Electrical Breakdown Triggered by a Free Conducting Spherical Particle in Saturated Liquid He I and He II under Uniform dc Field

This paper is concerned with the pre-breakdown phenomena and the breakdown voltage characteristics, in the presence of a free moving conducting spherical particle, of saturated normal liquid helium (He I) and saturated superfluid liquid helium (He II) under uniform dc field. Experiments show that the particle lifts off around the theoretical value of the lift-off electric field, and the particle oscillates between the electrodes at higher applied voltages. In that case, the microdischarge appears just before the charged particle collides with the oppositely charged electrode, and the bubble is generated at the moment of every collision of the particle with the electrode. It is confirmed theoretically as well as experimentally that the maximum bubble radius in He II is nearly proportional to the E_in^1/3, which is the released energy from the particle for the bubble generation. The insulation environment before the electrical breakdown suddenly changes at the λ-point since the bubble behavior in He I and He II greatly differs. In the characteristics of the breakdown voltage vs. liquid pressure, a clear discontinuity appears at the λ-point. Furthermore, the breakdown voltage in the parallel plane gap contaminated by a particle is lower than that in the rod-plane gap without a particle. It is found that the breakdown voltage characteristics are closely related to the trigger effect of the microdischarge and the bubble generation.