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Conductivity and ionization mechanisms in liquid helium have been investigated by using high field conduction phenomena occurring near a sharp needle electrode. When the voltage V applied between the needle and the plane is greater than some threshold voltage [V.sub.s], a corona-like discharge regime was observed. Charge carriers are generated in the high field region close to the point where an intense light emission is detected. Above [V.sub.s] the mean current is space charge limited and the mobility of charge carriers has been deduced from analysis of current-voltage characteristics. Some parameters of the corona-like region have been deduced from a spectral analysis of the light emitted during the discharge. Index Terms--liquid helium, corona, space charge, charge carrier mobility, optical spectroscopy, spectrum analysis.

This paper deals with the breakdown voltage characteristics of saturated liquid helium in the presence of a needle-shaped or spherical metallic particle to obtain insulation design data for pool-cooled, low-temperature superconducting coils and to find the predominant factor affecting the breakdown voltage of liquid helium at a given state. The results show the following. (1) The generation of bubbles at the instant of collision of a particle with an electrode is caused mainly by the kinetic energy released from the moving particle to the liquid helium, while the electrostatic energy accompanying the microdischarge between the particle and the electrode has a lesser effect on it. (2) The lowest breakdown voltage at different particle conditions appears in the case of a free needle particle. The low breakdown voltage is caused by the bubble triggered by the particle collision, the high electric field at the tip of the needle particle, and the rich initial electrons supplied by the microdischarge. Index Terms--Breakdown, microdischarge, bubble, liquid helium, particle, electrical insulation, superconducting coil

New data of the electronic properties of liquid helium and liquid neon are discussed with respect to their relevance for the initiation of electric breakdown. It is concluded that under ideal conditions the cathode plays a decisive role in the initiation of the breakdown process. It is speculated that a Townsend type breakdown mechanism is absent in the liquid phase of these elements. Index Terms--Liquid helium, breakdown, liquid neon, delocalized electrons, heat of vaporization, Townsend process, electron multiplication, radiation.

The paper deals with the bubble formation process in the presence of a free conducting particle in liquid helium to understand the electrical insulation environment in pool cooled superconducting devices under particle contaminated conditions. Experiments were conducted with dc stressed parallel plane electrodes containing a free spherical metallic particle of 1mm radius in saturated normal and saturated superfluid helium. Experimental results show that a single bubble is always generated at the moment of particle collision with the electrode and the bubble behavior depends strongly on the state of the liquid helium. The bubble expanding and shrinking processes were analyzed on the basis of energy balance among released electrostatic and kinetic energies of the particle at the moment of particle collision with the electrode, kinetic energy of liquid driven by bubble growth, work done on environmental pressure and internal energy of the bubble gas. The analysis shows a fairly good agreement with experiments.

We have been studying quench-induced 'dynamic' breakdown characteristics of [LHe.sup.*] and have already found that the electrical insulation performance of LHe was degraded drastically by the thermal bubble disturbance due to quench of superconductors. In this paper, in order to improve the insulation performance of LHe under quench conditions, we measured dynamic breakdown and prebreakdown characteristics of pressurized LHe. Experimental results revealed that breakdown voltage of LHe under quench conditions at 0.2 MPa was greatly improved, reaching 2 to 4x compared with that at atmospheric pressure under both uniform and non-uniform electric fields. Moreover, for practical and efficient insulation design of superconducting power apparatus, we investigated the dynamic breakdown voltage of LHe as functions of pressure and gap length.

The high dielectric dc strength of liquid helium can be explained by the effect of an energetic barrier against germinal vapor bubble elongation and runaway up to vapor discharge. The barrier normally rises in front of any cathode asperity where a germinal bubble may be produced from a collapsing vacuous cavity. After reaching a peak, the barrier changes into a decreasing tail, caused by significant vapor ionization. This so-called 'antibubble barrier' can be overcome at a sufficiently high field strength. For temperatures above the normal boiling point, the barrier peak must be lower than the threshold limit. At lower temperatures, the germinal bubbles may extend already into the falling tail, which favors runaway considerably. Nominal breakdown field strengths have been calculated assuming various surface roughness condition in the temperature range 2.5 to 4.5 K. A simple engineering formula for the roughness effect estimation is given.

Information about breakdown time lags under pulsed voltage stress is prerequisite to the insulation design of superconducting devices. Experiments have been carried out using a pulse generator which provides a step voltage with 200 ns rise time and a long decay time constant of more than 1 ms. The present experiments may bridge the information gap between time lag measurements in the ns range using extremely high field strengths, and existing data found with usual lightning impulse voltage. The test device is described in the paper, and some typical results are discussed in detail.