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TY - CONF AU - Fisher, A.S. AU - Benwell, A.L. AU - Feng, Y. AU - Jacobson, B.T. ED - Schaa, Volker R.W. ED - Decking, Winfried ED - Sinn, Harald ED - Geloni, Gianluca ED - Schreiber, Siegfried ED - Marx, Michaela TI - A Plasma Attenuator for Soft X-Rays in LCLS-II J2 - Proc. of FEL2019, Hamburg, Germany, 26-30 August 2019 CY - Hamburg, Germany T2 - Free Electron Laser Conference T3 - 39 LA - english AB - 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. PB - JACoW Publishing CP - Geneva, Switzerland SP - 553 EP - 555 KW - plasma KW - electron KW - FEL KW - cavity KW - controls DA - 2019/11 PY - 2019 SN - 978-3-95450-210-3 DO - doi:10.18429/JACoW-FEL2019-WEP103 UR - http://jacow.org/fel2019/papers/wep103.pdf ER -