Figure 1 shows a schematic diagram of the experimental
setup used in this study. A tungsten (W) needle electrode,
0.2mm in diameter, is placed in a glass or ceramic tube,
0.4 – 2.0mm in inner diameter (i.d.). A copper (Cu) plate
electrode with a hole (0.6– 2.2mm in diameter) is placed
beneath the nozzle of the tube. An arc discharge is formed
between the electrodes by applying high-voltage dc power
(Matsusada Precision HAR-6 kV50 mA) through a blocking
resistor, R1. By introducing dry air (N2 þ O2) into the tube,
the plasma and gas are spewed out from the nozzle,
generating a plasma torch at atmospheric pressure. The
applied voltage between the electrodes and the arc current
are measured using a voltage probe (Tektronix P6015A)
and a noninductive resistor (R2, 100), respectively. An
oscilloscope (Tektronix DPO4104, 1 GHz) records the
signals from the measuring devices.