Direct measurements of ions mobilities in gaseous helium for validation of their indirect determinations using corona discharges

An experiment carried out with the same He gas shows the distinction between negative ions mobility data obtained both by time-of-flight and corona current methods. The negative charge carriers mobility magnitude extracted from corona discharge experiments could be either very close to the one obtained by time of flight (for the more dirty gas) or greater by several fold than that from the time-of-flight experiments (for the smallest impurity level). This is interpreted as the difference in production and capture of electrons in the 2 methods: for plane-plane geometry the number of electrons produced by the photo-cathode is low and they are immediately captured by impurities near the electrodes. In corona the electrons are multiplied by avalanche process before being captured, the lifetime of electrons depending strongly on impurity level. The mixture of both electrons and negative ions in current transport is revealed in the obtained indirect mobility which conversely could be used to appreciate the purity level