Physical and technological issues of KrF laser drivers for inertial fusion energy

Abstract
Physics and technology of Krypton Fluoride (KrF) laser have been studied experimentally and theoretically to improve its efficiency and to increase a lifetime, and thus to verify the challenge of KrF laser for Inertial Fusion Energy (IFE). Experiments were performed with e-beam-pumped multistage 100-J output energy GARPUN KrF laser facility and 200-A/cm² current density EL-1 electron gun, both operating at P.N. Lebedev Physical Institute. They formed the database for verification of numerical codes capable to predict IFE-scale KrF drivers. Monte Carlo code was developed to calculate e-beam energy deposition inside GARPUN laser chamber while a quasistationary numerical KrF laser code based on generalized "forward-back" multi-direction approximation for radiation transfer equation was used to describe amplification of nanosecond pulses and amplified spontaneous emission (ASE). Long-lived absorption in UV optical materials induced by fast electrons and bremsstrahlung X-ray radiation was measured at EL-1 electron gun with total fluence of ionizing radiation up to 20.6 kJ/cm². Using these data together with measurements and scaling of bremsstrahlung X-ray yield, we can predict that the most stable windows of IFE-scale KrF laser driver would be able to withstand no less than 210⁶ shots. Fluorine-resistant coatings onto fused silica windows of KrF laser were developed and demonstrated damage thresholds as high as 29 J/cm² in test experiments with large 1313-mm uniformly irradiated spot.