Speaker: Bo Yang, Department of Mechanical Engineering, University of Texas at Arlington
Program Description
The LCLS-II project seeks to increase the repetition rate of the LCLS X-ray Free-Electron Laser by many orders, up to 1 MHz from the current 120 Hz maximum. It calls into question the performance of diagnostic/beamline devices such as gas detectors or attenuators. Previous steady-state studies considering only long time-scale thermal conduction (tens of ms and beyond) after the initial energy deposition by X-ray FEL pulses have revealed a significant nonlinear density depression or the so-called filamentation effect in a dilute gas attenuator, similar to that observed in the interaction of intense optical laser with a dense gas medium. As such, the achieved attenuation was found to exhibit rather complex correlations to many experimental parameters, including not only the gas pressure, but also the repetition rate, pulse energy, and the geometry of the beam and the gas tube. In a more recent study to be presented in my talk, a more complete Newtonian fluid mechanical simulation was performed to elucidate the gas dynamics on much shorter time-scales (< 10 ms) by taking into account previously neglected hydrodynamic motions (shock waves) of the gas molecules in addition to thermal conduction. It was found that shock waves plays the primary role in forming the initial density filament by convectively transporting the gas particles away from the FEL-gas interaction region, where the pressure gradient has been built upon instantaneous heating. The depth of the filament becomes deeper progressively by the shock waves launched from the trailing pulses until a dynamic equilibrium is established. By performing simulations at higher repetition rates but lower per pulse energy while maintaining a constant average power, the gas dynamics effects becomes less significant, with the temperature and density distributions asymptotically approaching, as expected, those calculated for a Continuous-Wave input of an equivalent power. A comparison study will also be discussed which goes beyond the classical Navier-Stokes-Fourier framework of fluid mechanics by including effects of a single component compressible fluid system with large temperature & density gradients such is the case for a FEL driven density filament.
