A New Mode to Excite a Gas-Discharge XeCI Laser

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.Gp The 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 [3-5]. 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 of an X-ray preionized XeCl-laser excited by a self-sustained discharge using a spiker-sustainer circuit. The circuit operates either in the resonant overshoot mode [5] or in the charge mode, a new mode based on the dynamics of the charging circuit. 1 Experimental Setup The experimental configuration is shown in, Fig. 1. The laser chamber is made of PVDF. The anode is a uniform field electrode according to Ernst [9] facing a flat aluminium cathode at a distance of 2.5 cm. A pulsed, collimated X-ray beam enters the discharge chamber through a 2 cm × 60 cm window of 1 mm thickness in the grounded aluminium plate. The X-ray preionization pulse, with a rise time of less than 10 ns and a duration of about 50 ns (FWHM), is produced by an X-ray generator using a corona plasma cathode [10]. Figure 2 shows the electrical circuit. This circuit will be used for a high repetition rate, high power (1 kHz, 1 kW) XeC1 laser, which is currently under construction in our laboratory as part of the EU213 Eureka project. A