Geoscience Reference
In-Depth Information
Fig. 6.16
Typical pictures of two plasma formations created by capacity-coupled high-frequency
discharge in high-speed swirl flow. Charged longitudinal plasmoid with
blue halo
at
P
st
300 Torr,
V
ax
90 m/s.
1
needle HF electrode;
2
plasma kernel;
3
plasma halo;
4
nozzle
The high-frequency plasma generator had the following parameters:
Pulsed high-frequency power
P
HF
D
200 W
Frequency
F
HF
D
1.2 MHz; 13.6 MHz
Operation regime
Continuous, pulse repetitive
Modulation frequency
F
M
< 20 kHz
50-500 s
The typical picture of the free longitudinal vortex plasmoid created by the
capacity-coupled high-frequency discharge in swirl high-speed airflow is shown in
Fig.
6.16
. One can see that this plasmoid consists of the bright plasma kernel and
the blue plasma halo around it. The typical plasma halo diameter
D
h
exceeds the
typical kernel diameter
D
k
by the factor
K
D
D
k
/
D
h
D
2-10. The value
K
depends
on the initial pressure
P
st
and the initial flow velocity
V
f
.
The electrical potential of this plasma halo was measured by the electric high-
voltage probe Tektronix P6018. The typical value of this electrical potential was
about ®
C
(1-4) kV (Fig.
6.17
). This value depends on the high-frequency power
input
P
HF
, the pulse repetition frequency
F
i
, and the airflow velocity
V
f
V
ax
V
t
.
The maximal value of ®
C
4 kV is measured at the maximal high-frequency power
input
P
HF
800 W, the maximal pulse repetition frequency
F
M
10 kHz, and the
maximal airflow velocity
V
ax
V
140 m/s.
It is not clear why the measured halo potential is positive. From a conventional
point of view, fast particles, that is, electrons, have to move from the kernel, so the
potential of the halo has to be negative. However, it is not so. We plan to investigate
this effect in future experiments.
Pulse duration
T
i
D
