27pA12
ヘリオトロンＪにおける高速イオン励起MHD不安定性発現時の周辺プラズマ応答

Edge Plasma Response to Fast-Ion Driven Instability in Heliotron J
大島慎介1、 橋本紘平2、小林進二1、山本聡1、長崎百伸1、坂本欣三1、水内亨1、岡田浩之1、
南貴司1、木島滋1、N. Shi1、向井清史3、H. Y. Lee2、L. Zang2、笠嶋慶純2、永榮蓉子2、松浦寛人4、竹内正樹5、佐野史道1
S. OHSHIMA1, K. HASHIMOTO2, S. KOBAYASHI1, S. YAMAMOTO1, et al.
京大エネ理工研1、京大エネ科2、NIFS3、大阪府大4、JAEA5
IAE, Kyoto Univ.1, GSES, Kyoto Univ2
Fast-ion driven instabilities such as Alfvén
Eigenmodes (AE) have been highlighted since theseinstabilities can cause loss of alpha particles and theresultant degradation of fusion plasma. Moreover,the possibilities of influence of these instabilities on
the bulk plasma confinement have been indicatedexperimentally and theoretically. For instance,energetic particle driven MHD activity can be a
trigger of internal transport barrier to produce
negative magnetic central shear plasma andgenerate poloidal flow shear through redistribution
process of energetic particles[1].
In this study, edge potential response
synchronized with a fast-ion driven MHD fluctuationat ~ 60 kHz was discovered in a helical-axis heliotron
device, Heliotron J [2]. Figures 1(a) and (b) show
band-pass filtered signals of magnetic probe in thefrequency range from 30-100 kHz and the floating
potential from 0 to 8 kHz around last closed flux
surface (LCFS). The periodic MHD bursts seen here
were reproduced every ~ 1 ms accompanying rapid
chirping up of the oscillation frequency. Clearly, slow
potential variation synchronized with the burst events
can be observed at ~ 1 kHz. This characteristic behavior
is observed only when cyclic, intermittent MHD burstsappear. From the measurement of triple probe, thisresponse does not appear in electron temperature,suggesting the response corresponds to the change ofspace potential. From the correlation measurement
among different probes located at different sections, it
was found that the potential changes symmetrically intoroidal and poloidal direction.
Typical structural change of potential around
LCFS on each burst is shown in Figs. 2 (a) and (b),which is obtained by radial array probe. Basically, thefloating potential deepens more negatively at the deeper
location into the plasma. When the potential dropoccurs, the potential become more negative insideLCFS, while becomes more positive outside LCFS.
Around LCFS and at the location far from LCFS, theresponse becomes small.
A candidates to explain this potential
response is the radial current due to anomalous fast ion
loss resulting from the MHD bursts because the loss offast ion should also be correlated with the growth anddecay of the MHD behavior. Indeed, the existence ofthe fast ion loss linked with the MHD bursts was
confirmed using directional probe measurement inother similar experiments in Heliotron J.
The details of the experiment and thecharacteristics of the response will be presented anddiscussed in this presentation.
[1] K. L. Wong et al, Nucl. Fusion, 45 30 (2004)[2] S. Ohshima, et al., 24th IAEA Fusion Energy
Conference, San Diego, 2012, EX/P4-17

Fig.1 (a) Band-pass filtered signal of MP from 30 to
100 kHz inside #14.5 probe and low frequency
potential responses at radially different positions
measured with radial multi-channel probe at #8.5section.

Fig. 2 (a) Structural change of potential profileand (b) perturbed potential profile at the timing ofpotential drop around LCFS.