Time-resolved RF-driven probe measurements of the plasma parameters in a pulsed RF argon discharge

This paper describes a three-harmonic active probe compensation technique, developed for obtaining time-resolved plasma parameter measurements in pulsed, low-pressure, RF discharges. Using a sample-and-hold circuit, the amplitude and phase of the compensation signals fed to the probe tip are tuned so that at each time point of the measurement, a maximum in the probe floating potential is achieved. The technique allows the accurate determination of the plasma parameters in transient phases of the discharge pulse (when the RF exciting voltage ramps up between 'off' and 'on' phases), not possible when the feedback signals are tuned for continuous wave (CW) operation. Comparison between this point-wise technique and CW tuning of the compensation signals has been carried out in a 1.3 Pa pressure argon discharge pulsed at frequencies between 200 and 2000 Hz. With tuning made in CW operation, the floating potentials are underestimated by about 10 V at the beginning of a pulse, while the electron temperature and plasma density are overestimated by a factor of 2, compared to the point-wise measurements. In the quiescent phases of the pulse (similar to100 mus after initiation), however, the two methods agree well.