Scalloped hibachi and vacuum-pressure foil for Electra: Electron beam pumped KrF laser

We are developing a new type of “scalloped” hibachi structure to be deployed on Electra, a 700 Joule/pulse electron beam pumped KrF laser system, to improve the durability and efficiency of the pressure foil. In an e-beam pumped laser, an electron beam is generated in a high vacuum diode, and then passed through a pressure foil to pump the gain medium in the gas laser cell. Previous hibachi structures used flat “picture frame” topologies in which the foil is laid flat on the frame. The natural bulging of the foils under pressure introduces significant stress concentrations at the corners of the rib openings. In our new design, the hibachi frame is scalloped, so the foil between the ribs approximates a section of a cylindrical pressure vessel. This arrangement eliminates these stress concentrations and, because the stress can in principle be made purely cylindrical, lowers the overall stress as well. This allows use of a thinner foil to transport the e-beam more efficiently. Two techniques were developed to seal this non-planar vacuum surface: utilizing a bonded gasket-foil fixture or employing a quad or double seal o-ring. The former is less expensive, but only proved viable for thicker foils. These methods have been shown to support foils of various materials including aluminum, stainless steel, and titanium with thicknesses ranging from 12 μm to 75 μm. Foils have been tested under high vacuum and with up to 30 psi differential applied to the foil.