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The interaction of weak shock waves with screens and honeycombs is examined to facilitate the design of a pulsed flowing gas laser system operating at a high repetition rate. Interactions with zero and finite base flow (M

A three-dimensional viscous cascade code has been developed for linear cascades with flat, parallel endwalls. It employs scalar implicit approximate factorization, a finite-volume formulation, second-order upwind differencing, and a two-equation q-& turbulence model based on integration to the wall. A special form of the thinlayer approximation for the compressible Navier-Stokes equations is used which gives accurate skin-friction predictions on highly skewed meshes, now based on sheared H-grids. This code has been validated by considering three-dimensional viscous flows in Langston's large-scale turbine blade geometry. This validation includes threedimensional heat-transfer predictions. Agreement with experiment is quite good.

This paper is motivated by a general interest in pulsed energy addition to a surface in vacuum at high power fluxes (> 1 MW/cm absorbed). The theoretical treatment assumes a perfect gas without plasma formation, and examples are for aluminum. Transient heat conduction into the surface, vapor flux through a Knudsen layer at the surface, and transient one-dimensional flow of vapor into the surrounding vacuum are included as part of the modeling. Results show that the Knudsen layer remains choked so long as the absorbed power is constant or increasing, and it is unchoked when the absorbed power is decreasing. The problem is fully coupled in the latter case. Quasisteady vapor efflux from the surface is established within a fraction of the pulse time at sufficiently high power fluxes. Impulse coupling coefficients due to vapor recoil are noted.

Thermal homogeneity in a single-pulse excimer laser cavity is of interest because it directly impacts medium optical quality. This study is primarily oriented toward an XeF* laser in which the cavity is hot (500 K) and is pumped by an e-beam from a cold cathode structure in a vacuum chamber next to the cavity. The foil and support structure are on a vertical wall between these hot and cold zones. In consequence, due to radiative heat transfer, the wall assumes a nonuniform termperature and induces natural convection and thermal inhomogeneity within the cavity. Studies of radiative, conductive, and natural convective heat-transfer processes are summarized which lead to a plausible engineering approach to achieving medium homogeneity of (6p/p)rms - 5 X10 ~ 5 in the closed XeF* laser cavity.

Transverse acoustic waves in pulsed lasers result from nonuniform pumping transverse to the flow direction. This theoretical study is based on linear acoustics and disregards hot/cold gas interfaces. First, transverse waves in an infinite parallel hard-walled duct are considered, in which case cavity medium inhomogeneity typically decays as t ~ ' / 2 . An idealized treatment is next given to show that exponential decay can be expected with an anode absorber. There is an optimum absorber resistivity and improved attenuation rate results with increased backing volume. Tilted sidewalls are the final topic. Decay is again exponential, and rapid attenuation requires fairly large tilt angles. Examples are chosen to apply to CO2 electric discharge, chemical, and excimer lasers.

Sidewall mufflers are used in pulsed exciplex lasers to rapidly attenuate pressure waves and restore cavity medium homogeneity at successive pulses. The technical basis for designing these is reviewed in the paper with primary focus on the near-cavity region and emphasis on physical understanding. The approach combines rudimentary theory and flow visualization, starting with simplest situation and building toward a more complete picture. Important results are as follows: longitudinal wave attenuation in a finite capacity muffler involves a balance between precursor pulse decay and recompression wave strength which defines an optimum configuration. Acoustic absorber is required in the backing volume, and a basis for choosing its resistivity is indicated. The contribution of a close-in muffler to transverse wave decay within the laser cavity is also characterized. Further, an entropy wave is shown to be generated within a muffler which is confined to the vicinity of the sidewalls. Other entropy sources in a closed loop flow system are noted.

Typical stagnation conditions required to simulate the orbital re-entry corridor are P0 > 1000 atm and T0 > 5000°K. Although it is not feasible to have such stagnation conditions in a reservoir operating under nearly continuous conditions, it is feasible to obtain high stagnation temperatures in a gas flowing from a relatively cool high-pressure reservoir by adding laser energy to the flow. In this new approach, two important advantages are realized: 1) the energy source (i.e., the laser) is physically separated from the wind tunnel and is not temperature-limited as compared to direct heating devices, and 2) the laser beam can be accurately aimed down the central core of the flow, avoiding direct heating of the tunnel walls. A conceptual wind-tunnel configuration is illustrated in Fig. 1. The laser beam enters the tunnel system through a window near the plenum chamber and propagates downstream through the subsonic flow region of the nozzle. Under reservoir con