Author: Fisher, A.S.
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MOPG58 Coherent Diffraction Radiation Imaging Methods to Measure RMS Bunch 198
  • R.B. Fiorito, C.P. Welsch
    Cockcroft Institute, Warrington, Cheshire, United Kingdom
  • C.I. Clarke, A.S. Fisher
    SLAC, Menlo Park, California, USA
  • A.G. Shkvarunets
    UMD, College Park, Maryland, USA
  The measurement of the RMS bunch length with high resolution is very important for latest generation light sources and also a key parameter for the optimization of the final beam quality in high gradient plasma accelerators. In this contribution we present progress in the development of novel single shot, RMS bunch length diagnostic techniques based on imaging the near and far fields of coherent THz diffraction radiation (CTHzDR) that is produced as a charged particle beam interacts with a solid foil or an aperture. Recent simulation results show that the profile of a THz image of the coherent point spread function (CSF) of a beam whose radius is less than the PSF, i.e. the image produced by a single electron, is sensitive to bunch length and can thus be used as a diagnostic. The advantages and disadvantages of near field and far field imaging are examined and the results of a recent high energy (20 GeV) CTHzDR experiments at SLAC/FACET are presented. Plans for experiments to further validate and compare these imaging methods for both moderate and high energy charged particle beams are also discussed.  
poster icon Poster MOPG58 [1.067 MB]  
DOI • reference for this paper ※ DOI:10.18429/JACoW-IBIC2016-MOPG58  
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WEPG23 Evaluating Beam-Loss Detectors for LCLS-2 678
  • A.S. Fisher, R.C. Field, L.Y. Nicolas
    SLAC, Menlo Park, California, USA
  The LCLS x-ray FEL occupies the third km of the 3-km SLAC linac, which accelerates electrons in copper cavities pulsed at 120 Hz. For LCLS-2, the first km of linac will be replaced with superconducting cavities driven by continuous RF at 1300 MHz. The normal-conducting photocathode gun will also use continuous RF, at 186 MHz. The laser pulse rate will be variable up to 1 MHz. With a maximum beam power of 250 kW initially, and eventually 1 MW, the control of beam loss is critical for machine and personnel safety, especially since losses can continue indefinitely in linacs and dark current emitted in the gun or cavities can be lost at any time. SLAC protection systems now depend on ionization chambers, both local devices at expected loss sites and long gas-dielectric coaxial cables for distributed coverage. However, their ion collection time is over 1 ms, far slower than the beam repetition rate. We present simulations showing that with persistent losses, the space charge of accumulated ions can null the electric field inside the detector, blinding it to an increase in loss. We also report on tests comparing these detectors to faster alternatives.  
poster icon Poster WEPG23 [6.589 MB]  
DOI • reference for this paper ※ DOI:10.18429/JACoW-IBIC2016-WEPG23  
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