RF pulse interaction with low density plasma diagnosed by gated Langmuir probe

Several experiments on interactions of centimetric microwaves with low density plasmas, in particular at low temperature and weakly ionized, have been performed in order to investigate the optimal conditions for the resonant absorption at various levels for the incident microwave power. Many of these experiments were made in equilibrium afterglow plasmas. Nevertheless experimental results concerning the temporal and spatial evolution of the plasma electron temperature, during and immediately after the interaction, do not seem available. In this paper some experimental results on this temperature evolution, obtained by using a biased Langmuir probe, are presented and commented. The probe current is gated by a boxcar amplifier in the presence of a repetitive pulsed microwave perturbation and, for a fixed delay of the gate aperture, the probe bias is slowly swept whereas the gated current is averaged over the many repeated pulses present during the time interval for scanning the whole Langmuir characteristic. For a scanning time 106 longer than the pulse duration time, an instantaneous I/V Langmuir characteristic may be plotted. Moreover by measuring the time-varying reflexion coefficient, during the pulse duration time, when the plasma is confined within a section of a high-Q resonator, the efficiency of the electron heating may be evaluated. However in order to control the accuracy of this probe diagnostic, our experiment was performed with a low density diffused plasma, transparent to the microwave, so avoiding the resonance absorption and the consequent instantaneous generation of fast resonant electrons which would make the diagnostic results more difficult to interpret. (author) 5 refs., 8 figs