Author: Cahill, A.D.
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MOPMW038 Measurements of Copper RF Surface Resistance at Cryogenic Temperatures for Applications to X-Band and S-Band Accelerators 487
 
  • A.D. Cahill, A. Fukasawa, J.B. Rosenzweig
    UCLA, Los Angeles, California, USA
  • G.B. Bowden, V.A. Dolgashev, M.A. Franzi, S.G. Tantawi, P.B. Welander, C. Yoneda
    SLAC, Menlo Park, California, USA
  • J. Guo
    JLab, Newport News, Virginia, USA
  • Y. Higashi
    OIST, Onna-son, Okinawa, Japan
 
  Funding: Funding from DOE SCGSR and DOE/SU Contract DE-AC02-76-SF00515
Recent SLAC experiments with cryogenically cooled X-Band standing wave copper accelerating cavities have shown that these structures can operate with accelerating gradients of ~250 MV/m and low breakdown rates. These results prompted us to perform systematic studies of copper rf properties at cryogenic temperatures and low rf power. We placed copper cavities into a cryostat cooled by a pulse tube cryocooler, so cavities could be cooled to 4K. We used different shapes of cavities for the X-Band and S-Band measurements. Properties of the cavities were measured using a network analyzer. We calculated rf surface resistance from measured Q0 and Q external of the cavity at temperatures from 4 K to room temperature. The results were then compared to the theory proposed by Reuter and Sondheimer. These measurements are a part of studies with the goal of reaching very high operational accelerating gradients in normal conducting rf structures.
 
DOI • reference for this paper ※ DOI:10.18429/JACoW-IPAC2016-MOPMW038  
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MOPMW039 TM01 Mode Launcher for Use in High Brightness Photoguns 491
 
  • A.D. Cahill
    UCLA, Los Angeles, California, USA
  • M. Dal Forno, V.A. Dolgashev
    SLAC, Menlo Park, California, USA
 
  Funding: DOE SCGSR and DOE/SU Contract DE-AC02-76-SF00515
Photo rf guns are a source of electron beams for X-ray FELs such as LCLS and European XFEL. In existing photoguns power is coupled into the cavity by waveguides through the cell walls, like LCLS, or through coaxial coupling, at the European XFEL. We are considering feeding a gun using a circular waveguide with the TM01 mode. To do that we need a mode launcher, a matched device that couples the rectangular TE01 mode waveguide to a TM01 mode in a circular waveguide. Use of the mode launcher reduces complexity of the gun cavity and increases flexibility of positioning the input waveguide relative to the gun body. Mode launchers have been successfully used at SLAC and elsewhere for X-band high gradient tests. Because the existing mode launchers were not built for high brightness guns, they have a significant quadrupole field component. High brightness rf guns have tight requirements on output beam properties, and this quadrupole component adversely affects the beam. We have designed a mode launcher free of this disadvantage. We present design considerations, methodology, and an example S-band mode launcher.
 
DOI • reference for this paper ※ DOI:10.18429/JACoW-IPAC2016-MOPMW039  
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THPOW028 Automated Design for Standing Wave Electron Photoguns: TOPGUN RF Design 3999
 
  • A.D. Cahill
    UCLA, Los Angeles, California, USA
  • M. Dal Forno, V.A. Dolgashev
    SLAC, Menlo Park, California, USA
 
  Funding: DOE SCGSR and DOE/SU Contract DE-AC02-76-SF00515
Systematic design of RF photoguns involves multiple RF simulations in conjunction with beam dynamic simulations. RF simulations include tuning gun frequency, matching the gun to the feeding RF circuit, balancing the on axis electric fields between gun cells, minimizing surface electric and magnetic fields and power consumption, and optimizing separation of resonant mode frequencies. We created a tool that allows this multiple parameter optimization to be done automatically. We used SUPERFISH to accomplish the RF simulations. We present an example of the rf photogun TOPGUN design using these tools.
 
DOI • reference for this paper ※ DOI:10.18429/JACoW-IPAC2016-THPOW028  
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