25aE12P
球状RFP装置RELAXにおけるMHDダイナミクス研究のための軟X線CT手法の開発
Development of Soft-X Ray Computed Tomography Diagnostic Systemfor MHD Dynamics Study in RELAX RFP
青木陽祐，三瓶明希夫，牧澤遼，長野友幸，樋口真行，軸原健太，坪井涼介，中ノ坊俊，比村治彦，政宗貞男
Y. AOKI, A. SANPEI, H. MAKIZAWA, H. HIMURA, S. MASAMUNE, et al.
京都工繊大・電子システム工学専攻
Department of Electronics, Kyoto Institute of Technology
The reversed-field pinch (RFP) is one of themagnetic confinement systems for high beta fusion
plasmas. One of the notable topics is quasi-single
helicity (QSH) state we called, spontaneous transitionof RFP configuration from axisymmetric tonon-axisymmetric helical structure. In this QSH state,
improved confinement was realized in the helical core,either inside a large magnetic island or helically
deformed nested flux surfaces having helical axis,
with increased electron temperature and/or higher
density [1].
RELAX is an RFP machine with major radius R =0.5 m and the minor radius a = 0.25 m. So thismachine has low aspect ratio A = 2, whose research
objectives include geometrical optimization of theRFP configuration [2]. Generally speaking, the q value
on axis increases as the aspect ratio is lowered, while
the edge value does not change so much because thefield reversal surface (q = 0) is close to the edge.
Therefore, if we could avoid the resonance near thecore, relatively large space is available in the coreregion without major resonance; the single island of acore resonant tearing mode can grow without
interacting neighboring islands associated with thetearing modes. This picture has been demonstrated
experimentally in RELAX in which transition to QSH
occurs at lower current density and lower Lundquist
number than in other machines such as RFX-mod andMST [3].
When the plasma is in equilibrium, the Soft-X
ray (SXR) emissivity contours coincide with magnetic
flux surfaces because both the electron temperatureand density can be regarded as surface quantities.
Therefore, the SXR computed tomography (CT) is a
powerful tool to identify the thermal or magnetic
structure inside high temperature plasmas. We aredeveloping a new SXR CT system consisting of two
20-channel photodiode arrays. The cross-sectionalview of the diagnostic system is shown in Fig.1 where
two arrays are set at the top and bottom at a single

toroidal location. Our interest includes time evolutionof the SXR emissivity contours at transition to andback-transition from the QSH in RELAX; the process
was studied by magnetic diagnostics and SXR camera,
the latter providing SXR images with 10-μs time
resolution. We expect to obtain time evolution of themagnetic flux surfaces with 1-μs time resolution(typical sampling time), which would be useful tostudy the dynamic processes of transition or
self-organization associated with QSH.
For this diagnostic system, we have verified ourCT algorithm based on the Fourier-Bessel
expansion using test SXR emissivity profiles. The
reconstructed SXR emissivity contours showed
good agreement with the test data. We will discuss
results from the initial experiments.

Fig.1 Poloidal cross-section ofRELAX with lines of sightcovered by two photodiodes.
[1] D. Escande, “What is a reversed field pinch?”2013 <hal-00909102>.
[2] S. Masamune et al., J. Phys. Soc. Jpn. 76, 123501(2007).
[3] K. Oki et al., PFR 7, 1402028 (2012); Fusion Sci.
Technol. 63, 1T, 386 (2013).