Positively charged ions were produced in supercritical helium at temperatures from 6 to\ud10 K and up to 2 MPa using a corona discharge. Their mobility was measured via currentvoltage\udcurves and the hydrodynamic radius was derived using Stokes law. An initial increase\udand subsequent decrease of hydrodynamic radius was observed and interpreted in terms of\udgrowth, compression and solidification of ion clusters. The mobility was modelled using a\udvan der Waals-type thermodynamic state equation for the ion-in-helium mixed system and a\udtemperature-dependent Millikan-Cunningham factor, describing experimental data both in the\udKnudsen and the Stokes flow region. Regions of maximum hydrodynamic radius and large\ud1\udcompressibility were interpreted as boiling points. These points were modelled over a large\udrange of pressures and found to match the Frenkel line of pure helium up to 0.7 MPa, reflecting\udsimilarity of density fluctuations in pure supercritical helium and gas-liquid phase transitions\udof ionic helium clusters.
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