Thermo-Mechanical Analysis of the Hibachi Foil for the Electra Laser System

Abstract

In support of the High Average Power Laser (HAPL) project the Electra Laser, a KrF Gas Laser system is being developed at NRL. The laser uses high voltage (500 - 800 keV), high current (100 - 500 kA), short pulse (100 - 600 ns) electron beams to pump the 0.14 MPa (20 psi) pressurized KrF gas cell, which is separated from the vacuum region by a 25 μm-thick stainless steel foil, the Hibachi Foil. The foil is made of SUS304, operates between 180 °C and 450 °C, and has typical dimensions of about 0.3 m × 1.0 m. The laser pulses at up to 5 Hz, and the foil is subjected to repetitive thermal and mechanical stresses. In typical experiments, the foil lasts 1000 - 20,000 shots before suffering a catastrophic failure. In an attempt to improve foil performance a variety of design modifications are being considered along with changes in foil material. Earlier Hibachi foil designs used flat foils resting on 0.3 m long square water-cooled supporting ribs (1 cm wide). There is a 3.4 cm gap between ribs. Advanced Hibachi foil concepts are under development using a scalloped foil design. In this paper we report on the comparative thermo-mechanical analysis between flat and scalloped foil geometries. It is demonstrated that the scalloped design reduces stresses to within yield limits of the stainless steel material.