Bernard M. Forestier

Two-dimensional numerical study of wave damping in a high-average-power excimer laser

To obtain a high average laser output power and a good quality laser beam in high repetition rate excimer lasers, it is necessary to swept the heated gas out of the laser cavity between two electrical excitations and to damp density fluctuations induced by acoustic and shock waves. In this aim, a numerical study of an unsteady two-dimensional flow inside the laser head has been undertaken at I.M.F.M. This study is extended to the case when mufflers are added in the surroundings of discharge zone. The work is achieved by means of a numerical model based on a finite difference scheme with a flux corrected transport algorithm (Shasta. F.C.T. method). This method allows to simulate the propagation of longitudinal (along the flow) and transversal (between the electrodes) shock waves induced by the active medium excitation, and the effects of acoustic dampers on the density fluctuations. Transversal acoustic waves are induced by the non-uniformity of energy deposited between the electrodes. The model uses two dimensional Euler equations in the laser flow channel and one dimensional Euler equations in the acoustic damper (grid and porous material). Several comparisons between experimental and numerical results have been performed to establish the model validity. The influence of the acoustic dampers on the transversal wave decay process in a laser cavity is presented.

High-Pulse Repetition Rate Limitations in a High Average Power XeCl Laser

Potential applications of high average power ultraviolet lasers are numerous in fields as different as semiconductor processing, photochemistry, long-range communications, laser-matter interactions… These applications stimulate a large amount of studies in U.S.A, U.S.S.R. and Europe(1),(2),(3),(4),(5).The scaling of ultraviolet and visible lasers to high output pulse energy and high pulse repetition frequency (prf) to achieve average powers in the range of 0.1 to 1 kW is under investigation at l’Institut de Mecanique des Fluides de Marseille in a closed cycle loop to conserve the expansive gas mixture typical of excimer lasers.

Parametric study of small-volume long-pulse x-ray preionized XeCl laser with double-discharge and fast magnetic switch

An experimental parametric study of a small volume x-ray preionized XeCl laser which uses the double discharge (spiker-sustainer) principle and a new fast ferrite magnetic switch is presented. This high-efficiency discharge excitation system, with a Ne/Xe/HCI mixture at relatively low pressure (P equals 2.5 atm.), in a 50 cm3 active volume (25(DOT)2(DOT)1 cm3), has made possible an efficiency of 3.2% in energy and 4% in power at maximum with up to 140 mJ of extracted optical energy in a 130 ns (FWHM) laser pulse in switch mode. The authors discuss laser characteristics dependence with magnetic switch and electrical circuit parameters, for switch mode. Future studies of the present advanced excitation system for conditions of very high PRF, in the frame of Eureka EU205 Eurolaser program, are presented. The experimental part of these planned studies will be made with LUX very high PRF high average power IMFM test-bed

Modeling of a long-pulse high-efficiency XeCl laser with double discharge and fast magnetic switch

A complete model of a long-pulse x-ray preionized self-sustained discharge XeCl excimer laser has been established for the case of excitation by an advanced electrical circuit associating a double discharge (spiker/sustainer) and a fast low-loss ferrite magnetic switch with pulsed reset. This model allows simulation of the potential working conditions of the IMFM high average power, very high repetition rate LUX test-bed. The model, which considers Ne as buffer gas and HCl as halogen donor, includes a rate equation for species with the most recent available rate constant values, circuit equations with time-dependent ferrite inductance and plasma impedance and rate equation for laser emission taking into account absorbtions. Modeling takes into account time delay between spiker and sustainer, low value of sustainer field and dynamics of magnetic switch. The model has been validated with a long- pulse x-ray preionized XeCl laser, with double discharge and magnetic switching, developed at IMFM, which has already obtained over 3% efficiency and 140 mJ extracted energy in 150 ns (FWHM) from a 50 cm3 active volume at 2.5 atm. pressure. Numerical results are in good agreement (within 30%) with IMFM experimental results. A parametric study of the XeCl laser with double discharge and fast magnetic switch is presented as well as a comparison with both experiments and numerical results of excitation by a C-L-C classical circuit for the same geometric and gas conditions and the same input energy.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Fast-flow gas-dynamic effects in high-pulse repetition-rate excimer lasers

The use of a high pulse repetition rate in a high average power excimer laser radicalizes the role of acoustic and aerodynamic phenomena. Accounts are presently given of the current understanding of interferometric wave analysis results, the damping of acoustic or shock waves, and the correlation between pressure perturbation level and energy in laser pulses

Effect of acoustic dampers on the excimer laser flow

It is well-known that to obtain a good quality laser beam in high repetitive rate excimer lasers, the acoustic waves due to a non uniform energy deposition between the electrodes, must be strongly damped. The unsteady two dimensional flow in eximer laser cavity has been described previously 1 by a numerical approach. The aim of this paper is to include in the numerical code a modelization of the sidewall mufflers in order to study the influence of the porous material on the behaviour of transverse and longitudinal waves in the excimer laser cavity. In the muffler, the flow is assumed one dimensional (y direction) and is coupled to the laser cavity flow. These flows are described by Euler equations and the numerical method is based on an explicit difference scheme assiociated with a corrected transport algorithm (Shasta method)2. The results show the propagation and the damping of longitudinal shock-waves and acoustic waves for different kind of mufflers. Different shapes of laser cavity are also investigated to study the influence of the electrode raduis curvature on the transverse waves. These results are compared to experimental results obtained from the L.U.X. laser of Institut de Mecanique des Fluides de Marseille.

Kinetics and electrical modeling of a long-pulse high-efficiency XeCl laser with double discharge and fast magnetic switch

A complete model of the long pulse XeCI excimer laser excited by a double discharge with fast magnetic switch has been established for an Ne/Xe/HCI active medium. This model simulate one of the potential working conditions of the IMFM high average power LUX test-bed. Model includes rate equations for species with most recent availiable rate constant values, circuit equations with time dependant ferite switch inductance and plasma impedance and rate equation for laser emission taking into account absorptions. The model allows to simulate very long laser pulses (150-200 ns) and numerical results are in good agreement (within 30 %) with experimental results obtained at IMFM.

Modelling of the X-Ray Preionised XeCl Self-Sustained Discharge Laser

A complete model of the long pulse X-ray preionised self-sustained discharge XeCl laser has been established for transfer capacitor and PFN circuits and Ne/Xe/HCl active medium. This model simulates the I.M.F.M. high average power Lux laser test-bed. The model includes rate equations for species with most recent rate constant values, rate equation for laser emission taking into account absorptions, and circuit equations with time dependant plasma impedance. The model shows the dominant effects of electron quenching and Xe2Cl absorption on laser output and efficiency. Results of the model are in good agreement (within 30%) with experimental results obtained with LUX device.

Flow Characteristics of Closed Loop High PRF E-Beam or X-ray-Assisted Discharge Exciplex Laser

The potential applications of high-average power short-wave-length lasers have led to intensive research in this field [1], and a study of methods for achieving such a laser using wellknown exciplexes capacities has been undertaken at I.M.F.M. The method chosen here is to excite a flowing active medium by means of a high-repetition-rate long-pulse avalanche discharge stabilized either by a low-density electron or X-ray beam. Specifically this paper is devoted to the numerical study of the transient effects of electrical excitation on flow behavior for typical conditions of exciplex laser at high loading (Q X100 J/∓). The experimental study is under development on a new device, “LUX”, described in a companion paper [5].

High-Repetition-Rate Long-Pulse-Discharge XeCl Laser Preionized by a Low-Density Electron or X-ray Beam

High average power short wavelength laser systems and particularly exciplex lasers are a very rapidly growing research field owing to their numerous potential applications: energetics, long range communications, material processing, etc. [1]. For further development of these systems, numerous problems remain to be solved. Among the most important of these problems are: n n nLaser discharge stability when long pulse, large volume, and high pulse repetition frequency (PRF) are considered; n n ndeposition at very high PRF of the necessary high specific power and energy [2,6]; n n nlaser medium homogeneity at very high PRF [3,4].