28E15P
RELAXにおける軟X線イメージング計測による2次元電子温度分布測定
2-D Electron Temperature DiagnosticUsing Soft-X Ray Imaging Technique in RELAX
西村香苗, 中木聖也, 出口和明, 田中裕之, 三瓶明希夫, 比村治彦, 政宗貞夫
Kanae Nishimura, Seiya Nakaki, Kazuaki Deguchi,Hiroyuki, Tanaka Hiroyuki,
Akio Sanpei, Haruhiko Himura, and Sadao Masamune
京都工芸繊維大学
Kyoto Institute of Technology
The reversed field pinch (RFP) is one of the magnetic
confinement systems for high beta plasmas. The RFP
was characterized by MHD relaxation to form andsustain the configuration, and associated MHD
activities played important roles in stochastization of
the magnetic field lines which is unfavorable toplasma confinement. As a result of recent efforts, two
methods have been established to avoid stochastic
field lines (magnetic chaos) in the core of the RFP.
One is the current profile control to stabilize all theotherwise unstable m=1 tearing modes to suppress
magnetic island overlap, and the other is to allow onlya single mode to grow in otherwise stochastic coreregion. The latter configuration is referred to as theQuasi-Single Helicity )QSH) state, and in the extreme
case, the island expels the original magnetic axis toform the new helical magnetic axis, which is referred
to as the Single Helical Axis (SHAx) state. Thesestates dominated by a single mode is self-organized inanaxisymmetricboundarygeometry.
In
low-aspect-ratio RFP configuration, it is shown that
we have more space in the core without major m=1
resonant surfaces than in medium- or high- aspectratio, with adequate choice of the safety factor on axis.
RELAX is a low-A RFP machine with circular crosssection. The major (minor) radius is 0.5m (0.25) with
A=R/a=2. We have observed easy transition to either
QSH or SHAx states in RELAX particularly inshallow-reversal regions, where the edge toroidal field
is weakly reversed (or almost no toroidal field) its
direction with respect to that inside the plasma. We
have developed soft-X ray (SXR) imaging diagnostics
particularly for the study of mechanism of transition
to QSH and helical structures in the core after
attaining to the QSH. In the present study, we reportinitial results from two-dimensional electron
temperature profile measurement along with thedevelopment of the system.
We have used two pin holes to form two SXR
pin-hole images on a single MCP plate, and twothin-foil films with different absorption coefficient areused at each pin hole. In the present system, we haveused a 1.0-μm thick aluminum filter and a 0.5-μm

thick Polyimide filter. Combination of the SXR
imaging and the double-filter techniques provide thechord-averaged 2-D electron temperature profile. The
experimental arrangement is shown in Fig.1. We haveinstalled the system to observe the vertical image ofthe plasma.
Figure 2 shows an example of the experimental
images from two different kind of plasmas. The
right-hand side figure was taken in a discharge withmany m=1 magnetic fluctuations excited (so called
multiple helicity (MH) state), where we cannot
identify characteristic structure. The electron
temperature is the highest in the center, decreasing
gradually towards the edge. On the other hand, theleft-hand side image, observed in a QSH discharge,
shows some structure. Detailed analysis of thisstructure has suggested that high temperature region isconcentrated in the core region, and the structure isdeformed. The initial results are consistent with ourprevious SXR diagnostics. Comparison of the SXR
electron temperature with the Thomson temperature
will also be discussed.
We have developed a SXR imaging system for 2-D
electron temperature measurement, and initial results
are reported. The results agrees well with the previous
SXR imaging diagnostics and other electron
temperature measurements in RELAX.

Fig.1

Arrangement of SXR pin-hole camera andhigh speed camera.

Fig.2

2-D electron temperature image: Right is MH
state. Left is QSH state.