Field emission and field ionization in liquid 4 he

Abstract

Spacecharge limited field emission and field ionization currents in liquid 4 He have been investigated for emitter potentials V s , temperatures T , and pressures p in the ranges 0 < |V s | < 3500V, 0.3 < T < 5 K, 1 < p < 25 × 105 Pa, respectively. In each case, for constant p and V s , the current i passes through a maximum at a temperature V max near 1.5 K. For T > T max it is possible to account for the characteristics on the basis of a simple theory which does not require a detailed knowledge of the current generating mechanisms, and which enables values of the ionic mobilities μ ± to be deduced from the data. The rapid decrease in i below T max is ascribed to a process in which ions become trapped on quantized vortices and subsequently escape again. A model is developed to enable values of the ionic escape probabilities to be derived from the experimental results. It was found that the current becomes temperature independent below 0.4 K. The low temperature i(V8) characteristics suggest that an effective mobility can be defined to describe ionic motion through a self generated vortex tangle. The negative ion current for T < 0.9 K showed a rapid increase with pressure for p > 10 6 Pa. This behaviour is ascribed to the presence of an increasing proportion of free ions travelling at the Landau velocity, owing to a decrease with p of the vortex nucleation probability v ; and a model is developed to enable values of v to be deduced from the experimental data. The conduction mechanism below 0.7 K is discussed, but is not understood in detail. It is inferred that Vinen’s F 2 parameter becomes temperature independent below 0.4 K.