Long-pulse KrCl laser with a high discharge quality

Abstract

The discharge quality and optimum pump parameters of a long-pulse high-pressure gas discharge excited KrCl laser are investigated. A three-electrode prepulse-mainpulse excitation circuit is employed as pump source. The discharge volume contains a gas mixture of HCl/Kr/Ne operated at a total pressure of up to 5 bar. For a plane-plane resonator, the divergence of both output laser beams is measured. A low beam divergence of less than 1 mrad is measured as a result of the very high discharge homogeneity. A maximum laser pulse duration of 150 ns (FWHM) is achieved for a pump duration of 270 ns (FWHM) and a power density of 340 kW cm −3 . Pumping the discharge under optimum conditions employing a stable resonator results in a maximum specific energy of 0.45 J/l with a laser pulse duration of 117 ns and an efficiency of 0.63% based on the deposited energy. High-pressure gas-discharge pumped rare-gas halide excimer lasers are sources of very powerful ultraviolet (UV) radiation. The wavelength range covered and the high pulse energies have made excimer lasers an extensively used tool for applications in science, industry and also in medicine [1,2]. Gas discharge pumping opened the way to UV-lasers with pulse repetition rates up to several kHz with output powers up to the kW-level. However, excimer lasers pumped by a simple charge-transfer circuit [3,4] are restricted to low efficiencies and short pulse durations of only a few tens of nanoseconds. Voltage matching between the storage capacitance used to pump the laser and the discharge steady-state voltage is hardly achievable with such circuits. The discharge current tends to oscillate under these conditions. Discharge instabilities grow rapidly due to the voltage mismatching and the current oscillation. The beam quality of these lasers is poor, as the required time to build up a beam with a low divergence is longer than the stability time of the discharge.An important step to increase the efficiency and pulse duration of excimer lasers has been the development of u Fax: +31 53 4891102, E-mail: l.c.casper@tnw.utwente.nl the prepulse-mainpulse excitation technique [5,6]. The prepulse, a very fast rising high-voltage pulse, is applied to the discharge electrodes within a few nanoseconds after preionization to initiate the discharge homogeneously. A lowimpedance circuit generates the mainpulse in order to sustain the discharge and to pump the laser. The mainpulse voltage can be chosen independently from the prepulse voltage. Therefore, matching between the sustaining voltage and the steady-state voltage can be realized. As a result, this excitation technique leads to an optimization of the pump efficiency as the voltage matching is possible and current ringing can be avoided. The discharge stability improves considerably and leads to an extension of the laser pulse duration. In the case of XeCl lasers (308 nm), laser pulse durations of several hundreds of nanoseconds with efficiencies up to 5% have been realized [7,8] and raised strong interest in science and indu...

Effects of tunable circuit parameters on pulsed discharge and input-output characteristics of a XeCl excimer laser

“…The time-varying nature of the discharge equivalent impedance R d may cause severe impedance mismatch, leading to polarity reversal (oscillation) of the gap voltage and the discharge current. This phenomenon has been observed in both experiments [17][18][19][20][21] and simulation [18,19,[22][23][24].…”

Nanosecond-pulsed dielectric barrier discharges in Kr/Cl2for production of ultraviolet radiation

“…where k (2) k is the two-body reaction rate coefficient. For each species the continuity equation is combined with the momentum equation in the drift-diffusion approximation n j w j = sgn q j µ j n j ∇ − µ j ∇ n j T j (7) describing the mean velocity w j under the influence of an electric field and collisions. In this equation, µ j is the species mobility, q j is the species charge and is the scalar potential, corresponding to an electric field E = −∇ , which describes the ambipolar plasma field, coupling together the transport of electrons and ions, combined with an applied field (voltage = en e w e • ∇ + e (8) where e is the density of the net power gained in collisions and e is the elementary charge.…”

“…Much of our present understanding about the formation of RgH exciplex molecules (electronically excited heteronuclear molecules) is derived from work carried out for laser applications [2][3][4][5][6][7][8][9]. Exciplex molecules have stable upper states with short radiative lifetimes (5-20 ns) and unbound (or weakly bound as is the case of XeF and XeCl) ground states, conditions that are ideal for creating a population inversion.…”