An international team comprising SLAC, KEK, FNAL, JLAB and DESY is collaborating on the design, fabrication and test of a low loss, 1.3 GHz 9-cell SRF structure as a potential improvement for the ILC main linac. The advantages of this structure over the TESLA structure include lower cryogenic loss, shorter rise time, and less stored energy. Among the issues to be addressed in this design are HOM damping, Lorentz force detuning and multipacting. We will report on HOM damping calculations using the parallel finite element eigenmode solver Omega3P and the progress made towards an optimized design. Studies on multipacting and estimates of the Lorentz force detuning will also be presented.
One of the approaches to power distribution system of a superconducting proton linac that is under discussion at Fermilab requires development of a fast-action, megawatt-range phase shifter. Using two phase shifters with a waveguide hybrid junction can allow independent control of phase and amplitude of RF power at the input of each superconducting cavity of the linac. This promises significant saving in number of klystrons and modulators required for the accelerator. A prototype of a waveguide version of a phase shifter that uses Yttrium-Iron Garnet (YIG) blocks was developed and tested. This report presents design concept of the device and main results of simulation and proof-of-principle tests.
The third harmonic 3.9 GHz superconducting cavity was recently proposed by DESY for a new generation of high brightness photo-injector (TTF photoinjector-2) to compensate nonlinear distortion of the longitudinal phase space due to RF curvature of the 1.3 GHz TESLA cavities [1,2]. Installation of the 3 rd harmonic cavity will allow us to generate ultra-short (<50 µm rms) highly charged electron bunches with an extremely small transverse normalized emittance (<1 µm). This is required to support a new generation of linear colliders, free electron lasers and synchrotron radiation sources. In this paper we present the current status of the 3 rd harmonic cavity being developed at Fermilab. We discuss the design procedure, the building and testing of the copper and niobium halfcells and components, the design of input and HOM couplers.
The proposed 8-GeV driver at FNAL [1] is based on ~430 independently phased SC resonators. Significant cost savings are expected by using an rf power fan-out from high-power klystrons to multiple cavities. Successful development of superconducting (SC) multi-spoke resonators operating at ~345-350 MHz provides a strong basis for their application in the front end of multi-GeV linear accelerators. Such a front-end operating at 325 MHz would enable direct transition to high-gradient 1300 MHz SC TESLA-style cavities at ~400 MeV. The proposed front end consists of 5 sections: a conventional RFQ, room-temperature (RT) cross-bar H-type (CH) cavities, single-, double-and triple-spoke superconducting resonators. It is effective to use short RT CH-cavities between the RFQ and SC sections in the energy range 3-10 MeV as is discussed below.
Over the last few years Fermilab has developed the superconducting third harmonic section for t he FLASH (TTF/DESY) upgrade. Initial v ertical tests of 9 -cell Nb cavities didn't reach the designed accelerating gradient. The main factor of gradient limitation was multipacting in the HOM coupler. In this paper we present the results of vertical tests ac companied with 3D analysis simulations of multipacting. Also we discuss the RF design of the new HOM couplers. The goal of the new design is to eliminate multipacting and to increase the second resonance frequency of the HOM coupler . Increasing this frequency reduces electric and magnetic fields , resulting in decreased thermal load on the antenna. Two cavities with modified HOM couplers have achieved operating gradients of 23MV/m.
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