<title>UV lasing in nitrogen pumped by a runaway electron preionised diffuse discharge</title>

Abstract

ABSTRACT UV lasing is studied in nitrogen and the N 2 -SF 6 mixture pumped by a volume discharge initiated by a runaway-electron preionised diffuse discharge (REP DD) produced in an inhomogeneous electric field. It is shown, that lasing at a wavelength of 337.1 nm is observed at pressures up to 2.5 atm without any preionisation source. At a pressure of 0.5 atm with the use of blade electrodes and the N 2 :SF 6 =10:1 active medium of length a 6 cm, the output laser energy of a 2 mJ was achieved for the pulse power of 0.55 MW. The REP DD pumping regime is compared with the regime of pumping by a volume discharge produced by a preionisation source. Keywords: volume discharge without a preionisation source, inhomogeneous electric field, nitrogen laser. 1. INTRODUCTION An important field of applications of various discharges and electron beams is laser technologies [1-4]. Volume discharges are used in various fields of science and technology, in particular, for the development the pulsed lasers on dense gases. High-pressure volume (diffusive) discharges in various gases have been investigated in many papers [4- 10]. Lasers were mainly pumped by discharges in a homoge neous electric field by using the discharge gap preionisation by an additional source [1-4]. Volume discharges at high pressures can be produced in comparatively strong electric fields because in this case a high enough initial electron concentration in the interelectrode gap is achieved [6]. At the given initial electron concentration, the heads of electron avalanches are overlapped to achieve the critical size of the avalanche and formation of a streamer. In strong electric fields the acceleration of a part of electrons in the electron-avalanche heads occurs, which does not restrict the formation of diffusive discharges [5, 10]; in this case, discharges consist of individual diffusive filaments, which lead to the inhomogeneity of the energy input and reduce the lasing efficiency [10]. Volume discharges formed in an inhomogeneous electric field are quite interesting. The possibility of producing a diffusive discharge in helium [11] and air [12] at the atmospheric pressure in an inhomogeneous electric field was first reported in the late 1960s. Nanosecond voltage pulses of amplitude above 100 kV were applied to the discharge gap. Investigations in this direction were stimulat ed by the fact that X-rays and runaway electron beams were observed during the discharge gap breakdown (see review [7] and references therein). However, this discharge regime was not used for pumping lasers, and then interest in the investigation of volume discharges in an inhomogeneous electric field at high pressures weakened. It was shown in recent papers [13, 14] that nanosecond voltage pulses applied to the discharge gap in an inhomogeneous electric field without any additional preionisation source produced a volume (diffusive) discharge in various gases at pressures above the atmospheric pressure. In [13], the record specifi c power inputs in the volume stage of the discharge were obtained (a 0.8 GW/cm