Probe detection of time-varying electron tails

Summary form only given, as follows. Acceleration of electrons by wave-particle interactions has been proposed as an ionization mechanism in helicon wave plasma sources, and a number of authors have shown distribution functions with a fast electron tail. Using a carefully RF-compensated Langmuir probe, we have reported the absence of such fast electrons. We find that the discrepancy lies in the delicate nature of RF compensation and in the time variation of the fast electrons, which appear only in the acceleration phase of the RF cycle. When the probe is made to follow RF fluctuations in the floating potential (by use of an auxiliary electrode), the electron tail cannot be seen on the time-averaged I-V trace because the floating potential itself shifts whenever the tail is present. On the other hand, if the probe is inadequately RF-compensated, the tail is masked by the apparent tail that is seen in the de average in the presence of RF. These effects are shown in I-V curves computed with model distribution functions. A properly designed probe should follow the fluctuations in space potential, not floating potential.