27D22P
LHDにおける共鳴摂動磁場印加実験
RMP Experiments in LHD
榊原 悟1, 鈴木康浩1, 成嶋吉朗1, 武村勇輝2, 渡邊清政1, 田中謙治1, 居田克巳1,山田弘司1, LHD実験グループ1
Satoru SAKAKIBARA1, Yasuhiro SUZUKI1, Yoshiro NARUSHIMA1, Yuki TAKEMURA2,Kiyomasa WATANABE1 et al.
核融合研1、総研大物理2
NIFS1, Grad. Univ. Advanced Studies2
The existence of an error field is one of thecommon problems in magnetic confinement
systems because it may degrade plasma
confinement and trigger MHD instabilities. The
RMP experiments have been done in many
tokamaks and RFPs to identify the threshold oferror field penetration, which is required for
estimating the tolerance to the error field inITER. Here we have investigated the thresholdof the perturbation field strength by changing
the RMP field in varying magnetic shear andmagnetic hill of LHD so as to find the common
physics of the effect of an error field in bothtokamaks and helical devices [1]. The magnetic
shear can be changed by controlling helical coilpitch parameter, c, where c is defined as 1.282ac
and ac is minor radius of helical coil. When the cdecreases from 1.20 to 1.13, the magnetic shear,(1/)d/d, is reduced from 2.03 to 1.08. The
magnetic well/hill is changed by shifting themagnetic axis position (Rax). In the experiments,
toroidal field at Rax was set at 0.9 T and balanced
neutral beams were applied for plasma heating
because of suppression of net toroidal current andtoroidal plasma flow. The c and Rax were set at
1.13-1.20 and 3.6-3.75 m, respectively. The RMP
field with m/n = 1/1 was ramped-up to 0.8 kA/T
during a discharge and both polarity was applied inthe experiments.
In the weak magnetic shear configuration with c =1.13, the magnetic island with m/n = 1/1 appeared
even in the case of no RMP, and flattening structureof electron temperature profile was observed aroundthe resonance. The island width increased withnegative RMP field. When the positive RMP wasapplied, the island was suppressed once andincreased again with the RMP field. In the strong
magnetic shear configuration, RMP field is shieldedtill RMP current exceeded a threshold. After thepenetration, the island had larger size than givenRMP field and reduced the total stored energy.
Results of different shear experiments indicate that(1) the threshold of the mode penetration linearly
increases with the magnetic shear, (2) the size ofpenetrated island is larger than that given by RMP
and (3) the thresholds obtained from both polarity
RMP field have an offset. The offset is almost
constant in any magnetic shear configurations andcorresponds to an error field in LHD. The plasma
flow was measured with Charge Exchange
spectroscopy in the experiments, and there is no
significant difference between different shear
discharges.
In order to investigate the effect of magnetic
well/hill on the penetration, the magnetic hill wasmitigated by shifting Rax to the outward withkeeping the strength of the magnetic shear (c =1.13). The experiments show that the error field
was shielded in the mitigated hill configuration (Rax= 3.75m), whereas it appeared in the enhanced hillcase (3.6 m). When the Rax was shifted from 3.75 m
to 3.6 m during a discharge, the island appeared atRax < 3.61 m. Finally, the RMP field was ramped upin the mitigated hill configuration and RMP field
was not penetrated in the range of IRMP/Bt < 0.8
kA/T, which means that the threshold of the mode
penetration is much higher than the enhanced
magnetic hill case.
These experiments show that the threshold of themode penetration has clear dependence of themagnetic shear and/or magnetic well/hill, whichsuggests that the stability parameter of theinterchange stability index (DI etc.) is one ofcandidates for the threshold of the penetration inaddition to the plasma flow.
[1] S.Sakakibara et al., Proc. Of 24th IAEA Fusion
Energy Conference (San Diego, USA, 8-13 Oct.
2012) EX/P4-30.