SLAC, Menlo Park, California
Under the support of the U. S. DOE SciDAC program, SLAC has been developing a suite of 3D parallel finite-element codes aimed at high-accuracy, high-fidelity electromagnetic and beam physics simulations for the design and optimization of next-generation particle accelerators. Running on the latest supercomputers, these codes have made great strides in advancing the state of the art in applied math and computer science at the petascale that enable the integrated modeling of electromagnetics, self-consistent Particle-In-Cell (PIC) particle dynamics as well as thermal, mechanical, and multi-physics effects. This paper will present 3D results of trapped mode calculations in an ILC cryomodule and the modeling of the ILC Sheet Beam klystron, shape determination of superconducting RF (SCRF) cavities and multipacting studies of SCRF HOM couplers, as well as 2D and preliminary 3D PIC simulation results of the LCLS RF gun.
SLAC, Menlo Park, California
Over the past years, SLAC's Advanced Computations Department (ACD) has developed the parallel finite element particle-in-cell code Pic3P (Pic2P) for simulations of beam-cavity interactions dominated by space-charge effects. As opposed to standard space-charge dominated beam transport codes, which are based on the electrostatic approximation, Pic3P (Pic2P) includes space-charge, retardation and boundary effects as it self-consistently solves the complete set of Maxwell-Lorentz equations using higher-order finite element methods on conformal meshes. Use of efficient, large-scale parallel processing allows for the modeling of photoinjectors with unprecedented accuracy, aiding the design and operation of the next-generation of accelerator facilities. Applications to the Linac Coherent Light Source (LCLS) RF gun are presented.
DESY, Hamburg
INFN/LNF, Frascati (Roma)
Jefferson Lab, Newport News, Virginia
SLAC, Menlo Park, California
SBUNSL, Stony Brook, New York
BNL, Upton, Long Island, New York
The Andrzej Soltan Institute for Nuclear Studies, Centre Swierk, Swierk/Otwock
SLAC, Menlo Park, California
The beam quality and operational requirements for the Linac Coherent Light Source (LCLS) currently being constructed at SLAC are exceptional, requiring the design of a new RF photocathode gun for the electron source. Based on operational experience at GTF at SLAC, SDL and ATF at BNL and other laboratories, the 1.6cell s-band (2856MHz) gun was chosen to be the best electron source for the LCLS injector, however a significant re-design was necessary to achieve the challenging parameters. Detailed 3-D analysis and design was used to produce nearly-perfect rotationally symmetric rf fields to achieve the emittance requirement. In addition, the thermo-mechanical design allows the gun to operate at 120Hz and a 140MV/m cathode field, or to an average power dissipation of 4kW. Both average and pulsed heating issues are addressed in the LCLS gun design. The first LCLS gun is now fabricated and has been operated with high-power RF. The results and analysis of these high-power tests will be presented.
LLNL, Livermore, California
SLAC, Menlo Park, California
Stanford University, Stanford, Califormia
Fundamental power couplers for superconducting accelerator applications like the ILC are complicated RF transmission line assemblies due to their having to simultaneously accommodate demanding RF power, cryogenic, and cleanliness constraints. When these couplers are RF conditioned, the observed response is an aggregate of all the parts of the coupler and the specific features that dominate the conditioning response are unknown. To better understand and characterize RF conditioning phenomena toward improving performance and reducing conditioning time, a high-power coupler component test stand has been built at SLAC. Operating at 1.3 GHz, this test stand was designed to measure the conditioning behavior of select components of the TTFIII coupler independently, including outer-conductor bellows, diameter changes, copper plating and surface preparations, and cold window geometries and coatings. A description of the test stand, the measurement approach, and a summary of the results obtained are presented.
SLAC, Menlo Park, California
LLNL, Livermore, California
The TTF-III coupler adopted for the ILC baseline cavity design has shown a tendency to have long initial high power processing times. A possible cause for the long processing times is believed to be multipacting in various regions of the coupler. To understand performance limitations during high power processing, SLAC has built a flexible high-power coupler test stand. The plan is to test individual sections of the coupler, which includes the cold and warm coaxes, the cold and warm bellows, and the cold window, using the test stand to identify problematic regions. To provide insights for the high power test, detailed numerical simulations of multipacting for these sections will be performed using the 3D multipacting code Track3P. The simulation results will be compared with measurement data.
SLAC, Menlo Park, California
KEK, Ibaraki
Jefferson Lab, Newport News, Virginia
DESY, Hamburg
The Low-Loss shape cavity design has been proposed as a possible alternative to the baseline TESLA cavity design for the ILC. The advantages of this design over the TESLA cavity are its lower cryogenic loss, and higher achievable gradient due to lower surface fields. High gradient prototypes for such designs have been tested at KEK (ICHIRO) and JLab (LL). However, issues related to HOM damping and multipacting (MP) still need to be addressed. Preliminary numerical studies of the prototype cavities have shown unacceptable damping for some higher-order dipole modes if the typical TESLA HOM couplers are directly adapted to the design. The resulting wakefield will dilute the beam emittance thus reduces the machine luminosity. Furthermore, high gradient tests on a 9-cell prototype at KEK have experienced MP barriers although a single LL cell had achieved a high gradient. From simulations, MP activities are found to occur in the end-groups of the cavity. In this paper, we will present the optimization results of the end-groups for the Low-Loss shape for effective HOM damping and alleviation of multipacting. Comparisons of simulation results with measurements will also be presented.
SLAC, Menlo Park, California
The actual cell shape of the TESLA cavities differ from the ideal due to fabrication errors, the addition of stiffening rings and the frequency tuning process. Cavity imperfection shift the dipole mode frequencies and alter the Qext's from those computed for the idea cavity. A Qext increase could be problematic if its value exceeds the limit required for ILC beam stability. To study these effects, a cavity imperfection model was established using a mesh distortion method. The eigensolver Omega3P was then used to find the critical dimensions that contribute to the Qext spread and frequency shift by comparing predictions to TESLA cavity measurement data. Using the imperfection parameters obtained from these studies, artificial imperfection models were generated and the resulting wakefields were used as input to the beam tracking code Lucretia to study the effect on beam emittance. In this paper, we present the results of these studies and suggest tolerances for the cavity dimensions.
SLAC, Menlo Park, California
Fermilab, Batavia, Illinois
Cockcroft Institute, Lancaster University, Lancaster
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
The FNAL 9-cell 3.9GHz deflecting cavity designed for the CKM experiment was chosen as the baseline design for the ILC BDS crab cavity. Effective damping is required for the lower-order TM01 modes (LOM), the same-order TM11 modes (SOM) as well as the HOM modes to minimize the beam loading and beam centroid steering due to wakefields. Simulation results of the original CKM design using the eigensolver Omega3P showed that both the notch filters of the HOM/LOM couplers are very sensitive to the notch gap, and the damping of the unwanted modes is suboptimal for the ILC. To meet the ILC requirements, the couplers were redesigned to improve the damping and tuning sensitivity. With the new design, the damping of the LOM/SOM/HOM modes is significantly improved, the sensitivity of the notch filter for the HOM coupler is reduced by one order of magnitude and appears mechanically feasible, and the LOM coupler is simplified by aligning it on the same plane as the SOM coupler and by eliminating the notch filter. In this paper, we will present the coupler optimization and tolerance studies for the crab cavity.
SLAC, Menlo Park, California
DESY, Hamburg
Fermilab, Batavia, Illinois
UMAN, Manchester
Higher Order Modes (HOMs) excited by the passage of the beam through an accelerating cavity depend on the properties of both the cavity and the beam. It is possible, therefore, to draw conclusions on the inner geometry of the cavities based on observations of the properties of the HOM spectrum. A data acquisition system based on two 20 GS/s, 6 GHz scopes has been set up at the FLASH facility, DESY, in order to measure a significant fraction of the HOM spectrum predicted to be generated by the TESLA cavities used for the acceleration of its beam. The HOMs from a particular cavity at FLASH were measured under a range of known beam conditions. The dipole modes have been identified in the data. 3D simulations of different manufacturing errors have been made, and it has been shown that these simulations can predict the measured modes.
SLAC, Menlo Park, California
Royal Holloway, University of London, Surrey
LAL, Orsay
BNL, Upton, Long Island, New York
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
UMAN, Manchester
CERN, Geneva
Fermilab, Batavia, Illinois
DESY, Hamburg
OXFORDphysics, Oxford, Oxon
Cockcroft Institute, Warrington, Cheshire
ANL, Argonne, Illinois
JAI, Oxford
CEA, Gif-sur-Yvette
STFC/RAL, Chilton, Didcot, Oxon
KEK, Ibaraki
UU/ISV, Uppsala
LLNL, Livermore, California
Kyoto ICR, Uji, Kyoto
DESY Zeuthen, Zeuthen
STFC/DL, Daresbury, Warrington, Cheshire
UBC, Vancouver, B. C.
University of Tokyo, Tokyo
BINP SB RAS, Novosibirsk
University of Oregon, Eugene, Oregon
Colorado University at Boulder, Boulder, Colorado
Birmingham University, Birmingham
Tohoku University, Sendai
BNL, Upton, Long Island, New York
ITEP, Moscow
UW-Madison/PD, Madison, Wisconsin
Yale University, Physics Department, New Haven, CT
UCR, Riverside, California
RBRC, Upton, Long Island, New York
Kyoto University, Kyoto
IUCF, Bloomington, Indiana
The spin physics program at the Relativistic Heavy Ion Collider (RHIC) requires knowledge of the proton beam polarization to better than 5%. To achieve this goal, a polarized hydrogen jet target was installed in RHIC where it intersects both beams. The premise is to utilize the precise knowledge of the jet proton polarization to measure the analyzing power in the proton - proton elastic scattering process in the Coulomb Nuclear Interference (CNI) region at the prescribed RHIC proton beam energy, then use the reverse reaction to measure the degree of the beam polarization, and finally confront the results with simultaneous measurements by the fast high statistics polarimeter that measure the p-Carbon elastic scattering process in the CNI region to calibrate the latter. In this presentation, the polarized jet target mechanics, operation, detector systems and the p-Carbon polarimeter are described. The statistical accuracy attained as well as the systematic uncertainties will be discussed. Such techniques may well become the standard for high energy polarized proton beams planned elsewhere in Russia and Japan.
SLAC, Menlo Park, California
LBNL, Berkeley, California
One of the action items for the damping rings of the International Linear Collider (ILC) is to compute the broad-band impedance and, from it, the threshold to the microwave instability. For the ILC it is essential that the operating current be below threshold. Operating above threshold would mean that the longitudinal emittance of the beam would be increased. More seriously, above threshold there is the possibility of time dependent variation in beam properties (e.g. the "sawtooth" effect) that can greatly degrade collider performance. In this report, we present the status of our study including calculations of: an impedance budget, a pseudo-Green's function suitable for Haissinski equation and instability calculations, and instability calculations themselves.
SLAC, Menlo Park, California
The effects of the round edge beam hole on the frequency and wake field are studied using variational method, which allows for rounded iris disk hole without any approximation in shape treatment. The frequency and wake field of accelerating mode and dipole mode are studied for different edge radius cases, including the flat edge shape that is often used to approximately represent the actual structure geometry. The edge hole shape has weak effect on the frequency, but much effect on the wake field. Our study shows that the amounts of wake fields are not precise enough with the assumption of the flat edge beam hole instead of round edge.