Evaluation of materials proposed for the construction of the plasma electrode Pockels cell (PEPC) on beamlet

摘要

The proposed upgrade of the NOVA laser system at Lawrence Livermore National Laboratory employs a multipass architecture that requires an optical switch to emit the laser light at the appropriate fluence. This Pockels cell-based optical switch does not use traditional ring or thin-film electrodes because of the large aperture and high fluence of the laser system. Rather, it uses a plasma electrode Pockels cell with a KD*P crystal as the electro-optical medium. A discharge plasma is formed on each side of the electro-optic crystal and high voltage is applied across the crystal through the plasma electrode to initiate optical switching. In October 1991 we began evaluating materials suggested for the large aperture plasma electrode optical switch. Previous experiments suggested that switching performance could be significantly affected by the deterioration of cell materials. The final prototype switch tested used polyethylene for the switch body, Mykroy for the mid-plane and a silicone vulcanite to encapsulate the KD*P crystal. The encapsulant easily compensated for the effect of assembling the optical switch and we measured no strain-induced birefringence in the crystal after encapsulation. Oxygen was eventually added to the plasma to react with the sputtered carbon from the cathode and produce a gaseous effluent. As an added benefit, the production of ozone absorbed most of the ultra violet radiation affecting the encapsulant. All the materials tested decomposed and produced volatiles, although we have seen no change in the damage threshold of exposed optical surfaces tested to date. The following is an evaluation of the recommended materials for major cell components using published manufacturers data, experimental results from our Material Evaluation Apparatus, and outgassing performance and sputtering data produced at the Laboratory's Vacuum Process Lab.