Critical Velocities of Ions in Liquid Helium II Detected by Heat Flush

The discontinuity in the drift velocity of ions in liquid helium, previously detected by the time-of-flight method, is confirmed by the heat-flush method in a wide channel. The critical velocity is found to be $〈v_c〉=4.96\mathrm{}\pm 0.35$ m/sec independent of beam geometry and of ionic density. The detailed analysis of the beam displacement in the two-fluid counterflow process allows one to conclude that the new dissipation suffered by the beam at $〈v_c〉$ is essentially due to its interaction with the normal fluid.