Application of convection heat transfer in near-wall jets to electron-beam-pumped gas lasers

Heating of the transmission foil separating the vacuum diodes from the laser gas in electron-beam-pumped gas lasers due to high-energy electron beam attenuation necessitates an external cooling scheme to prevent its failure under repetitively pulsed operating conditions. Attenuation of the electron beam (typically 500 kV, 100 kA and 100 ns pulse duration) produces a strong and pulsed volumetric heat source in the relatively thin stainless-steel foil (thickness of &sim25 mum) causing it to fail. An experimental and numerical investigation has been conducted to study the cooling effectiveness of near-wall high-speed jets for a single stainless-steel foil strip that simulates the actual foil geometry between two neighboring support ribs in the Electra KrF gas laser developed by the Naval Research Laboratory. The foil is placed inside a rectangular channel with continuous gas flow to simulate the circulating laser gas. The foil is electrically heated with the heating power input adjusted to achieve the same foil temperatures observed in Electra when no active cooling is applied. Detailed studies include two jet geometries (planar and circular) and two injection methods (tangential/parallel or obliquely impinging jets) for two hibachi foil structure designs (flat and scalloped). The planar jet of &sim1mm thickness flows parallel to the circulating laser gas across the entire foil span. The other configuration uses circular jets of small diameters (0.8 mm, 1.2 mm and 1.6 mm) positioned in two staggered rows located on the foil's two vertical edges with a pitch of 1.25 cm over the entire height of the foil. For both configurations, experiments have been conducted at various jet velocities (or jet Reynolds numbers), impingement angles and jet-foil spacing with an aim to identify the optimal operating parameters for the actual hibachi foil cooling.Numerous investigations have been performed that covered a wide range of operating parameters. Local and average heat transfer coefficients for the foil were obtained and the data indicate that near-wall jets can enhance convection heat transfer by more than one order of magnitude when compared to forced convection that only uses the laser gas recirculation. The locally injected jets only affect the flow field close to the foil and the effect on the bulk flow is very limited, which is crucial to preserve the laser quality. The experimental data will provide the designers of the gas laser system an efficient and cost-effective option for foil cooling for the anticipated operating conditions.Experimental results have also been compared to the predictions from CFD simulations using FLUENTRTM with a well-established k-&egr turbulence model. Good agreement has been observed for the planar jet cooling experiments in terms of foil temperature and heat transfer coefficients. For impinging jet cooling experiments, a simplified three-dimensional model was developed and qualitative agreement was observed.The results show that near-wall jets can effectively cool the foil that separates the vacuum diodes from the laser gas under prototypical pulsed (5Hz) operating conditions. The jets can prolong the lifetime of the foil and also minimize impact on electron beam quality and laser efficiency.