X radiation from a laser-triggered spark gap

Abstract

Laser triggering of spark gaps is now widely used for low-jitter firing of one or several spark gaps [1–5]. The most important advantage of laser-triggered high-voltage spark gaps is no galvanic coupling between the gap electrodes and the laser. This allows low-jitter firing of one or several spark gaps to which electrodes a dc or a pulsed voltage is applied. Conduction between the spark gap electrodes is initiated by the plasma produced at one of the electrodes or in the gas-filled gap. For a laser-triggered spark gap, the trigger delay and jitter depend on the gas mixture composition and pressure, laser beam focusing site, electric field strength, laser wavelength, radiation power density and pulse duration, and laser beam divergence. For low radiation pulse energies, the shortest trigger delays are achieved for systems with uv lasers, though in some cases, pulsed lasers operating in other spectral ranges are used. Elegas (SF6) or its mixtures with nitrogen are generally used for the working gas in high-voltage spark gaps. SF6 is electronegative gas possessing high electric strength. In optimum conditions (high electric fields, optical breakdown of the gas in a gap at the cathode) the delay time of the spark gap triggering after application of a laser pulse is a few nanoseconds, and the jitter is no more than 1 ns even for a rather short length of the laser spark (about ten gap spacings). Recently it has been shown [6] that the spark preionization systems that are widely used to initiate a volumetric discharge in pulsed lasers generate not only uv and vuv radiation, but also x rays. This makes preionization more uniform and improves the lasing performance for some lasers. There are many reports on x radiation and runaway electron beams in nanosecond discharges initiated in a nonuniform electric field (see, e.g., [7, 8–14]). Supershort avalanche electron beams (SAEBs) where produced in gas diodes at a pressure of 12 atm for helium, 4 atm for nitrogen, and 2 atm for xenon and elegas [8, 10, 14]. It was natural to suggest that runaway electrons and x radiation are generated in a laser-triggered discharge gap as well, and it is just the runaway electrons and x radiation that provide low-jitter laser triggering. However, we have found no information on this subject in the available literature. Note that in conditions of laser triggering, the threshold of optical breakdown for gases used in spark gaps is in the range 5⋅10– 5⋅10 W/cm [1, 15], which is several orders of magnitude lower than the threshold for the appearance of runaway electrons and x radiation in a laser plasma [16]. The goal of the work under discussion was to detect x radiation from a laser-triggered spark gap and to demonstrate that the generation of x radiation in the gap is due to the electric field enhancement by the laser-produced plasma.