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=10-17V cm2), 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.