23P088-P
Studies of GAMMA 10 Central ECRH Heating Characteristics
by X-ray Measurement
X線計測によるGAMMA 10 セントラル部ECRH加熱特性の研究
Hideaki Iizumi, Ryutaro Minami, Tsuyoshi Imai,Tsuyoshi Kariya, Tomoharu Numakura, Hitomi Aoki, Hidetaka Nakabayashi et al.
飯泉英昭，南龍太郎，今井 剛，假家 強，沼倉友晴，
青木 瞳，中林英隆，市村 真，中嶋洋輔，吉川正志，GAMMA 10 グループ
Plasma Research Center, University of Tsukuba
1-1-1, Tennodai, Tsukuba, Ibaraki 305-8577, Japan筑波大学プラズマ研究センター 〒305-8577 茨城県つくば市天王台 1-1-1

The new ECRH antenna system is installed and the improvement of c-ECRH is expected. But
in some cases, the plasma collapse is caused by the c-ECRH. The one reason of the plasma
collapse might be the degradation of the plasma stability by non-axisymmetric heating. Theremay be many other causes. The ECRH in the mirror plasma has to be careful to heating
position and heating electron energy range and more for the plasma confinement.
1. Introduction
The tandem mirror plasma confinement device
GAMMA 10 studies the plasma physics utilizing
the simplicity of linear device for the physics
understanding. Figure 1 shows the schematic
view of the GAMMA 10. The main plasma isconfined in the central cell by the mirror
magnetic field and the confining electric potential.
The GAMMA 10 is ordinarily operated in thehigh ion temperature mode [1] for aiming thehigher plasma performance by the Ion Cyclotron
Resonance Heating (ICRH), which produces andsustains the plasma. Since, the electron
temperature is far lower than the ion, the rise ofion temperature is limited by the electron drag. It
is necessary to confine the high temperature ionsfor achieving the nuclear fusion. Furthermore, thehigh temperature ions are needed to obtain highheat flux for the recent GAMMA 10 PWI studies.
It is very important to suppress the electron drag.
So, the Electron Cyclotron Resonance Heating(ECRH) is employed for raising the electron
temperature and suppressing the electron drag inthe central cell [2].

Fig.1. The schematic view of the GAMMA 10

The soft X-rays are radiated from EC heatedelectrons by the bremsstrahlung. The soft X-ray
intensity has the information of electron
temperature and density. The effects of ECRH canbe clarified by measuring the soft X-ray intensity.
In case of some experimental settings, by thecentral cell (c-) ECRH, the plasma collapse iscaused by degrading the plasma comfinement inspite of raising the electron temperature. Forsolving the issue, this study aims to clarify theheating characteristics and optimize the c-ECRH
by the X-ray measurement.
2. Experimental Apparatus
For c-ECRH, The 28GHz wave propagates thecorrugated waveguide as HE 11 mode fromgyrotron. The wave polarization is controlled by
the miter bend polarizers before the EC wave
reaches vacuum vessel. The EC wave is polarized
to 100% X-mode in the section 3. Figure 2 showsside view of the c-ECRH antenna system. The EC
wave which is polarized to X-mode propagates tothe axis of cylindrical plasma at B=1T fromstrong field for fundamental resonance heating.
The soft X-ray measurement is employed forinvestigating the effect of ECRH by the Micro
Channel Plate (MCP) in the central cell. The MCP
is composed of a lot of channels that is thesecondary electron multiplier and installed at theupper side of the central cell. The MCP investigates
the time evolution of the integrated soft X-rayintensity and has radial resolution of about 1.8 cm
because it employs the separated MCP and thecollimator.

c-ECRH in #219359 but the negative side IsxL
increase more than the positive side in #219410
as shown in Fig. 5.

Fig.2. The side view of the c-ECRH antenna system.

3. Experimental Results
The new direct injection antenna system (Fig.
2) is installed in this year because the wave
polarization changed due to the wave passproblem and could not be controlled in theprevious antenna system. In addition, the O-mode
micro wave generates the hot electrons anddegrades the plasma comfinement [3]. The
improvement of c-ECRH by the new antenna
system is expected.
The experimental results where the c-ECRH
injects the two similar plasmas are examined.
Figure 3(a) shows the time evolution of thediamagnetism and the line density. In Fig. 3, thec-ECRH is injected during the shaded region.
By the c-ECRH, the diamagnetism increases in#219359. On the other hand, in #219410, theplasma collapse is caused in spite the similar
plasma. Figure 3(b) shows the time evolution ofLine Intensity of Soft X-ray (IsxL). In #219410,
the IsxL increases immediately after the injection
and then decreases by the plasma collapse. So,
the c-ECRH raises the electron temperature but
the plasma confinement degrades.

Fig.3. The time evolution of (a) Diamagnetism, Line densityand (b) IsxL. The c-ECRH is injected at shaded region.

Figure 4 shows the time evolution of radial
IsxL distribution in the range of arrow interval inFig. 3(b). In #219410, the radial IsxL distribution
is oscillated spatially in contrast to #219359 asshown in Fig. 4. The plasma stability degrades
and then the plasma collapse is caused by thec-ECRH. The one reason of the plasma collapse
might be non-axisymmetric heating because theIsxL profile increases symmetrically by theFig.4. The time evolution of radial IsxL distribution
seems to be oscillated spatially in #219410.

Fig.5. The radial IsxL distribution at immediately
before (149ms) and after (151ms) the c-ECRH.

4. Summary
The improvement of c-ECRH by the newantenna system is expected. In some cases, thediamagnetism increases but in other cases, theplasma collapse is caused by the c-ECRH in spite ofthe injection into similar plasmas. The one reasonof the plasma collapse might be the degradationof the plasma stability by non-axisymmetricheating. There may be many other causes like theplasma confinement degradation by the differenceof heated electron energy range and/orenhancement of the ion transport due to thenon-axisymmetric potential formation. The
ECRH in the mirror plasma has to be careful toheating position and heating electron energy
range and more for the plasma confinement. Itwould be required to analyze data in more detailto see these physics and to optimize the ECRH.
Acknowledgments
The author thanks the member of the GAMMA
10 group of the University of Tsukuba for theircollaboration and discussion. This work is partially
supported by NIFS collaboration research program(NIFS11KUGM050 and NIFS11KUGM053).
References[1] Y. Kiwamoto et al.: Plasma phys. Control. Fusion 39
5A (1997) 381.
[2] T. Imai et al.: Trans Fus. Sci Tech. 51 2T (2007) 208.
[3] H. Iizumi et al.: 27th JSPF Annual Meeting 30P29.