A new mode to excite a gas-discharge XeCl laser

Abstract

Abstract.The charge mode is a new mode to excite a discharge XeC1 laser based on the dynamics of a spikersustainer circuit with a magnetic pulse compressor. The breakdown voltage is higher than in any other mode, due to a very fast rise time. The higher breakdown voltage provides a wider discharge and we found that more energy can be deposited before discharge instabilities terminate the optical output. PACS: 42.55.GpThe efficiency of high-pressure, self-sustained, gas-discharge excited lasers, like excimers, can be improved by separating the electrical circuit in a high-impedance, high-voltage spiker circuit and a low impedance sustainer circuit which deposits the main part of the stored energy into the discharge [1]. The spiker circuit provides a fast rising voltage pulse to increase the electron density from its preionization value to its quasi steady state value. Once the discharge has been excited, the sustainer circuit can deposit the stored energy efficiently into the discharge because impedance matching is achieved by charging a Pulse Forming Network (PFN) to twice the dc-breakdown voltage of the gas. To separate the two circuits from each other at least one low inductance switch is needed to prevent the spiker energy from flowing into the low impedance sustainer circuit. Use can be made of a rail-gap switch [1] or an array of switches like thyratrons [2]. Very promising is the application of fast saturable inductors enabling high repetition rate operation. Several circuits have been proposed in which low-loss, high-frequency ferrites are applied to operate the spiker-sustainer circuit in a variety of modes [3][4][5]. Also saturable inductors made of glassy alloys have been used, however, these appear to have higher losses [3].There is a number of reasons why the rise time of the spiker voltage should be as short as possible. Firstly, besides making an electron avalanche, the spiker circuit must also bring the saturable inductor into saturation when a magnetic switch is used. A short rise time lowers the amount of magnetic core material needed and therefore decreases the loss in the saturable inductor.Secondly, a short rise time increases the breakdown voltage of the gas and this provides longer stability. Dyer [6] showed for a TEA CO 2 laser, that small perturbations in the electrical field during the avalanche phase cause fluctuations in the electron density which in case of an excimer laser can initiate long term instabilities like the halogen depletion instability [7].Thirdly, a short rise time prevents preionisation electrons from drifting away from the cathode during the preavalanche phase, which can cause a region with insufficient overlap of the avalanche heads that leads to the onset of streamers [8].Finally, a high breakdown voltage ignites a larger part of the preionized volume as will be shown in this paper. The width of the discharge also depends on the curvature of the electrodes and on the spatial distribution of the preionization electron density.In this paper we describe the performance ...

<title>High-average power XeCl laser with x-ray preionization and spiker-sustainer excitation</title>

“…We also investigated the performance of the pre-pulse-main-pulse circuit. This circuit, figure 5, can operate in two modes called the 'resonant overshoot' mode [13] and the 'charge' mode [15] respectively. 'Resonant overshoot' mode is obtained when the pre-pulse is triggered at about the reversal of the charging current.…”

“…A number of electrical circuits and modes of operation has been studied during the course of the project [13,14,15]. Application of x-ray switching [16], where the pre pulse circuit and the main switch can be omitted simplifies the circuit significantly.…”

The influence of charge-mode operation of a XeCl laser on the beam profile

“…2 shows the electrical circuit schematically. The laser could operate in the 'resonant overshoot mode' [6] and in the 'charge mode' [7]. The spiker circuit includes a one-step pulse compression circuit that has two functions: Firstly, it increases the rise time and duration of the current pulse through the spiker thyratron which helps to increase the life time of this thyratron at high repetition rate operation and secondly it plays an important role when the circuit operates in the charge mode.…”

“…3 that for charge-mode operation the discharge current starts a few tens of nanoseconds after the breakdown caused by the spiker pulse. This can be explained as follows [ 7,8] : The saturable inductors, L, and L,, are initially in the reversed saturated state caused by the PFN-charging current that still has a non-zero value. For charge mode operation it is essential that the spiker is fired a few hundred of nanoseconds -- before the charging current of the PFN capacitor, Q,,, crosses zero.…”

A compact spark pre-ionized pulser sustainer TE-CO2 laser

“…Here, the laser is operated by switch in the primary part of the pulser which handles much lower voltages, longer pulses and smaller energies. The successful operation of pulser sustainer TEA-CO 2 laser (Taylor & Lepold 1985Sylvan et al 1991) and excimer laser (Timmeraans et al 1993) using ASSE has been reported.…”