Determination of charge mobility in He gas from current-voltage measurements in point-plane geometry

In order to clarify whether or not is it possible to extract reliable values of positive and negative mobility for ions and even for electrons in gas when using sharp point-plane geometry, we have undertaken a systematic study using He gas. Monitoring the purity level of the same gas filling two different cells (point-plane and plane-parallel), we have compared mobility values deduced from current-voltage curves (mobility K) with time of flight measurements (actual mobility mu) as a function of gas pressure (0.1-15 MPa) and purity. For negative charge carriers (ions or electrons), we have directly measured K- and mu (-) and then compared the results. We observed, as expected, that the measured negative charge carriers mobility was strongly dependent on gas purification. For commercial gas (O-2 of the order of 100 ppm), K- was very close to the data obtained for negative O-2(-) ions (i.e., K- congruent to mu (O-2(-))) but, for better purification level, K- increases quickly and can reach electronic values p, (K- --> mu (e).). For positive ions, in all the range of studied pressure, it was shown that one can extract easily from I(V) measurements, mobility values K+ in accordance with the values of mu (+) of He+ previously given in the literature.