Acoustic Suppression in a Pulsed Laser System

Abstract

The paper describes experimental and theoretical studies aimed at achieving acoustic suppression in a high- energy pulsed electric discharge laser system. Small-scale experiments were conducted for an open-cycle system having a sonic orifice plate upstream of the cavity and an acoustic horn and sidewall muffler downstream. The theoretical studies address the performance of the attenuators used for the experiments in both the linear and nonlinear acoustic response regimes. The linear regime is investigated by developing analytical solutions; whereas for the nonlinear regime, a ''floating shock-fitting technique is applied. Comparisons of the theoretical predictions with the experimental results are presented. LOWING gas laser systems were a necessary step toward generating high average output power from a single cavity with good beam quality. In this paper we will be discussing a pulsed CO2 gas laser configuration in which laser initiation energy is added in a short duration e-beam sustained electric discharge. In practice, there could be a large number of pulses spaced a fraction of a second apart in a pulse train. During the intrapulse time, waste-heated gas must be removed from the cavity and acoustic disturbances, caused by waste-heat deposition during the initiation process, must be damped to a low level in order to assure the medium will produce a high quality output beam at the succeeding pulse. For a CO2 laser system operating at room conditions and with a 1 m path length between mirrors, a typical density homogeneity requirement is (<Wp) rms < 1 x 10 ~ 3. This paper summarizes an experimental and theoretical study of an attenuator configuration meant to achieve the desired performance. There are two basic categories of pulsed laser systems, depending on the system optimization criterion. Maximizing the average output power per unit mass in a pulse train is appropriate for open-cycle or blowdown facilities in which lasing mixture is stored at high pressure in tanks and one wants to get as much power from that fixed gas supply as possible. It typically involves low base flow Mach numbers (e.g., M, —0.1). More recently, interest has focused on closed- cycle systems in which maximizing average output power in a pulse train is the appropriate goal. This paper is concerned only with open-cycle systems. The generic elements in an open-cycle configuration are shown in Fig. 1. Taking advantage of the high pressure at which the gas will typically be stored, a sonic orifice plate with highly underexpanded jets is used to define the upstream boundary of the flow channel. The mass flow through the channel is controlled by fixing the plenum pressure and temperature just upstream of the orifice plate. The pressure drop across the orifice plate is chosen to insure that no ob- jectionable acoustic disturbances are transmitted into the plenum. Downstream of the laser cavity, there will be an