Underwater Electrical Discharges: Temperature, Density and Basic Instability Features with Different Anode Materials

Abstract

Plasma temperature, density and arc dynamics have been investigated in underwater arc discharges under different anodic environments. Distinct differences in the behaviour with different anode material and rich variety of interesting dynamics have been directly revealed through fast photography and emission spectroscopic analysis. Four different anode materials of high technological relevance, namely SS304, copper, zircaloy, and P91 [ASTM387-Gr91] have been used as anode material. Instabilities in the plasma channel with alternate expansion and contraction zones, highly prominent magnetic pumping near the anode arc root, formation of strong anode jet, dynamic branching of arcs, formation of distinct shock boundaries, and coexistence of multiple anode and cathode arc roots are some of the interesting features observed. Observation of Hα line and absence of Hβ line in the emission spectra is an interesting observation. Emission lines, characteristic to chosen anode material, have been used in determining the axial plasma temperature using a modified Boltzmann plot technique, suitable for spatially unstable arc discharges and pulsating arcs. Electron density inside the plasma has been determined from the width of Stark broadened Lorentzian profile of Hα line. Typical temperatures for discharges with P91, Zr, SS304 and Cu anodes are estimated around 8159 K, 14525 K, 6151 K, and 10534 K respectively under similar operating conditions. Respective electron densities per m3 in the discharge zone are estimated to be 2.97 × 1023, 3.76 × 1023, 0.41 × 1023 and 6.21 × 1023. Estimated plasma coupling parameter finds the discharge in the regime of weakly non-ideal plasma.