Dynamics of a self-sustained volume discharge in laser-shock-disturbed SF₆-based mixtures

Abstract

This paper reports on first systematic investigations into the dynamics of a self-sustained volume discharge (SSVD) in SF6-based mixtures pre-irradiated by a pulse CO2 laser. Two different illumination geometries are considered, one with a diaphragm slit arranged normal to the central electric field line of the discharge gap and the other with the discharge gap being wholly irradiated. The pattern of a laser-induced gas dynamic disturbance has been theoretically drawn starting from a shock wave theory and visualized by taking SSVD photographs. The delay times between the laser and voltage applications were varied from a few to several tens of microseconds. It has allowed us for the first time to obtain information, based on measuring the discharge voltage, about how long the mixtures considered are capable of being exposed to high temperature with no dissociation. The dynamic barrier effect found in our previous works is thoroughly investigated in both illumination geometries and the observed discharge patterns are qualitatively treated in terms of the electric field distortion by the surface charges at the shock wave fronts. The corresponding surface charge density is numerically estimated. The concept of controlling the electric field distribution over the discharge gap using the dynamic barrier effect is put forward.

Electron detachment instability and self-organization in strongly electronegative polyatomic gases

“…Thus none of the reasons mentioned can be invoked to explain the filamentary pattern observed. It should be noted that a quasi-periodic structure of a pre-irradiated SSVD manifests itself at larger delays τ too when the laserinduced shock waves advance towards the electrodes (see figure 5 in our recent paper [6]).…”

“…The set-up and experimental procedures are generally identical to those of [5,6], so that only some relevant points will be touched on below.…”

“…The negative ion densities in SF 6 -based mixtures in the absence of an external electric field are solely determined by the natural ionizing background and estimated as being not larger than 10 4 cm −3 . In a quasi-stationary phase of an SSVD followed by the post-breakdown sharp drop in the discharge voltage (see typical oscillograms in [5,6]), the negative ion density rapidly increases by many orders of magnitude. We can therefore put N n (0) = 0, the time being kept from the instant of establishing the quasi-stationary phase.…”

“…That is typical of an SSVD in SF 6 -based mixtures at W el values less than 50 J l −1 even in the laser-heated regions. The second possibility (k d ≈ β ei ) is a characteristic of the shock-compressed gas regions sandwiched between the shock wave fronts and the contact discontinuity surfaces wherein the gas temperature is nearly that of the undisturbed regions (see figure 2(d) in [6]).…”

“…Among other things, it is triggered by recent experiments on a self-sustained volume discharge (SSVD) in laser-heated SF 6 -based mixtures wherein a fundamentally new self-organization phenomenon (formation of filamentary current structures) in the SSVD plasma has been discovered [1]. Another relevant aspect is concerned with the mechanism of growing conducting channels in SF 6 -based mixtures at room temperature. It is specifically of great interest for understanding the physical processes in working media of electric discharge non-chain chemical HF lasers.…”