Gas - discharge XeF * laser with high specific output energy

The discharge characteristics of the XeF* (B~X) laser are investigated. The NF3 and Xe partial pressure of the laser gas mixture and the total gas pressure have been varied. A highest specific output energy of 4.7 J/1 with an efficiency of 0.5% was obtained from a X-ray preionized Ne/Xe/NF3 gas mixture at 6 bar with single-pulse excitation through a multichannel spark gap. PACS: 42.55.Gp, 42.60.By, 52.80 The technology of electrical discharge lasers has been improved greatly in the last decade. High repetition-rate excimer lasers operating with average powers of 100-300 Watt are commercially available now. Most of the scientific research on discharge-excited excimer lasers has been concentrated on the XeCl* laser, from small systems working at high gas pressures [1, 2] to very large systems producing output powers in the kW range [3]. Far less results have been published on excimers with F2/NF 3 halogen-donor-based gas mixtures as KrF* or ArF*, although these lasers have at least the same or even better output characteristics as the XeCI* laser [4]. This is probably caused by the fact that it is more difficult to make a homogeneous discharge in such gas mixtures compared to HCl-doped gas mixtures. Especially the discharge stability is a real problem due to the stronger electron affinity of the F2/NF3 halogen donor in these gas mixtures [5]. Results on the XeF* (B~X) discharge laser are even more scarce although e-beam pumping of this laser transition has been studied very extensively [6, 7]. The fact that in the past these type of F2/NF3-doped laser-gas mixtures have not been investigated extensively in gasdischarge devices certainly has to do with the enhanced probability for discharge instabilities. In the last decade, * Permanent address: Institute of Electronics, Academia Sinica, Beijing, P.R. of China however, the techniques to excite laser-gas mixtures by an electrical discharge have been improved considerably. The use of X-ray preionization and pre-pulse-main-pulse excitation makes it easier to handle these strong electronegative gas mixtures. Kumagai and Obara [8] investigated the role of the buffer gases He/Ne and of the Fa/NF3-halogen donor in laser-gas mixtures for the discharge-pumped XeF* (B~X) laser at low gas pressures. In their experiments they used a more or less conventional electrical excitation circuit with a spark-gap switch and UV preionization. They obtained an output-pulse length of about 20 ns and a maximum specific output energy of 1.2 J/1. In this paper, we report on a high-pressure XeF* (B~X) discharge laser preionized with X-rays and with the electrical circuit connected to the laser head by means of a low-inductance multichannel spark gap. With this device it was possible to produce laser output-pulse lengths of 85 ns (FWHM) at a total pressure of 6 bar. Under optimised conditions a highest output energy of 350 mJ (4.7 J/l) was measured. 1 Experimental setup The experimental setup is described in detail elsewhere [9, 10]. A cross-sectional view of the system is shown in Fig. 1. The transfer capacitor Ct, which is an array of small (TDK, 2.7 nF, 30 kV) capacitors, is charged by a low-inductance storage capacitor Cs (Maxwell, 80 nF, 50 kV). In order to improve the voltage rise time on the anode, the transfer capacitor C, is connected to the anode by a low-inductance multichannel spark gap (Fig. 1). The MultiChannel Spark Gap (MCSG) is not externally triggered, its voltage-breakdown value is controlled by the gas pressure inside the gap. The MCSG consists of a brass rod and knife mounted in a perspex housing. The gas pressure, the gas usually is nitrogen, can be varied from 1 to 2.5 bar absolute and the electrode distance between 5 and 20 mm. The peaking capacitor C; consists of 8 capacitors (TDK, 0.7 nF, 40 kV each) in parallel. 534 P.J.M. Peters et al.