A schematic drawing of the experimental setup is shown
in Fig. 1. The common ground of the main structure of PJ-1 and PJ-2 is that they both consist of two concentric electrodes.
The outer electrode is coupled to an audio-frequency power
supply at variable frequency range from 5 kHz to 20 kHz, and
the inner one is grounded with the discharge gas such as argon
or helium passing it through. The key differences between
these two structure types of plasma jet source are the surfaces
of inner electrode and the shapes of outer electrode. For PJ-1,
the grounded inner electrode is a stainless steel pipe closing
on one end and only opening the other end as gas inlet. There
are several holes used as spray nozzle of gas along a circular
helix on the wall of the inner electrode having 8 mm inner
diameter. The outer electrode of PJ-1 is a metal pipe (inner
diameter 15 mm) covered with both sides insulating coat. For
PJ-2