05pA01
低磁気シアヘリカルプラズマにおける高速イオン励起MHD不安定性研究	 
Studies of energetic-ion-driven MHD instabilities
in helical plasmas with low magnetic shear	 山本 聡1), 小林進二1), 長崎百伸1), E. Ascasíbar2), 永岡賢一3), 井戸 毅3),大島慎介1), B. Blackwell4), D. Pretty4), D. Spong5), A. Melnikov6), 磯部光孝3),
小川国太3), 佐野史道1), 水内 亨1), 岡田浩之1), 南貴司1), 門信一郎1), 中村祐司7),
木島 滋1), 史 楠1), T. Estrada2), M. Ochando2), 臧 臨閣7), 佐野 匠7), 中山裕介7)
S. Yamamoto1), S. Kobayashi1), K. Nagasaki1), E. Ascasíbar1), K. Nagaoka1) et al.,1)京大エネ研, 2)CIEMAT, 3)核融合研, 4)ANU, 5)ORNL, 6)KI, 7)京大エネ科
1)IAE, Kyoto Univ., 2)CIEMAT, 3)NIFS, 4)ANU, 5)ORNL, 6)KI, 7)GSES, Kyoto Univ.
The features of energetic-ion-driven MHD
instabilities such as Alfvén eigenmodes (AEs) inthree-dimensional magnetic configuration with low
magnetic shear and low toroidal field period thatare characteristics of advanced helical plasmas areintroduced. Moreover, the external control of AEs
and the effects of AEs on energetic ion
transport/loss in low magnetic shear helical plasmas
are also introduced.
Most concepts of advanced helical plasmas havea low magnetic shear in combination with amagnetic well for good stability againstpressure-driven MHD instabilities and low toroidal
field period for both good transport and MHD
stability. The existence and stability ofenergetic-ion-driven MHD instabilities, which
would affect the transport of energetic ions
including alpha particles in a fusion reactor, arecharacterized by the magnetic configuration
including the magnetic shear. We haveexperimentally and numerically investigated theenergetic-ion-driven
MHDinstabilities
inhelical-axis heliotron Heliotron J with low magnetic
shear and medium rotational transform (iota). And
we have compared the results of Heliotron J withresults of TJ-II, which has low magnetic shear andhigh iota. We have effectively utilized thesimilarities and differences between both devices toclarify the characteristics of AEs in helical plasmas
with low magnetic shear.
Characteristics of AEs in low shear helicalplasmas: The MHD instabilities destabilized bythe energetic ions are observed in NBI-heatedplasmas of Heliotron J [1] and TJ-II [2]. The
frequency of observed modes is in the range ofAlfvén frequency and inversely proportional to
the square root of electron density ne. In order toidentify the observed mode, we have compared
the experimental results and numerical simulations
by STELLGAP and AE3D code [3] where we took
into
account
three-dimensional
magnetic
configuration which leads to both the poloidal andtoroidal mode coupling of each shear Alfvén wave.
In the case of Heliotron J, we only found the GAE,
whose frequency lies just below the shear Alfvén
spectra of m=2/n=1 as shown in Fig.1. The
frequency, mode number and radial structure ofcalculated GAE agree with the experimental results
obtained from Mirnov coils and soft X-ray (SX)
measurements. The observed modes are identified
as GAE in the Heliotron J. It is assumed that theeffect of toroidal mode coupling for GAE is fairly
weak although Np is small. HAE frequency is
predicted to be above 500 kHz in the Heliotron J
because of medium iota. On the other hand, in theFig.1. Shear Alfvén spectra for mode family
Nf=1 of Heliotron J. Inserted plot means
profileofGAE
whosefrequency
corresponds to broken line in spectra.

case of TJ-II, we found GAEs and HAEs, whosefrequency is similar to the observed frequencies, insimulations. It seems that the observed mode in theTJ-II is HAE although there are uncertainties inboth toroidal and poloidal mode numbers.
Moreover, we also observed energetic particle
modes (EPMs) whose frequency does not depend
upon the square root of electron density. Theobserved EPMs are localized at plasma edge region
obtained from beam emission spectroscopy (BES)[4] and SX. The observed frequency, which isdetermined by the characteristic frequency of
passing energetic ions, intersects with shear Alfvén
continua at plasma edge region.
To make more clear the effect of iota on AEs,
we have performed iota scan experiments in bothdevices where the iota is scanned in the range of
0.47~0.63 in Heliotron J and 1.53~1.85 in TJ-II.
Figures 2 (a) and (b) respectively show the GAE
frequency obtained from numerical simulations andexperiments as a function of iota in Heliotron J.
Experimental results show that the GAE frequency
increases with an increase of iota. This is the sametendency found in the numerical simulations. In thecase of TJ-II, we did not see the clear dependenceof the observed frequency on iota. However, GAEand HAE can exist in the frequency range ofobserved mode.

(a)

(b)

1 and 2 are observed in NBI-heated plasma in therange of ne < 1x1019 (m-3) where the ECCD cansuccessfully drive plasma current. When therefractive index of ECH N// is changed in order toinduce ECCD, the mode amplitude of the observed
EPMs with n=0 and 1 is decreased as the EC
induced plasma current increases as shown in Fig. 3.
The mode amplitude is strongly linked to themagnetic shear because the continuum damping
rate, which is the main damping mechanism ofEPMs, is related to the magnetic shear. The
magnetic shear of Heliotron J is low in low beta
plasma where energetic ion beta is comparable tobulk plasma beta and we often observed EPMs.
Therefore the control of EPMs is successfullydemonstrated by the plasma current control due tothe ECCD in Heliotron J [5].
In TJ-II, when ECH is applied into NBI-heatedplasmas, the initially continuous AEs display
bursting behavior with rapid frequency chirping.
The mode amplitude of the bursting AEs is weaker
than that of the continuous AEs. The ECH powerand ECH deposition profile affect the mode
amplitude of the observed bursting AEs. A
candidate to explain the mitigation of AE by ECH
in TJ-II is the increment of Landau and/orcollisional damping whose damping date is related
to the electron temperature.

(a)

(b)

Fig.2. Frequency of GAE obtained from (a)numerical simulations and (b) experiments inHeliotron J.
External control of AEs in low shear helicalplasmas: Some way of external control of MHD
instabilities destabilized by energetic ions will be
required for a fusion reactor. We choose
ECH/ECCD to control energetic ion driven MHD
instabilities because ECH/ECCD is an ideal tool tocontrol MHD instabilities since it can provide
highly localized ECH power /EC current with aknown location and good controllability. In
Heliotron J, we have attempted to control the mode
amplitude of the observed EPMs by the change ofplasma current induced by ECCD. EPMs with n=0,Fig.3. Dependence of (a) n=0 and (b) 1 EPMs on
plasma current induced by ECCD in Heliotron J.
References[1] S. Yamamoto, et al., Fusion Science andTechnology 51 (2007) 92.
[2] R. Jimenez-Gomez, et al., Nuclear Fusion 51(2011) 033001.
[3] D. Spong, et al., Physics of Plasmas 17 (2010)
022106.
[4] S. Kobayashi, et al., Review of Scientific.
Instruments 81 (2010) 10D726.
[5] K. Nagasaki, et al., to be published in NuclearFusion.