22F12
Equilibrium Characteristics in Toroidal ECR Plasmas in the LATE Device
LATE装置におけるトロイダルECRプラズマの平衡特性
Kuroda Kengoh, Katsuma Jun ,Kanemitsu Toshiyuki, Hayashi Ryota, Fukunaga Tadahiko,Mizogami Hikaru, Omi Shota, Noguchi Yuto, Watanabe Fumitake, Uchida Masaki, Tanaka Hitoshi
and Maekawa Takashi
黒田賢剛, 勝間淳, 金光俊幸, 林良太, 福永忠彦, 溝上晃, 逢見翔太, 野口悠人, 渡辺文武,打田正樹, 田中仁, 前川孝
Graduate School of Energy Science, Kyoto University
Kitashirakawaoiwaketyou, Sakyo-ku, kyoto-city 606-8502, Japan京都大学 〒606-8502 京都市左京区北白川追分町
In toroidal ECR plasmas without vertical field, the charge separation current is caused by the vertical driftof the electrons and the ions due to the field gradient and curvature. The present experiments show that theradial distributions of the charge separation currents flowing to the top and bottom electrodes and in themain plasma are all the same, namely, the currents flow only in the vertical direction. And we partly verify
the model for a ion-flow toward the top surface that was proposed in the previous paper[1].

1. Introduction
The electron cyclotron resonance (ECR)
pre-ionization is a standard method of initiation oftokamaks discharge. There is considerable interest
in ECR plasmas and investigations of the behavior
of it are very important.
In ECR toroidal plasmas immersed in a simple
axisymmetric toroidal magnetic field theelectrons and ions drift vertically due to the field
gradient and curvature in the opposite direction
each other. The vertical drift causes the charge
separation current in the vertical direction. We
found that the current circulated via theconducting vessel in previous paper[1]. This
indicated that the equal currents of electrons andions flowed to the top and bottom surface
respectively. However the ions drift much slower
than electrons because the ion temperature ismuch lower than the electrons temperature in theECR plasmas. We proposed the model for a
ion-flow toward the top surface to resolve thediscrepancy.
In this paper we address whether the ion-flow
model is the case or not.
2.Experiment Description
The previous paper showed that the total charge
separation current flowing to the top and bottom
surfaces were the same and the current circulated
via the conducting vessel. We proposed the model
for a ion-flow toward the top surface. The modelis that the ions drift toward the top surface by theE×B drift and the vertical drift of the ions areaccelerated in the vicinity of the top surface dueto the steep space potential gradient.
We investigate the radial distributions of thecharge separation current and examine the model
for a ion-flow in the present experiments.
The experimental apparatus is shown in Fig.1.

Fig.1. Experimental apparatus

The vacuum vessel of the LATE device is acylinder with an inner diameter of 1000mm and aheight of 1000mm. We produce the hydrogenplasmas using microwaves at 2.45GHz and 0.5～
2.0kW. The toroidal field is 480G(R=25cm) and novertical field is applied.

Firstly, we investigate the radial distributions of
the charge separation current with three electrodes,
the top divided panels, the top divided limiters andthe bottom divided limiters. The panels arehorizontal and the limiters are vertical, which bothare divided into twelve at intervals of 3cm in theradial. They are set at the top and bottom of thevessel but the bottom panel is omitted because it
detected no current in the previous experiment.
Figuers.2(c) and 2(e) show the current density
flowing to the top and bottom electrodes,respectively. We suppose that the panel detects thecurrents flowing to the area of the electrode plate
and the limiter detects the current flowing to theannular area of the width of the plate. We alsomeasure the plasma profiles of the electron
temperature, density and space potential in the same
plasmas with the Langmuir probe. Figuer.2(d) show
the current density obtained by using the profiles ofthe electron pressure in the main plasmas.
As shown in Figs2.(d) and 2(e), the radial
distributions of the current density in the main
plasma (Z=240,-8,135,240,390mm) are the same, andthey are also coincident with the current density
flowing to bottom limiter.
The result is consistent with the picture that thecharge separation current is caused by the verticaldrift due to the field gradient and curvature. That is,the charge particles drift due to the field gradientand curvature and due to the E×B drift, and only
the former causes a current. The current flows onlyin the vertical direction in the main plasma and tothe bottom limiters. Then its radial distribution isindependent of Z-position with no horizontal flow.
In this experiment the picture is confirmed.
As shown in Figs2.(c), the currents flowing to thetop limiters is quite small just as the previousexperiments. The current density to the top panels
are larger than the current density to the bottomlimiters, which is suggested that the panels detect
the current flowing to a larger area than the plate.
The previous paper showed that the total current
flowing to the top and bottom surfaces were thesame. Figuer2.(c) shows that the radial distributionof the current density of ions flowing to the top
electrodes are also the same.
Secondly, in order to examine the energy of theions in the vicinity of the top electrodes, wemeasure V-I characteristics of the electrodes(panels). Figuer.2(a) shows the radial distribution of
the floating potential of the V-I characteristics.
Figuer.2(b) shows the profile of the space potential
in the vicinity of the electrodes. Both radial
distributions suggest that the ions energy increases
at R=14cm～20cm in the vicinity of the top surface

due to the steep space potential gradient.

Fig.2. Comparison radial distributions of (a)-(e).
(a): Floating potential measured with the top panels.
(b): Profile of the space potential in the vicinity of thetop panels. (c): Current flowing to the top divided panels.
(d): Current obtained by the profiles of the electronpressure in main plasma(Z=240,-8,135,240,390mm).
(e): Current flowing to the bottom divided limiters

3. Summary
The present experiments show that in the toroidal
ECR plasmas without a vertical field the charge
separation current flows only in the verticaldirection throughout the plasma from bottom to top.
The results also suggest that the ions energy
increases at R=14cm～20cm in the vicinity of thetop surface, which is consistent with the model for aion-flow toward the top surface.
References[1] S. Nishi, T. Sakabe, M.Uchida, H. Tanaka and T.
Maekawa: Plasma Phys. Control. Fusion 52 (2010)
125004