Imaging of Electric-Field-Free Gas Breakdown

Abstract

Images of electric-field-free gas breakdown are presented. Possible mechanism of the breakdown is proposed.Index Terms-Explosions, electric breakdown, shock waves. G AS BREAKDOWN and gas discharge have been detected in magnetic flux compressor generators (FCGs) since 1960s [1]. Recently [2], we have reported that gas breakdown in an FCG can occur without electric field. In this paper, we present a series of high-speed images of this type of gas breakdown and discuss a possible mechanism of the plasma initiation.In these studies, we used a loop flux compression generator (LFCG) [3]. The operation of an LFCG is similar to that of other types of FCGs, and it is based on magnetic flux compression inside a stator due to the explosively driven expansion of metallic armature. The unique feature of an LFCG is the opportunity to observe the processes that occur inside the generator from the very beginning until the final stage of its operation. To avoid any shock-induced glow in the atmosphere around the LFCG, we put the generator in a wooden box purged with helium to prevent afterglow from shocked nitrogen in the air. We used a Cordin 10A high-speed framing camera at 500 000 frames per second. Other experimental details are given in [2].A schematic diagram of LFCG is shown in Fig. 1. An external seed source generated the initial current of 2.6 kA before the explosive operation of the LFCG. The two detonators of the LFCG were initiated at t = 0, and one can see the light coming from the detonation of the C-4 high explosives (HE) inside a cylindrical aluminum armature of the LFCG [ Fig. 1 (t = 0 µs)]. As the LFCG armature started its expansion [a dark contour in Fig. 1 (t = 2 µs)], gas breakdown occurred within the LFCG and plasma appeared in the gap between the armature and the crowbar. Based on high-speed photograph images, the speed of the expansion of the armature was 1.6 ± 0.1 mm/µs at the initial stage and 2.9 ± 0.1 mm/µs at the Manuscript final stage. The electric field strength between the armature and the crowbar contacts was negligible during operation of the system; it did not exceed 7 V/mm. This electric field was not high enough to initiate gas breakdown and form plasma. Further expansion of the armature in Fig. 1 (t = 4 µs) was accompanied by the generation of intense plasma that filled the gap between the armature and the stator. The plasma front propagated along the inner perimeter of the stator [ Fig. 1 (6 µs) and Fig. 1 (8 µs)].The formation of plasma and the propagation of a plasma front with no electric field can cause the development of intense electric discharges in gas within the generators, even at low intensity of electric field. Electrical insulation of the FCG winding would limit electrical contact between the initial plasma and the FCG stator, and correspondingly would avoid the development of plasma formation. Nevertheless, research reports [1], [4] present puzzling evidence that even heavy insulation of windings of FCG stators does not help to solve the problem.One can better understand the pr...