Study of energy deposition in the electron-beam-pumped laser facility HAWK

Abstract

The HAWK facility has been developed as a tool to investigate the kinetics of electron-beam-pumped lasers at a pressure of 1 atm for pump rates of 0.040–1 kW/cm3 and pump times of 0.05–1 ms. The highly collimated (1 cm FWHM within the lasing medium) relativistic electron beam propagates through fast valves that separate the accelerator vacuum from a 2-m-long vacuum isolation section, a 0.4-m-long gas-filled buffer section, and 0.7 m along a lambda-geometry laser cell. A 6.5 kG magnetic field confines the 1-MeV electron beam laterally and guides it around a 30° bend into the 3.3-cm-diam laser cell. A magnetic mirror at the far end of the laser cell reflects a large portion of the transmitted electron beam, thereby protecting the laser optics from the electron beam and making the axial deposition more uniform. In this paper we describe modeling of the electron beam energy deposition in HAWK using the three-dimensional Monte Carlo electron/photon transport code acceptm and compare our results with the measured energy deposition. These numerical simulations were begun during the design phase to define the operational range of the facility. There is good agreement between the calculated energy deposition in the laser cell and the deposition inferred from measurements with a segmented, totally stopping calorimeter at different axial locations in cases where hydrodynamic effects are unimportant. ln cases where these effects are important, however, the calculations can predict too large an amount of energy deposited in the laser gas. The numerical simulations are being used to infer the radial and axial deposition profiles of the electron beam energy as an aid in determining the electron-beam-pumped laser performance.