Ionization and charge transport phenomena in liquid helium induced by corona discharge

Corona discharge around a point electrode has been used to study ionization and charge transport phenomena in liquid helium (LHe) in a wide range of hydrostatic pressure (0.1-10 MPa). All experiments have been done by using 0.45 and 2.5 mu m point electrode radii for negative and positive polarities of the electrode. Corona threshold voltages have been found to be pressure independent which indicates that corona initiation takes place in the liquid phase of He. Extinction of the corona discharge occurred for a voltage lower than that for corona initiation. The space-charge limited corona current depended on applied pressure both negative and positive point electrode polarities. This involves considering the charge carriers in LHe as "micro-bubbles" for negative carriers and "snowballs" for positive ones. The parameters of the corona plasma generated close to the negative point electrode have been analyzed using a quasi-one-dimensional numerical model for the simulation of the ionization and the drift regions. An evaluation of the Townsend ionization coefficients is proposed for liquid He. The model provides the space and time variations of the electric field, the electron and positive ion densities in the vicinity of the point cathode. The formation of a steady ionization zone occurs in some nanoseconds due to the multiplication of the cathode-emitted electrons. The electric field strength in the ionization zone of the burned corona is less than the space charge free field before the corona onset. This explains the lower observed corona extinction voltages compared with the initiation ones. (c) 2008 Elsevier B.V. All rights reserved.