia
The planned luminosity upgrade to LHC is likely to necessitate a large crossing angle and a local crab crossing scheme. For this scheme crab cavities align bunches prior to collision. The scheme requires at least four such cavities, a pair on each beam line either side of the interaction point (IP). Upstream cavities initiate rotation and downstream cavities cancel rotation. Cancellation is usually done at a location where the optics has re-aligned the bunch. The beam line separation near the IP necessitates a more compact design than is possible with elliptical cavities such as those used at KEK. The reduction in size must be achieved without an increase in the operational frequency to maintain compatibility with the long bunch length of the LHC. This paper proposes a suitable superconducting variant of a four rod coaxial deflecting cavity (to be phased as a crab cavity), and presents analytical models and simulations of suitable designs.
Imperial College of Science and Technology, London
Imperial College of Science and Technology, Department of Physics, London
Cockcroft Institute, Lancaster University, Lancaster
The ever-growing demand for higher beam energies has dramatically increased the risk of RF breakdown, limiting the maximum achievable accelerating gradient. Field emission is the most frequently encountered RF breakdown where it occurs at regions of locally enhanced electric field. Electrons accelerated across the cavity as they tunnel through the surface in the presence of microscopic defects. Upon Impact, most of the kinetic energy is converted into heat and stress. This can inflict irreversible damage to the surface, creating additional field emission sites. This work aims to investigate, through simulation, the physics involved during both emission and impact of electrons. A newly developed 3D field model of an 805 MHz cavity is generated by COMSOL Multiphysics. Electron tracking is performed using a Matlab based code, calculating the relevant parameters needed by employing fourth Order Runge Kutta integration. By studying such behaviours in 3D, it is possible to identify how the cavity surface can alter the local RF field and lead to breakdown and subsequent damages. The ultimate aim is to introduce new surface standards to ensure better cavity performance.
UMAN, Manchester
DESY, Hamburg
Fermilab, Batavia
Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock
We analyse the higher order modes in the 3.9GHz bunch shaping cavities recently installed in the XFEL/FLASH facility at DESY. We report on recent experimental results on the frequency spectrum, both beam and probe based. These are compared to those predicted by finite element computer codes, globalised scattering matrix calculations and a two-band circuit model. This study is focused on the dipole component of the multiband expansion of the wakefield.
UMAN, Manchester
We present an optimized geometry for a 1.3 GHz superconducting cavity in which the surface electromagnetic fields have been minimized and the bandwidth of the fundamental mode has been maximized. We refer to this design as the New Low Surface Field (NLSF) cavity*. Earlier work* focused the fundamental mode properties. Here we study higher order modes (HOMs), means of damping them, and short range wakefields. A two-band circuit model is employed in order to facilitate rapid characteristic of the HOMs in the cavity.
* N. Juntong and R.M. Jones, High-Gradient SRF Cavity with Minimized Surface E.M. Fields and Superior Bandwidth for The ILC, SRF2009, THPPO024, 2009.
Royal Holloway, University of London, Surrey
BNL, Upton, Long Island, New York
The Superconducting Proton Linac (SPL) is part of the proposed upgrade of the LHC injection chain, intended to significantly improve the characteristics of the beam circulating in the collider. SPL will rely on two classes of superconducting cavities; beta=0.65 and beta=1; each containing 5-cells resonant at 704 MHz. Presented here are the results of some initial simulations of the beta=1 design, performed at the NERSC supercomputing facility with the highly-parallelised ACE3P codes released by the Advanced Computations Department at SLAC National Accelerator Laboratory. The HOM spectrum in the baseline design has been calculated, and dangerous modes identified by their high R/Q value. In addition, perturbations due to the location of the various couplers, and the structure of the beampipes have been investigated, and are presented here.
Fermilab, Batavia
In the Project X facility a 2.6 GeV, H- CW beam is delivered to three users simultaneously by way of selectively filling appropriate RF buckets at the front end of the linac and then RF splitting them to three different target halls. With the desire to split the H- beam three ways, an RF separator directs two quarters of the beam to one user (Mu2e), one quarter to another user (Kaon), and one quarter to the third (unidentified) user. The natural way is to use a SC structure with the deflecting TM110 mode. Basic requirements to the deflecting RF structure are formulated and design of the deflecting SC cavities is presented.
Muons, Inc, Batavia
CLASSE, Ithaca, New York
Superconducting RF (SRF) systems typically contain unwanted frequencies or higher order modes (HOM). For storage ring and linac applications, these higher modes must be damped by absorbing them in ferrite and other lossy ceramic materials. Typically, these absorbers are brazed to substrates that are strategically located, often in the drift tubes adjacent to the SRF cavity. These HOM loads must have broadband microwave loss characteristics and be robust both thermally and mechanically, but the ferrites and their attachments are weak under tensile and thermal stresses and tend to crack. Based on existing work on HOM loads for high current storage rings and for an ERL injector cryomodule, a HOM absorber with improved materials and design will be developed for high-gradient 750 MHz superconducting cavity systems for storage ring and linac radiation sources. This work will build on novel construction techniques to maintain the ferrite in mechanical compression without brazing. 750 MHz RF system designs will be numerically modeled to determine the optimum ferrite load required to meet broadband loss specifications.
Private Address, Freeville
CLASSE, Ithaca, New York
Ferrites and lossy ceramics used in HOM (higher order mode) load for superconducting accelerators, have shortcomings such as poor batch-to-batch reproducibility of electromagnetic properties, extremely low electric conductivity at cryogenic temperatures leading to accumulation of charge on the material surface, brittleness, which may cause contamination of the nearby SRF cavities by lossy dust, etc. A proposal to use a resistive material free of these shortcomings is presented.
CLASSE, Ithaca, New York
Cornell University has developed and fabricated a SCRF injector cryomodule for the acceleration of a high current, low emittance beam in the Cornell ERL injector prototype. This cryomodule is based on superconducting rf technology with five 2-cell rf cavities operated in the cw mode, supporting beam currents of up to 100 mA. After a rework of this cryomodule in 2009 to implement several improvements, it is now in beam operation again. In this paper we report on latest results and discuss future test plans.
CLASSE, Ithaca, New York
This paper discusses the optimization of superconducting RF cavities to be used in Cornell's Energy Recovery Linac, a next generation light source. We discuss the determination of a parameter corresponding to beam break-up current and the results of introducing a realistic higher-order-mode absorber constructed of carbon nanotubes rather than a ferrite based absorber. We conclude by comparing the threshold current of the new design and show differences are due to the new absorber material.
JLAB, Newport News, Virginia
JLab has designed and fabricated several prototype SRF cavities with cell shapes optimized for high current beams and with strong damping of unwanted higher order modes. We report on the latest test results of these cavities and on developments of concepts for new variants optimized for particular applications such as light sources and high-power proton accelerators, including betas less than one. We also report on progress towards a first beam test of this design in the recirculation loop of the JLab ERL based FEL. With growing interest worldwide in applications of SRF for high-average power electron and hadron machines, a practical test of these concepts is highly desirable. We plan to package two prototype cavities in a de-mountable cryomodule for temporary installation into the JLab FEL for testing with RF and beam. This will allow verification of all critical design and operational parameters paving the way to a full-scale prototype cryomodule.
JLAB, Newport News, Virginia
ANL, Argonne
A squashed, elliptical supercondconducting (SC) cavity with waveguide dampers on the beam pipes has currently been chosen as the baseline design [1] for the Short Pulse X-ray (SPX) project at the Advanced Photon Source (APS). An alternate cavity design, with a waveguide damper located directly on the cavity cell for improved damping characteristics, has also been designed and cold-tested with promising results. In either case, eight cavities would be operated CW in a single cryomodule at 2K to produce an electron bunch chirp of 4MV at a frequency of 2.815 GHz. Detailed analysis of multipactoring (MP), lorentz force detuning (LFD), and the thermal properties of the baseline design has led to an engineering specification of the basic parameters of the cryomodule.
JLAB, Newport News, Virginia
PKU/IHIP, Beijing
NSU, Newport News, Virginia
Field emission is still one of the major obstacles facing Nb superconducting radio frequency (SRF) community for allowing Nb SRF cavities to reach routinely accelerating gradient of 35 MV/m that is required for the international linear collider. Nowadays, the well know low temperature backing at 120 oC for 48 hours is a common procedure used in the SRF community to improve the high field Q slope. However, some cavity production data have showed that the low temperature baking may induce field emission for cavities treated by EP. On the other hand, an earlier study of field emission on Nb flat samples treated by BCP showed an opposite conclusion. In this presentation, the preliminary measurements of Nb flat samples treated by BEP, EP, and BCP via our unique home-made scanning field emission microscope before and after the low temperature baking are reported. Some correlations between surface smoothness and the number of the observed field emitters were found. The observed experimental results can be understood, at least partially, by a simple model that involves the change of the thickness of the pent-oxide layer on Nb surfaces.
* L.C. MacIntyre, R. Ike, and A.T. Wu, 2005, unpublished
ODU, Norfolk, Virginia
JLAB, Newport News, Virginia
The superconducting parallel-bar cavity* has properties that makes it attractive as a deflecting or crabbing rf structure. For example it is under consideration as an rf separator for the Jefferson Lab 12 GeV upgrade and as a crabbing structure for a possible LHC luminosity upgrade. Initial cavity shape optimization has been performed to obtain a high transverse deflecting voltage with low surface fields. We present here a study of the Higher Order Mode (HOM) properties of this structure. Frequencies, R/Q and field profiles of HOMs have been evaluated and are reported.
* J.R. Delayen and H. Wang, Phys. Rev. ST Accel. Beams 12, 062002 (2009).
BNL, Upton, Long Island, New York
The damper of 56 MHz cavity is designed to extract all modes to the resistance load outside, including the fundamental mode. Therefore a high-pass filter is required to reflect the fundamental mode back into the cavity. A preliminary design of the filter was previously done. In this paper, we optimize all elements to eliminate the poor filter performance above 1 GHz. The circuit diagram is extracted from microwave lumped elements that reproduce the frequency spectrum of the finalized filter. We also show mode damping results with dampers and filters in the desired configuration, determining the final performance of the cavity.
BNL, Upton, Long Island, New York
A 56 MHz cavity was designed for a luminosity upgrade of the Relativistic Heavy Ion Collider (RHIC), including requirements for Higher Order Mode (HOM) damping. A preliminary design of the HOM damper was previously done without optimization. In this paper, we describe our optimization of the damper's performance, and modifications made to its original design. We also show the cavity damper efects with different geometries. Magnetic field enhancement at the ports is reduced to a value less than the highest field in the cavity to eliminate electrical breakdown. All HOMs up to 1 GHz are simulated with their frequencies, mode configurations, R/Qs and shunt impedances, and all modes are well-damped with the optimized design and configuration.
CIEMAT, Madrid
CERN, Geneva
The goal of the present CLIC Test Facility (CTF3) is to demonstrate the technical feasibility of specific key issues of the CLIC scheme. The extracted drive beam from the combiner ring (CR), a pulse of 35 A magnitude and 140 ns duration, is sent to the new CLic EXperimental area (CLEX). A Tail Clipper (TC) kicker is required, in the CR to CLEX transfer line, to allow the duration of the beam pulse to be adjusted: the unwanted bunches are kicked into a collimator. The TC must have a fast field rise-time, of not more than 5 ns, in order to minimize uncontrolled beam loss. Striplines are used for the TC: to establish the required fields, the applied pulse wave front must fully propagate along the striplines. To reduce the wave front propagation time, the overall length of the stripline assembly is sub-divided into 4 sections. The TC has been designed with the aid of detailed numerical modelling: the stripline cross-section and coaxial-to-stripline transitions were carefully optimized using a 3D code. The results of simulations and the measured behaviour of the striplines are presented; in addition measured current pulses are shown.
KEK, Ibaraki
In our high gradient R&D of ICHIRO cavities at KEK, we have found some problems related to HOM coupler and high power RF input coupler port on beam tube: end group. One is the difficulties of rinsing in complex structures like HOM coupler. The other is Q-slope at high filed more than 40MV/m. The cavities without end group did not show such a high field Q-slope. At first step, we tested much stronger and aggressive rinsing method; wiping, brushing, and mega-sonic rinsing, against end group. The details and results of these rinsing effects will be reported.
KEK, Ibaraki
We have continued high gradient R&D of ICHIRO 9-cell cavities at KEK. The maximum gradient of ICHIRO 9-cell cavity #5 that has no end groups on beam tube was still limited around 36MV/m so far. The 9-cell performances were sometimes limited by triggered field emission (FE) by multipactings. We suspected the residual gas in the cavity might be one of the sources of triggered FE. The cavity was closed during vertical test in our system. Other labs evacuated cavity during vertical test. In order to improve the vacuum of cavity during vertical test, we made evacuation system in our cavity test stand. The comparison of results for vertical test with and without evacuation will be reported.
KEK, Ibaraki
We are developing LL high gradient SRF cavity for ILC. Recently we have observed a Q-slope problem at higher gradient over 35-40MV/m on the full end single cell cavities, which have a HOM coupler and an input coupler on a beam tube. This problem might be due to poor rinsing in such a complicate structure. We have studied to strengthen cleaning by improvement of the nozzle shape used high pressure water rinsing, inside ultrasonic cleaning, steam cleaning, and so on. In this paper we will report these results.
KEK, Ibaraki
We are developing large grain/single crystal niobium material for ILC collaborating with Tokyo Denkai. These materials are very much promising to obtain high SRF cavity performance with cost-effective production. We have fabricated two 9-cell cavities from these large grain niobium materials and made cold test to evaluate the SRF performance. In this paper, we will report cavity fabrications and preparations and cold test results.
Sokendai, Ibaraki
KEK, Ibaraki
We have designed Demountable Damped Cavity (DDC) for ILC main linac. DDC has two design concepts. One is the coaxial waveguide for HOM damping, which can strongly couple HOM's. Accelerating mode is reflected by a choke filter. The axial symmetry can reduce the beam kick effect. The other concept is demountable structure which can make easy cleaning of end group in order to suppress the Q-slope problem at a high field. In this paper we will report the RF design and measurement results in model cavity.
MHI, Kobe
MHI has supplied superconducting cavity for the ILC R&D project to KEK in Japan for the last few years. We are improving the technology to design and fabricate the superconducting cavities. We can present some example of our work that have improved the productivity of the superconducting cavities.
Fermilab, Batavia
In the paper the results are presented for calculation of the transverse wake and RF kick from the power and HOM couplers of the ILC acceleration structure. The RF kick was calculated stand-alone by HFSS, CST MWS and COMSOL codes while the wake kick was calculated by GdfidL. The calculation precision and convergence for both cases are discussed and compared to the results obtained independently by other group.
KEK, Ibaraki
A damping ring of positron beams is under consideration for the upgrade of KEKB (SuperKEKB) because low emittance of beams injected to the main rings is required by the SuperKEKB optics in the nano-beam scheme. We present the design of the RF accelerating structure, especially on the higher-order-mode (HOM) damped structure. This structure is based on the normal-conducting accelerating cavity system ARES, which has successful records of the long-term stable operations so far with low trip rates at KEKB. All the HOM absorbers are made of silicon carbide, bullet-shaped, and to be directly water cooled, so that the structure presented in this paper can be also a prototype for accelerating beams of the order of 10A in the SuperKEKB main ring in the high-current scheme.
KEK, Ibaraki
Linear accelerator systems with superconducting RF technology have become increasingly important to reach high-quality, high current beam conditions required by the high-energy physics and photon science communities. The International Linear Collider, for instance, calls for very challenging (beam conditions). Similarly, the XFEL requires (beam conditions) and future ERLS require (high average current). In this talk, we review the needs and challenges of SCRF linac beam physics and technology for present and future applications.
DESY, Hamburg
INFN/LASA, Segrate (MI)
The preparation phase for the European XFEL cavity production includes a number of actions. Material issues: qualification of high purity niobium vendors, verifying of large grain material as a possible option, construction of the scanning device for the niobium sheets. Mechanical fabrication issues: accommodation of the TESLA cavity design to the XFEL demands, device construction for RF measurement of components, integration of the helium tank and it's welding to the cavity into the fabrication sequence, documentation and data transfer, application of a new high resolution camera for inspection of the inside surface. Treatment and RF measurement: establishing the XFEL recipe, in particular the final surface treatment (final 40 μm EP or short 10 μm Flash BCP), and the cavity preparation strategy (vertical acceptance test with or without helium tank welded, with or without assembly of HOM antennas), construction of the cavity tuning machine. About 50 prototype cavities are produced at the industry, treated (partially in industry and partially at DESY) and RF-tested at DESY. The XFEL requirements are fulfilled with a yield of approx. 90%.
KEK, Ibaraki
JAEA/J-PARC, Tokai-mura
The RF cavity using Magnetic Alloy (MA) cores has been developed for achieving the high field gradient in J-PARC. For reducing the beam loading effects, the Q-value of the RF cavities in the Main Ring (MR) is controlled by using the cut-core configuration. In order to check the effect of HOMs between the cut-core gap, a simulation method of MA cores was studied and electromagnetic fields of excitation modes have been calculated by HFSS. We present the detail of the simulation method of MA cores and the HOM analysis of the cavity with the cut-cores.
KEK, Ibaraki
Under the CERN-SLAC-KEK collaboration, we have been developing the high gradient TW accelerator structures. One of the main focuses is the feasibility study of CLIC accelerator structure at X-band. A high power facility, Nextef*, was established at KEK in 2007. A few structures have been tested, including an un-damped disk-loaded structure successfully tested beyond 100 MV/m, a heavily damped structure to be tested from late 2009 and a structure made in a quadrant configuration. These structures follow the same accelerating-mode RF parameter profile, called CLIC-C**, but show different features at high gradient operation. Various observables, such as dark current, vacuum activities, light emission, breakdown rate, and so on, are measured. We discuss the high gradient phenomena related to these observables and the possible improvement for stable operation at a higher gradient.
* T. Higo et al., THP038, LINAC06,2006.
**A. Grudiev, http://indico.cern.ch/conferenceDisplay.py?confId=30911
KEK, Ibaraki
SLAC, Menlo Park, California
CERN, Geneva
A CERN-SLAC-KEK collaboration on high gradient X-band accelerator structure development for CLIC has been ongoing for three years. The major outcome has been the demonstration of stable 100 MV/m gradient operation of a number of CLIC prototype structures. These structures were fabricated basically using the technology developed from 1994 to 2004 for the GLC/NLC linear collider initiative. One goal has been to refine the essential parameters and fabrication procedures needed to realize such high gradient routinely. Another goal has been to develop structures with stronger dipole mode damping than those for GLC/NLC. The latter requires that surface temperature rise during the pulses be higher, which may increase the breakdown rate. Structures with heavy damping will be tested in late 2009/early 2010, and this paper will present these results together with some of the earlier results from non-damped structures and structures built with a quadrant geometry.
Toshiba, Yokohama
JASRI/SPring-8, Hyogo-ken
TOSHIBA is manufacturing the L-band acceleration system for the SPring-8 Joint Project for XFEL. We have developed a new type duct-shaped SiC dummy load for its power distribution system. The load terminates a WR650 waveguide and can absorb the maximum mean power of 10kW. In order to reduce VSWR less than 1.1 in the frequency range of 1.428GHz, we shaped the SiC absorber into a 35cm long tapered cylinder and mounted matching stubs in the waveguide near the inlet of the load. The SiC absorber was fit into a cylindrical copper with efficient water-cooling channels. The design and manufacture and the low-power tests of our original dummy load are described in this paper.
TUB, Beijing
CAEP/IFP, Mainyang, Sichuan
Mini-LIA was a miniature linear induction accelerator designed and manufactured by China Academy of Engineering Physics and Tsinghua University. To investigate the higher order mode (HOM) of Mini-LIA cavity, especially the frequency and quality factor Q of the TM110 and TM120 in it, both numerical simulation and experiments were performed. Several models of the cavity were established and calculated by using E module of MAFIA code. Network analyzer was applied to measure the frequency and Q in cavity. Both the simulation results and the experiment results are presented in this paper. The results of the experiments were coincident with the calculated results. Finally, The HOM characteristic of Mini-LIA cavity with metglass core in it was explored, and some interesting results was obtained.
TUB, Beijing
The application of the genetic algorithm in the diagnosis of the coupled cavity chain is investigated in this paper. One program named GANL2 has already been developed based on the genetic algorithm at Tsinghua University. The cell frequencies, quality factors, and coupling between the cells can be estimated by GANL2 if the pass-band reflection curve is known. This method has been applied in the diagnosis of the S-band and X-band standing-wave linac cavities. In this paper we present the preliminary investigation of the genetic algorithm in the diagnosis of the L-band 9-cell superconducting copper cavity model. The result of the calculation and measurement are compared. Not all the cells are diagnosed well. More precise measurement is needed for further study.
USTC/NSRL, Hefei, Anhui
UW-Madison/SRC, Madison, Wisconsin
A radio frequency system with a higher-harmonic cavity will be used to increase the beam lifetime and suppress coupled-bunch instabilities of the upgrade Hefei Light Source. In the paper, the simulated results confirm that tuning in the harmonic cavity may suppresses the parasitic coupled-bunch instabilities. The higher-harmonic cavity has been designed and the calculated optimum lifetime increase ratio is 2.58.
ANL, Argonne
Euclid TechLabs, LLC, Solon, Ohio
As the dimensions of accelerating structures become smaller and beam intensities higher, the transverse wakefields driven by the beam become quite large with even a slight misalignment of the beam. These deflection modes can cause inter-bunch beam breakup and intra-bunch head-tail instabilities along the beam path. We propose a built-in tunable absorption mechanism for damping the parasitic transverse modes without affecting the operational modes in dielectric loaded accelerating (DLA) structures and wakefield power extractors. The new principle for HOM absorption is based on electron paramagnetic resonance. The dielectric tube of the DLA has to be doped with a material exhibiting high EPR, for example ruby, Al2O3 overdoped ~1% with Cr3+. The absorption frequency can be tuned by an external DC magnetic field to match the frequency of the transverse mode. At the resonance imaginary part of permeability becomes significant and the dielectric tube acts as an absorber for the transverse modes. The external DC magnetic field is solenoidal and has to have a magnitude of about 3 kG. This configuration in fact is desirable to focus the beam and provide additional control of beam break up.
JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken
MHI, Nagoya
KEK, Ibaraki
At the 3GeV RCS (Rapid Cycling Synchrotron) in J-PARC (Japan Proton Accelerator Research Complex), the scheme of H- charge exchange injection using stripping foils is adopted. The charge exchange system is composed of three stripping foil devices. The first stripping foil device, which converts the H- beam from the 181MeV LINAC into the H+ beam, can replace the broken foil with new one in vacuum remotely and automatically. In September 2007, mechanical trouble with the first stripping foil device had occurred just before the RCS beam commissioning was started. The magnetic coupling of the transfer rod had been decoupled and the transfer rod had been broken which was caught in the vacuum gate valve. We studied the trouble cause, re-examined the structural design and the selection for the material, and then verified the specification from endurance tests with sample pieces. Then the improved device was installed in the ring in September 2008. In this presentation, we report the mechanical trouble and that countermeasure, including the improvements of the charge exchange system.
DESY, Hamburg
JLAB, Newport News, Virginia
This paper reports about our efforts to develop a flangeable coaxial coupler for both HOM and fundamental coupling for 9-cell TESLA/ILC-type cavities. The cavities were designed in early 90‘s for pulsed operation with a low duty factor, less than 1 %. The proposed design of the coupler has been done in a way, that the magnetic flux B at the flange connection is minimized and only a field of <5 mT would be present at the accelerating field Eacc of ~ 36 MV/m (B =150 mT in the cavity). Even though we achieved reasonably high Q-values at low field, the cavity/coupler combination was limited in the cw mode to only ~ 7 MV/m, where a thermally initiated degradation occurred. We have improved the cooling conditions by initially drilling radial channels every 30 degrees, then every 15 degrees into the shorting plate. The modified prototype performed well up to 9 MV/m in cw mode. This paper reports about our experiences with the further modified coaxial coupler and about test results in cw and low duty cycle pulsed mode, similar to the TESLA/ILC operation conditions.
DESY, Hamburg
High frequency Higher Order Modes (HOM) propagating in the beam line of a superconducting linac can carry a substantial fraction of the energy deposited in accelerating structures by the beam. In this contribution, we report test results of the beam line absorber (BLA), which was designed and fabricated at DESY, and installed in the FLASH accelerator to absorb the HOM energy generated by high current beams. Two tests were carried out, in September 2008 and September 2009, during so called high current runs. The experiments confirmed the concept of the BLA design and showed remarkable agreement with computer modeling of the HOM energy absorption.
DESY, Hamburg
Fermilab, Batavia
Uni HH, Hamburg
Ultra short bunches with high peak current are required for efficient creation of high brilliance coherent light at the free electron laser FLASH. They are obtained by a two stage transverse magnetic chicane bunch compression scheme based on acceleration of the beam off the rf field crest. The deviation of the rf field's sine shape from a straight line leads to long bunch tails and reduces the peak current. This effect will be eliminated by adding the Fermilab-built third harmonic superconducting accelerating module operating at 3.9 GHz to linearize the rf field. The third harmonic module also allows for the creation of uniform intensity bunches of adjustable length that is needed for seeded operation. This paper summarizes the results from the first complete rf system test at the crymodule test bench at DESY and the first experience gained operating the system with beam in FLASH.
PKU/IHIP, Beijing
As part of the updated DC-SC injector, a 3.5cell cavity has been fabricated at Peking University, which includes two Coaxial High Order Mode (HOM) couplers. The effect of the HOM couplers has been studied by numerical simulation and measurement. The results are highly uniform and show that the two couplers do effectively damp the HOMs.
Lancaster University, Lancaster
The possibility of achieving higher accelerating gradients at higher frequencies with the reduction of the effect of HOMs, compared to conventional accelerating structures, is increasing interest in the possible use of Photonic Crystals (PC) for accelerator applications. In this paper we analyze how the properties of the lattice of a PC resonator can be engineered to give a specific band structure, and how by tailoring the properties of the lattice specific EM modes can either be confined or moved into the propagation band of the PC. We further go on to discuss the role of disorder in achieving mode confinement and how this can be used to optimize both the Q and the accelerating gradient of a PC based accelerating structure. We also examine the use of high disorder to give rise to Anderson Localization, which gives rise to exponential localization of an EM mode. Discussing the difference between the extended Bloch wave, which extends over the entire PC, and the Anderson localized mode.
MIT/PSFC, Cambridge, Massachusetts
Photonic bandgap (PBG) structures show promising results for use in future collider applications. Both acceleration and wakefield damping have been demonstrated experimentally. The breakdown performance of a single cell PBG structure was tested at X-band at SLAC and found to have significant contributions from magnetic field effects. A new structure has been designed at 17.1 GHz to be tested at MIT to investigate the scaling of these and other breakdown effects with frequency. The 17.1 GHz structure will also use the open nature of the PBG lattice to greatly improve the breakdown diagnostics. Finally, a novel PBG structure has been designed for testing at SLAC using elliptical inner rods. This design significantly reduces the pulsed heating in the structure and should therefore improve the breakdown performance.
Euclid TechLabs, LLC, Solon, Ohio
ANL, Argonne
TUB, Beijing
Tsinghua University, Beijing
Wakefield excitation has been experimentally studied in a 3-cell X-band standing wave Photonic Band Gap (PBG) accelerating structure. Major monopole (TM01- and TM02-like) and dipole (TM11- and TM12-like) modes were indentified and characterized by precisely controlling the position of beam injection. The quality factor Q of the dipole modes was measured to be ~10 times smaller than that of the accelerating mode. A charge sweep, up to 80 nC, has been performed, equivalent to ~30 MV/m accelerating field on axis. A variable delay low charge witness bunch following a high charge drive bunch was used to calibrate the gradient in the PBG structure by measuring its maximum energy gain and loss. Experimental results agree well with numerical simulations.
CERN, Geneva
MPI-K, Heidelberg
Cockcroft Institute, Warrington, Cheshire
Higher Order Modes (HOMs) can severely limit the operation of superconducting cavities in a linac with high beam current, high duty factor and complex pulse structure. The full HOM spectrum has to be analyzed in order to identify potentially dangerous modes already during the design phase and to define their damping requirements. For this purpose a dedicated beam simulation code focused on beam-HOM interaction was developed, taking into account important effects like the HOM frequency spread, beam input jitter, different chopping patterns, as well as klystron and alignment errors. Here this code is used to investigate in detail the HOM properties of the cavities foreseen in the Superconducting Proton Linac (SPL) at CERN and their potential to drive beam instabilities. A special focus is set to HOM excitation by chopped pulses with high repetition rate and on the influence of HOMs on recirculating electron beams in the high-energy part of the SPL. Finally, the HOM characteristics of similar linac designs are presented and compared to the SPL.
CERN, Geneva
The CLOUD Project, where a high-energy physics accelerator is being used to study atmospheric and climate science for the first time, will be described.