Mobility of positively charged ions in supercritical helium

The mobility of positively charged ions in supercritical helium at temperatures from 6 to 11 K and up to 20 bar was measured using corona discharges. Regions of increased and decreased hydrodynamic radius are interpreted in terms of growth, compression and solidification of ion clusters. Modelling of the mobility using a thermodynamic state equation for the ion-helium mixed system and a temperature dependent Cunningham correction provides best match with experimental data for a van der Waals helium-ion interaction term. Regions of large hydrodynamic radius and large compressibility are attributed to liquid-like behaviour.