26pC08
大気圧ミリ波放電の伸展モデルとその数値解析
Modeling of Millimeter-Wave Discharge Extension under Atmospheric Pressure and Its
Numerical Analysis
中村 友祐、福成 雅史、山口 敏和、小紫 公也、小泉 宏之
Yusuke Nakamura, Fukunari Masafumi, Yamaguchi Toshikazu,
Komurasaki Kimiya, Koizumi Hiroyuki
東京大学
The University of Tokyo
1. Introduction
Nowadays development in high power millimeterwave oscillator Gyrotron enables observations ofseveral new phenomena caused by high power
millimeter-wave. One of these are propagation ofmillimeter-wave discharge. In this phenomenon a seedplasmoid irradiated with subcritical power-density
millimeter-wave extend toward the beam source. This
is attracting interest because propagation velocity is
one order faster than laser discharge, however anapplicable physical model describing the relation
between the discharge phenomenon and wave
frequency/ power density is yet to be discovered.
In previous experiments, the plasma which is divided
into small pieces during its extension is observed (Fig.
1 (a)). The pitch of each plasma pieces is around 0.8
while  is the wavelength of incident beam. Thissuggests that some beam energy concentration occur at
the plasma front. The purpose of this study is toreproduce this plasmoids by computation and to reveal
the cause and effect of this phenomenon.
Plasmamotion
Flat
top
beam

5λ

5λ(a)

2

Pitch size [mm]

Flat
top
beam

Plasmamotionsimilar to well-known ones, but parameters for lower
intensity case are refitted. Electro-magnetic field arecalculated with S24 FDTD method which is 2nd orderaccurate in time and 4th order accurate in space3).
Plasma is calculated with fluid model equation.
The calculation is conducted on a plane containing
electric field direction and beam direction whose sizeis 7×7.5. The computational domain is reproduced
using one of the long side boundaries as the center line.
One of the short side boundaries generates uniform
electro-magnetic waves and the other short side
together with the adjacent long side act as theabsorbing boundaries. Initial conditions are set withseed plasmoids on the opposite area of the beam
generating boundary. Many computations areconducted changing seed-plasma distribution,
propagation velocity and frequency.
3. Results and Conclusion
One of the results is shown in Fig. 1 (b). Regardlessof seed plasma distribution, extension velocity andbeam frequency, extending plasma are separated intopieces and finally lined up with the same pitch size isof approximately 0.8. This result showed good
agreement with the experimental results. Fig. 2 shows
the relationship between the pitch size and thepropagation velocity obtained from the computation at
170 GHz. This shows that the pitch size is independentof propagation velocity.

(b)Fig. 1 (a) A flaming camera image of millimeter-wave discharge.
170 GHz, ca. 1 GW/m2, exposure time 100 ns, discharge
propagation velocity ca. 1 km/s. (b) Computed plasma density
contours. Maximum density ca. 2 × 1021 /m3, 170 GHz,
discharge propagation velocity ca. 4 km/s.

2. Computational Method
In spite of many researches which computepropagation of millimeter wave discharge1), 2), there is
no research which reproduce the situation shown in Fig.
1 (a). This is because the computational method used isapplicable to the case where the beam intensity is very
close to that of the discharge threshold and moreover
the experimental value of beam intensity is much lower
than the threshold. In this study, the method used is

1.6
1.2

0.8
0.4
00102030
Propagation velocity [km/s]
Fig. 2 Plasmoids pitch size vs. Propagation velocity. λ=1.76 mm.
One plot is ca. 4 km/s and the other is ca. 27 km/s.

References
1)
2)
3)

J. P. Boeuf et al., Phys. Rev. Lett., 104(2010), 015002
Q. Zhou et al., Appl. Phys. Lett., 98(2011), 161504
M. F. Hadi et al., IEEE Trans. Antennas Propagat. 45(1997),254-264