Modeling of an Electron-Beam Pumped Arf Excimer Laser *

Abstract

We present here initial efforts to advance the modeling of electron-beam-pumped ArF lasers. These efforts will be tested against experiments using the NRLElectrafacility. The advantages of using ArF as a driver for direct drive laser fusion compared to KrF [1]are: shorter wavelength (193 nm for ArF* vs. 248 nm for KrF*) and broader bandwidth. Use of ArF driver should lead to more robust higher-energy-gain fusion implosions than KrF, with an even larger advantage over the 351 nm laser technology used on NIF. The smaller absorption cross section by F2 with ArF [2]is an advantage in constructing large, high-energy amplifiers. In addition, it may have higher intrinsic efficiency than KrF [3]. A theoretical model of an e-beam pumped ArF* excimer laser is under development. One of the goals of this work is to understand the energy deposition in Ar-F2mixture for ArF lasers and compare to Ar-Kr-F2mixture for KrF lasers [4, 5]. The collisional rates with electrons are obtained as a function of F2concentration and power deposition by solving the steady-state Boltzmann equation for the electron energy distribution function. We use the concept of excitation-to-ionization ratios to obtain slowly varying rates over a wide range of input parameters such as beam power, gas pressure, and initial gas composition. These rates are coupled to a 1D time-dependent plasma chemistry based on NRLOrestessuite of numerical models. It includes plasma chemistry reactions and vibrational population kinetics of ArF* molecules, coupled to a 3D-radiation transport for the amplified spontaneous emission (ASE). The input parameters are the e-beam temporal profile, gas composition, and system geometry (type of laser amplifier configuration with initial laser seed characteristics). Measurable plasma parameters, such as species concentrations, electron and gas temperatures, as well as laser parameters, such as small signal gain, non-saturable absorption, saturated laser intensity, and AES are calculated as a function of input power and gas composition in the e-beam high-power regime. The model results are compared with the limited experimental measurement literature for ArF lasers.