27D13P
テスト粒子法によるトロイダル磁場中での高周波グロー放電の挙動解析
Test particle simulation for high frequency glow discharge in magnetic field
朝比	  祐一、筒井広明、飯尾俊二、嶋田隆一

Yuuichi ASAHI, Hiroaki Tsutsui, Shunji Tsuji-Iio and Ryuichi Shimada
東京工業大学 〒152-8850	  東京都目黒区大岡山2-12-1-N1-11
Tokyo Institute of Technology 2-12-1-N1-11, Ookayama, Meguro-ku
In future fusion devices with superconducting coils,
it is requried to perform wall conditioning under theinfluence of strong magnetic fields. High frequency glow
discharge cleaning (HF-GDC) is a candidate which has
been shown to be available with the presence ofmagnetic fields, and possibly be applicable to future
superconducting devices, such as ITER.
This work reports on numerical studies on the
pre-breakdown process of high frequency glow discharge
plasma in strong toroidal magnetic field. In this work, theparticle-induced electric fields are assumed to benegligible since the degree of ionization is low at thebeginning of the discharge. Thus, the test particle method
is used, wherein the particle trajectories are calculatedusing the guiding center equation and electron-neutral
collisions are treated based on Nanbu’s method [1].
According to the experiments in EAST, the highfrequency glow discharge plasma with toroidal magnetic
field becomes more uniform at lower filling pressure [2].
This indicates that the mechanism of plasma uniformity
would be clear at the low-pressure limit, wherein theelectron mean free path is sufficiently long incomparison with the size of the torus. So, firstly, thecollision-less electron trajectories in toroidal magnetic
field are calculated to find out the difference between theconstant voltage and the pulsed voltage discharges.
For the constant voltage case, the electrons show
periodic motion along the magnetic field line. Therefore,
their trajectories in phase space are closed as shown in
Fig. 1.

the magnetic field line for the pulsed voltage case.

Fig. 2 Electron trajectories in phase space(Pulsed voltage case)Secondly, to understand the impact of the pulsed
voltage discharges on ionization process, the test particle
simulation has been carried out. For the constant voltage
case, it turned out that the multiplication factor became
progressively saturated (See Fig. 3). This is because
there are only energy loss processes, such as excitationand ionization, in the constant voltage case. In the pulsed
voltage case, however, the number of electrons continued
to increase during the active periods. As shown in Fig.2,
all electrons can move freely during the off periods andtherefore some of them can get away from the electrodes.
These electrons in turn have larger potential energy at thebeginning of the next pulse. Thus, there is a process for
obtaining energy in the pulsed discharge case.

ONFig. 1 Electron trajectories in phase space(Constant voltage case)In the pulsed voltage case, all electrons follow theopen-straight trajectory during the inactive periods ofpulses as shown in Fig. 2. By contrast with the constantvoltage case, the energies of some electrons can be high,and these high-energy electrons can move freely along

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Fig. 3 Electron multiplication rate
References[1] K. Nanbu, Jpn. J. Appl. Phys., vol. 33, pp.4752 4753 , 1994.
[2] J. Li et al, Wall conditioning towards the utilizationin ITER, J. Nucl. Mater (2010)