25aE07P
RELAXにおけるMHDフィードバック制御システムの改良とRFPプラズマへの効果
Improvement of Active MHD Control System andIts Effect on Plasma Performance in RELAX
長野 友幸1, 政宗 貞男1, 青木 陽祐1, 樋口 真行1, 軸原 健太1,坪井 涼介1, 中ノ坊 俊1, 三瓶 明希夫1, 比村 治彦1, Roberto Paccagnella2
Tomoyuki Nagano1, Sadao Masamune1, Yosuke Aoki1, Masataka Higuchi1,Kenta Jikuhara1, Ryosuke Tsuboi1, Syun Nakanobo1, Akio Sanpei1, Haruhiko Himura1,Roberto Paccagnella2
1

京都工芸繊維大学, 2Consorzio RFX
1Kyoto Institute of Technology, 2Consorzio RFX
1. Introduction and BackgroundIn reversed-field pinches (RFPs), current driven
non-resonant kink modes can grow as resistive wallmodes (RWMs), leading to degradation of plasma
performance or discharge termination mainly due toenhanced plasma-wall interaction associated with
deformation of the edge magnetic structures. In theRFP, much effort has resulted in successful
stabilization of the RWM by means of saddle coilarrays. The number of saddle coils is determined by
the mode spectrum of the unstable RWMs; thenumber of saddle coils is relatively large because of
the low-q (safety factor) nature of the RFP
configuration.
2. Active MHD Control System in RELAXIn RELAX [1] (R/a = 0.51 m/0.25 m, Ip ~ 100kA,Te(0) ~ 100eV, ne ~ 1019m-3, τ ~ 3ms), 64 (4 inporoidal, 16 in toroidal) saddle coils are attached on
the outer surface of the vacuum vessel for active
feedback control of MHD instabilities. The radial
sensor coils with similar saddle shape are located at
the same locations as the saddle coils. In the first step
experiment for stabilization of RWM, all coils wereconnected in series to provide the m/n = 1/2 externallynon-resonant helical field. The sensor coils wereconnected in the same manner, and two
IGBT-controlled power supplies were used to provide
actuator current. The on-off feedback control worked
well to suppress the growth of this mode, resulting in
longer discharge duration exceeding 3 ms, which was~ 20 % shorter without feedback [2].
3. Improvement of MHD Control System andEffect on Plasma results
On the other hand, we observed small discrepancy
between the m/n = 1/2 fourier amplitude and sensor
amplitude, the former provided by 16 sine- andcosine-radial coils set outside the vessel, while thelatter by sensor saddle coil arrays. We have attributed

this discrepancy to the toroidal non-uniformity of thefeedback control field arising from the flanges withlower poloidal resistance at the two insulated poloidal
gaps. To compensate for this non-axisymmetric effect,
we applied separate control of the magnetic boundary
conditions at the two gap sectors. At each gap sector,
the inner and outer side saddle coils were controlled
independently such that the radial flux penetrating thesensor coil remains zero, while the top and bottom
saddle coils are connected in series to control the m =1 vertical structure. Therefore, 6 additional power
supplied were used three power supplies are used ateach of the two sectors for separate control of themagnetic boundary conditions at the two toroidal
sectors which include the poloidal insulated gas andflanges. The remaining saddle coils were connected inseries to provide the m/n = 1/2 helical field as before.
We observe further improvement in discharge duration
far exceeding 3 ms [3].

Fig: Time evolutions of Ip, Vloop, Br sensor signal,without (blue) and with (red) active control.
References[1] S. Masamune et al., J. Phys. Soc. Japan. 76, 2007,123501.
[2] H. Tanaka et al., Plasma and Fusion Res. 9, 2014,1302057.
[3] T. Nagano et al., Proc. IEEJ. 2015, 1131.