Author: Cahill, A.D.
Paper Title Page
TUPAB129 Optimization of Beam Dynamics for an S-Band Ultra-High Gradient Photoinjector 1626
 
  • A.D. Cahill, A. Fukasawa, J.B. Rosenzweig
    UCLA, Los Angeles, California, USA
  • C. Limborg, W. Qin
    SLAC, Menlo Park, California, USA
 
  Funding: Work Supported by DOE/SU Contract DE-AC02-76-SF00515, US NSF Award PHY-1549132, the Center for Bright Beams, and DOE SCGSR Fellowship. Travel to IPAC'17 supported by the Div. of Phys. of the US NSF (Accel. Sci. Prog.) and the Div. of Beam Phys. of the APS
New electron sources with improved brightness are desired to enhance the capabilities of FELs, making them more compact and fully coherent. Improvements in electron source brightness can be achieved by increasing electric fields on the cathode of photo-emitted electron guns. Recent developments in pulsed RF accelerator structures show that very high gradient fields can be sustained with low breakdown rates by operating at cryo-temperatures, which when applied to photoguns will lead to a large increase in the electron beam brightness. In particular, our simulations show that when operating with a peak gradient field of 240 MV/m on the cathode of an S-band, electron beam brightness of 80~nC/(mm· mrad)2/mm can be achieved with 100~pC bunches. In this paper, we present the design and optimization of an 1.x cell S-Band RF photoinjector, where the x varies from 4-6. The optimization in brightness has been obtained by using a multi-objective genetic algorithm on the solutions calculated with the ASTRA code. We calculate the optimum length of the rf gun, position of accelerating structure, and laser pulse dimensions for a variety of charges.
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2017-TUPAB129  
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THPIK125 Ultra High Gradient Breakdown Rates in X-Band Cryogenic Normal Conducting Rf Accelerating Cavities 4395
SUSPSIK097   use link to see paper's listing under its alternate paper code  
 
  • A.D. Cahill, J.B. Rosenzweig
    UCLA, Los Angeles, California, USA
  • V.A. Dolgashev, S.G. Tantawi, S.P. Weathersby
    SLAC, Menlo Park, California, USA
 
  Funding: Work Supported by DOE/SU Contract DE-AC02-76-SF00515, US NSF Award PHY-1549132, the Center for Bright Beams, and DOE SCGSR Fellowship.
RF breakdown is one of the major factors limiting the operating accelerating gradient in rf particle accelerators. We conjecture that the breakdown rate is linked to the movements of crystal defects induced by periodic mechanical stress. Pulsed surface heating possibly creates a major part of this stress. By decreasing crystal mobility and increasing yield strength we hope to reduce the breakdown rate for the same accelerating gradient. We can achieve these properties by cooling a copper accelerating cavity to cryogenic temperatures. We tested an 11.4 GHz cryogenic copper accelerating cavity at high power and observed that the rf and dark current signals are consistent with Q0 changing during rf pulses. To take this change in Q0 into account, we created a non-linear circuit model in which the Q0 is allowed to vary inside the pulse. We used this model to process the data obtained from the high power test of the cryogenic accelerating structure. We present the results of measurements with low rf breakdown rates for surface electric fields near 500 MV/m for a shaped rf pulse with 150 ns of flat gradient.
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-IPAC2017-THPIK125  
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