Author: Benwell, A.L.
Paper Title Page
WEP103 A Plasma Attenuator for Soft X-Rays in LCLS-II 553
  • A.S. Fisher, A.L. Benwell, Y. Feng, B.T. Jacobson
    SLAC, Menlo Park, California, USA
  Attenuation of X-ray FEL beams is often required to avoid damaging optics and detectors during alignment, and to study fluence-dependent effects. Soft X-rays are commonly attenuated by photoabsorption in a gas such as argon. However, absorbing a mJ pulse along a meter creates a pressure wave that drives gas away from the X-ray propagation axis, until equilibrium recovers in ~1 ms. This timescale matched the 120-Hz pulse spacing of LCLS, but at the high repetition rate (up to 1 MHz) and power (up to 200 W) of LCLS-II, the attenuation of subsequent pulses is reduced. Simulations demonstrate hysteresis and erratic attenuation from gas-density depletion. Instead, we propose to replace the gas column with an argon plasma in a TM010 RF cavity. The density profile then is largely set by the RF mode. X-ray absorption becomes a perturbation compared to the energy in the plasma. An LCLS-II solid-state RF amplifier, generating up to 4 kW at 1.3 GHz, can provide the drive, and the FPGA-based low-level RF controller can be programmed to track tuning with plasma density. Several diagnostics are planned to monitor plasma properties over a fill-pressure range of 10 to 1000 Pa.  
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About • paper received ※ 16 August 2019       paper accepted ※ 28 August 2019       issue date ※ 05 November 2019  
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