H+and D+ions in He: observations of a runaway mobility

The mobilities of H + and D + ions in He have been measured over a range of ratios of electric field strength E to helium number density N with a selected-ion flow-drift tube. It is found that, at E/N greater than about 40Td (1 Td=10 -17 V cm 2 ), the apparent mobilities exhibit an unusual increase with increasing E/N that is attributed to the breakdown of a steady-state drift velocity, i.e. a runaway mobility. Furthermore, the widths of the arrival-time histograms indicate that the same effect is occurring for the diffusion coefficients. These observations support the predictions of this effect by Lin et al. (1979) from their kinetic-theory calculations using an ab initio potential for the H + -He interaction.