Final optics protection in laser inertial fusion with cryogenic liquid droplets

Abstract

A burst of x rays and vaporized debris from high yield targets can damage the final optics in laser inertial fusion energy (IFE) power plants and in laboratory experimental facilities such as the National Ignition Facility (NIF) or Laser MegaJoule (LMJ). Noble gases such as Xe or Kr have been proposed to protect final optics from target-produced x rays and debris. Some problems with the use of such ambient gas fills are the large amount of gas involved, heat transfer to a cryogenic target, potential resonant reradiation of x rays absorbed, and a nonuniform index of refraction due to turbulence interfering with the focusing of laser light. Also the fast igniter laser intensity may be too great for propagation through an ambient gas. We propose to provide the gas in the form of many small closely spaced liquid droplets injected in front of the optics. In the case of NIF, the droplets would be injected only when needed just before a high yield shot. The laser light that is absorbed will cause evaporation of the liquid and spreading of this gas. The liquid droplets intercept only {approx}5% of the laser light allowing {approx}95% to pass through to the target. The light absorbed in the NIF example (assumed to be 50% of the intercepted light, whose intensity is 3.6 x 10{sup 9} W/cm{sup 2}) would cause the xenon droplets to evaporate and spread uniformly such that the x rays from 10 eV to 2 keV are appreciably attenuated when they arrive 40 to 70 ns later at the optical surface. X rays above 3 keV and below 10 eV are not attenuated very much but their intensities are rapidly falling off in this range anyway. Typical droplet sizes are {approx}10 {micro}m radius with a spacing of {approx}0.4 mm. The gas would also protect vaporized target debris from condensing on the optics due to the 0.2 mg/cm{sup 2} of xenon (5 x 10{sup 17} cm{sup -2} or 8 Torr-cm for l-e-folding of 1 keV x-rays). These droplets might be produced with technology similar to ink jet technology and photo-etching of silicon to make nozzle plates. The xenon would be easily pumped away from the chamber. The vapor pressure from the evaporating droplets needs to be studied to see if the laser beam will be disturbed. We recommend further analysis and experimental investigation of this new idea.