Curling probe measurement of a large-volume pulsed plasma with surface magnetic confinement

Abstract

A curling probe (CP) based on microwave resonance is applied to the measurement of electron density in a pulsed DC glow discharge under surface magnetic confinement (SMC) provided by a number of permanent magnets on a chamber wall. Owing to the SMC effects, a 1 m scale large-volume plasma is generated by a relatively low voltage (~1 kV) at low pressure (~1 Pa) in various gases (Ar, CH4, and C2H2). Temporal variation of the electron density is measured for pulse frequency f = 0.5–25 kHz for various discharge-on times (TON) with a high resolution time (~0.2 µs), using the on-point mode. In general, the electron density starts to increase at time t = 0 after turn-on of the discharge voltage, reaches peak density at t = TON, and then decreases after turn-off. The peak electron density is observed to increase with the pulse frequency f for constant TON owing to the residual plasma. This dependence is successfully formulated using a semi-empirical model. The spatio-temporal evolution of the cathode sheath in the pulsed discharge is revealed by a 1 m long movable CP. The measured thickness of the high-voltage cathode fall in a steady state coincides with the value of the so-called Child–Langmuir sheath.