Numerical and experimental studies of Electraʹs scalloped transmission foil cooling with small impinging jets

The 5 Hz rep-rate operation of the Electra KrF laser necessitates the cooling and protection of the transmission foil that is subject to the pulsating electron beam bombardment. The pulsed volumetric heating from the e-beam attenuation heats up the foil (∼2.54 × 10−5 m thick) rapidly and often causes the foil to fail, increasing the operation cost and down time for the laser. Various methods have been investigated forheat transfer enhancement. While elevated heat transfer was achieved, the previous methods assume a flat foil shape. The actual foil shape is scalloped due to the pressure difference across the foil during the laser operation. Also a new “scalloped” foil design was proposed for thermal stress reduction. This paper investigates the applicability of small locally impinging jets to cooling the scalloped-shaped foil. The jets were formed through a line of small circular openings on two stainless-steel jet tubes aligned with the foil edges having the two columns of jets impinging on the foil obliquely in a staggered pattern for improved coverage. CFD simulations were used to optimize jet configurations. Experiments were performed that utilize a scalloped foil strip which matched the foil shape between two neighboring supporting ribs in the Electra hibachi. Jet diameters and jet velocities were varied at a surface heat flux greater than 20.0 kW/m2. Substantial heat transfer enhancement with impinging jets was observed. Average Nusselt numbers were correlated with jet Reynolds number and the normalized jet-to-foil distance. The study indicates that the impinging jets can effectively enhance heat transfer for the scalloped foil and can be a promising method for actual foil coolingof KrF lasers, including Electra.