Foil cooling for REP-rated electron beam pumped KRL lasers

Summary form only given. The Electra program at the Naval Research Laboratory is developing the science and technologies for implementation of krypton-fluoride (KrF) lasers in inertial fusion energy. Large aperture (ges30times30 cm²) KrF lasers are pumped by electron beams which transit a foil separating the gas target at ges1 atm pressure from the vacuum diode. A fraction of the beam energy on each shot is deposited in the foil and thus long term (ges108 shots), rep-rated (5 Hz) operation requires active cooling of the foil to prevent thermal yield relaxation and cycling fatigue. This paper will report on experimental data and theoretical analysis of three diverse approaches to foil thermal management: convective, conductive, and mist cooling. Forced convective cooling has been operational on the Electra main amp through the use of louvers within a gas recirculator containing the pumped lasing region. At 5 Hz the foil temperature (Tf) can be maintained at ~400degC for a 1 mil SS foil. Conduction cooling provides a conceptually simple configuration with only the need for water channels in the ribs of the hibachi. In this approach Tf increases with the rib separation and hibachis with narrow rib openings (2 cm) are under presently construction. The narrow rib opening requires a narrow pattern for the beam emission from the cathode in order to maintain transmission efficiency. The third approach of mist cooling has been measured to be very effective. Saturated air is injected between two foils separated by the hibachi ribs. The atomized gas-liquid mixture forms a thin layer on the inside of each foil and Tf can be maintained below ~150degC at 5 Hz. The rawback of mist cooling is the double foil configuration and high pressure pumps, which extracts additional energy from the beam. Additional experimental results will also be reported. Finally, advanced materials combining high thermal conductivity with strength, such as Al-Ti-Al sandwich foils will be discu- sed