CERN, Geneva
SLAC, Menlo Park, California
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.
LANL, Los Alamos, New Mexico
SLAC, Menlo Park, California
ORNL, Oak Ridge, Tennessee
To facilitate a rapid response to the International Linear Collider (ILC) L-Band development program at SLAC, a spare converter-modulator was shipped from Los Alamos. This modulator was to be a spare for the Spallation Neutron Source (SNS) accelerator at ORNL. The ILC application requires a 33% higher peak output power (15 MW) and output current (130 Amp). This presents significant design challenges to modify the existing hardware and yet maintain switching parameters and thermal cycling within the semiconductor component ratings. To minimize IGBT commutation and free-wheeling diode currents, a different set of optimizations, as compared to the SNS design, were used to tune the resonant switching networks. Additional complexities arose as nanocrystalline cores with different performance characteristics (as compared to SNS), were used to fabricate the resonant "boost" transformers. This paper will describe the electrical design, system modifications, modeling efforts, and resulting electrical performance as implemented for the ILC L-band test stand.
SLAC, Menlo Park, California
The nominal design gradient for the ILC is 31.5 MV/m, but the L-band superconducting cavities built to date have demonstrated a range in sustainable gradient extending below this goal, limited by Q-dropoff and quenching. An economically feasible cavity acceptance rate will include in the linacs a certain percentage of sub-performing cavities. We examine how, with a customizable RF distribution scheme, one can most efficiently distribute power from one klystron amongst 24 nine-cell cavities. The nominal cavity fills to the design gradient at the time the beam arrives, after which the beamloading voltage exactly cancels any further rise, yielding constant gradient during the bunch train. Along with adjustable RF power, we assume adjustable cavity coupling, or loaded quality factor, so that the gradient can be leveled in non-nominal cavities, to avoid quench-inducing overshoots. We explore these and related issues for the ILC linac high-power RF.
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
An ILC R&D facility is being constructed in the NML building at Fermilab which, in addition to an injector and beam dump with spectrometer, will contain up to three cryomodules worth of ILC-type superconducting 9-cell cavities, 24 in all. This linac will be powered by a single klystron. As part of SLAC?s contribution to this project, we will provide a distribution network in WR650 waveguide to the various cavity couplers. In addition to commercial waveguide components and circulators and loads developed for TESLA, this sytem will include adjustable tap-offs, and customized hybrids. In one configuration, the circulators will be removed to test pair-wise cancellation of cavity reflections through hybrids. The system will be pressurized with nitrogen to 3 bar absolute to avoid the need for SF6 at windows or circulator. The full distribution for the first cryomodule will be delivered and installed later this year. We describe the design of the system and completed RF testing.
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
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
The ILC Linac Group at SLAC is actively pursuing a broad range of R&D to improve the reliability and reduce the cost of the L-band (1.3 GHz) rf system and normal-conducting accelerators. Current activities include the development of a Marx-style modulator and a 10 MW sheet-beam klystron, operation of an L-band (1.3 GHz) rf source using an SNS HVCM modulator and commercial klystron, construction of an rf distribution system with adjustable power tap-offs and custom hybrids, tests of cavity coupler components to understand rf processing limitations, simulation of multipacting in the couplers, optimization of the cavity fill parameters for operation with a large spread in sustainable cavity gradients and operation of a 5-cell prototype positron capture cavity. This paper surveys the results from the past year and reviews L-band R&D at other labs, in particular, that at DESY for the XFEL project.
UCL, London
SLAC, Menlo Park, California
Royal Holloway, University of London, Surrey
UCB, Berkeley, California
JINR, Dubna, Moscow Region
Notre Dame University, Notre Dame, Iowa
DESY Zeuthen, Zeuthen
University of Cambridge, Cambridge