Observation of a second critical field for negative ions in He II

Abstract

The drift velocity nu of negative ions has been measured in He II near 1K, under a pressure of 25 bar, for electric fields E up to 17 kV cm-1. Beyond Ec1, the critical field for creation of stable charged vortex rings, a second critical field Ec2 has been observed at which nu passes through a minimum and then starts to increase again. Numerical values of the rate nu at which rings are nucleated and of the rate P at which the ions escape from the rings again when E>or=Ec2 are calculated from the experimental data.