M. S. Tillack and F. Najmabadi
University of California, San Diego
mtillack
ucsd.edu
Experimental studies of IFE chambers and optics have been performed in the Laser-Matter Interactions Laboratory at UC San Diego. In this presentation we will report on the latest results of our studies of laser-induced damage to grazing-incidence metal mirrors, dry chamber wall responses to short-pulse energy deposition, and laser propagation through chamber media.
Grazing incidence metal mirrors (GIMM’s) have been proposed as a robust, neutron damage resistant alternative to dielectric mirrors. Used at grazing angles, they offer relatively high laser-induced damage threshold. However, operation for long periods of time in an environment contaminated with aerosol and dust could reduce their damage threshold substantially. Damage in pure surfaces occurs due to accumulated thermomechanical damage resulting from the large temperature swings and thermal stress gradients in the near-surface region. Damage from contaminants occurs due to localized absorption and subsequent growth of initial defects. Experimental studies of the long-term behaviour of Al GIMM’s have been performed for both clean and contaminated surfaces fabricated by thin film deposition and by diamond turning of bulk metal.
Similar phenomena may occur in solid surfaces exposed to the pulsed ion and x-ray fluxes from IFE targets. Although the absorption mechanisms are quite different, long-term response to intense short-pulse thermomechanical phenomena share some similarities. We are using our high power Nd:YAG and KrF lasers to test candidate wall materials at high shot counts. A range of diagnostics have been developed for both time-averaged and time-resolved surface studies, including a multi-colour optical thermometer, quartz crystal microbalance and residual gas analyzer. These high rep-rate experiments are being used in conjunction with modelling and experiments on pulsed ion and x-ray sources to help determine the operating limits and long-term behaviour of IFE chamber walls.
The requirements on target illumination for direct-drive laser-IFE are very stringent: pointing accuracy of 10-100 mm and spatial uniformity of ~1%. We have performed detailed laser propagation studies in order to further understand the limitations on chamber gas pressure and composition. Previous studies have measured the "breakdown threshold" for high-power lasers by measuring the occurrence of visible emission. We have performed more detailed measurements of the changes in spatial profile, temporal pulse shape and wavefront in order to determine the acceptable operating regime with more precision.
