Fast time resolved measurements of the EEDF in a helicon wave plasma

Summary form only given. One of the biggest problems encountered when using electric probe diagnostics in RF plasmas is the high frequency fluctuation of the plasma potential. This fluctuation makes the probe sheath voltage drop an unknown quantity and in general makes the analysis of the probe characteristic impossible. In addition, the entire integrated volume of the fluctuating sheath will add displacement current to the probe signal causing further distortion. In the past, we have used high impedance mini-inductors built into the probe tip to reduce the RF sheath voltage drop. This scheme is easy to implement but has two major drawbacks: (1) the effectiveness of this filtering method requites a low ratio of sheath to circuit impedance, which limits the range of plasma parameters over which the probe is reliable, and (2) this method cannot measure fast variations of the EEDF, which have been inferred from fast fluctuations of the plasma optical emission. For these reasons, we have used a gridded energy analyzer with a 250-MHz digitizing oscilloscope to measure the instantaneous electron current over an entire RF period. Each signal can be recorded, and by varying the DC grid bias the current-voltage characteristic can be constructed at any phase of the RF cycle. To test this system, an electron gun has been constructed to generate known RF-modulated EEDFs. Special considerations in the analyzer design, measured EEDFs under various discharge conditions, and comparison with results from RF-compensated Langmuir probes will be discussed.