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Performances of Gas Devices for High Rep. Rate X-ray FEL’s using Thermodynamic and Hydrodynamic Studies
(SLAC National Accelerator Laboratory)
E1.173 (Campus Schenefeld, Main Building)
Campus Schenefeld, Main Building
Numerical simulations were performed to investigate the short time-scale (ms) hydrodynamics and thermodynamics in gas devices for high repetition rate X-ray Free-Electron Lasers. It was found that the shock wave motions are primarily responsible for creating a density filament by pushing gas molecules away from the X-ray laser-gas interaction region, where a large pressure and temperature gradient has been built up upon the initial energy deposition and subsequent thermalization. Concurrent outward heat conduction tends to reduce the pressure in the filament core region, generating a counter gas flow to backfill the filament, but on a slower time scale. If the inter-pulse separation is sufficiently short, the depth of the filament progressively increases as the trailing pulses remove additional gas particles. Since the rate of hydrodynamic removal decreases while the rate of heat conduction back flow increases as time elapses, the two competing mechanisms ultimately reach a dynamic balance, establishing a repeating pattern for each pulse cycle, asymptotically approaching the behavior predicted by thermodynamic studies óf longer time scales (ms or greater). The impact of this gas filamentation on the performance of the gas devices will be discussed. In addition, the findings from a recent experimental measurement using a two-bunch FEL beam of LCLS will be presented.