23D10
Experimental evaluation of fast ion confinement by fast ion charge exchange
spectroscopy in LHD
LHDにおける荷電交換分光計測による高速イオン閉じ込めの実験的評価
Takafumi Ito1, Masaki Osakabe1, Katsumi Ida1, Mikiro Yoshinuma1, Masahiro Kobayashi1,
Motoshi Goto1, Sadayoshi Murakami2, Mitsutaka Isobe1, Ryosuke Seki1,
Shinji Kobayashi3, Kazuo Toi1 and the LHD experimental group1
1
伊藤隆文 , 長壁正樹1, 居田克己1, 吉沼幹朗1, 小林政弘1, 後藤基志1, 村上定義2, 磯部光孝1,
關良輔1, 小林進二3, 東井和夫1, LHD実験グループ1
1

National Institute for Fusion Science322-6 Oroshi-cho, Toki-city, Gifu 509-5292, Japan1
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
2
Graduate School of Engineering, Kyoto University
Yoshida Honmachi, Sakyo-ku, Kyoto, 606-8501, Japan2
京都大学大学院工学研究科 〒606-8501 京都市左京区吉田本町3Institute of Advanced Energy, Kyoto University
Gokasho, Uji-city, Kyoto, 611-0011, Japan
3
京都大学エネルギー理工学研究所 〒611-0011 京都市宇治市五ヶ庄
Spectroscopic investigation for H-alpha lights from reneutralized fast ions via charge exchange process is
an useful method to identify spatial density profile of fast ions for each energy range. Tangential viewinggeometry is suitable to observe properties of circulating fast ions in the scheme because the line of sighthas quite large sensitivity and resolution for the fast ion velocity component parallel to the magnetic fieldline. We have successfully identified spatial profile of circulating fast ions on two energy ranges in Large
Helical Device(LHD) plasmas which is sustained by the tangential negative-ion based neutral beam. The
slowing down process of the fast ions has been examined by the evaluation of time evolution of theDoppler-shifted H-alpha emission.

1. IntroductionFor sustaining fusion reaction and heatingplasma, fast ions including the fusion-born alphaparticles and the externally-injected particles arerequired to be confined because their loss might
degrade the performance of fusion plasmas andmight damage the fast wall of the fusion
reactor[1]. In particularly, circulating fast ions
are important because they can give their energy
and momentum to the fusion plasma for heating
and current drive(CD).
However, thedegradation of fast ion confinement might occur
by collective instabilities depending on the fastion distribution in real or phase space.
Therefore, monitoring motion of supra-thermal
ions is one of the key issue for understanding fastion confinement and fast-ion-driven instabilities
in high performance fusion devices.
Fastionchargeexchange
spectroscopy(FICXS)[2]
isapplicabletoinvestigate fast ion profiles because of its energyand spatial-resolved measurement in Large
Helical Device(LHD). This diagnostic is one of
the active beam spectroscopy based on theanalysis of Doppler-shifted spectrum fromreneutralized fast ions via charge exchange(CX)
processes between the probe beam and the fastion[3]. This kind of diagnostic can measure thelocal properties of fast ions for each line ofsight(LOS) because the measurement area can bedetermined by the spatial profile of probe beamand the observation LOS. An arrangement ofobservation geometry is important if focusing on
the specific velocity component of fast ion isrequired.
On LHD, various types of Alfvénic phenomena
driven by circulating fast ions have beenobserved[4]. To study these modes, we haveadded the tangential viewing geometry to theFICXS diagnostic and the initial observation of
fast ion signal in LHD plasma has beenobtained[5]. In this report, the validation of theobserved fast ion charge exchange(FICX)
spectrum compared with the calculation will bepresented. Moreover, the evaluation of slowing
down process of fast ions in the quiet LHD
plasma will also be shown.
2. Experimental setup

The lines of sights(LOSs) for tangential FICXS
diagnostic on LHD are arranged as shown in Fig. 1.
The LOSs are horizontally aligned for the spatial
measurementofpassing-particles
atahorizontally-elongated poloidal cross section where
the diagnostic NB is injected. The measurement
direction is set counter clockwise(CCW) of thetorus. Thus, the particle which moves to theclockwise direction in the tourus emits blue shiftedH-lights,
while
that
moves
tothecounter-clockwise direction does the red-shifted
lights. The observation areas on the LOS depends
on the choice of NBs for diagnostic usage.
When
the radial-NB is used as a diagnostic-NB, t the area
covers between R~2.8m to ~3.8m in major radii.
A modulation of the probe beam is necessary toestimate the background spectra because thetangential FICXS diagnostic does not have any
background LOS.
3. Experimental results
The FICX spectrum is obtained where no
significant instabilities are observed, so that theobserved spectrum can be compared to thecalculated spectra which is evaluated from theneoclassical transport simulation code for fast-ions.
The
calculated
FICX-spectrum
andtheexperimental observation are well agreed for the
shape of spectrum. This agreement satisfies thatthe assumption of the fast ion velocity distribution
and its spatial profile based on the realistic
equilibrium used for the simulation is reasonable.
The time evolution of FICX signal derived by theintegration of the observed spectrum on the limited
wavelength range has also been examined. The
intensity of FICX component in the specificwavelength range projects an window due to its
intensity onto the phase space of fast ion. This
profile on the phase space can be considered as asensitivity of measuring fast ion density for eachLOS. Therefore, it can be considered that a spatial
profile of intensities of FICX component in thelimited wavelengths indicates a density profile offast ion with the specific energy range. By using
this scheme, the slowing down time is evaluated by
the time-evolved FICX signal. On the LOS faced
the core region, the evaluated slowing down time
and the calculation is well agreed as shown in Fig.2.
This result indicates that the anomalous transport offast ion for the slowing down time scale scarcely
appeared in the experiment. More details will beshown in the presentation.
Acknowledgments
The support of the LHD experiment group isFig. 1 Hardware apparatus of tangential
FICXS diagnostic on the midplane of LHD.

R~3.7mFig. 2 Comparison between slowing downtime obtained by calculation and observed
FICX signal.

gratefully acknowledged. This work is supportedby the LHD project budget NIFS11ULRR006 andNIFS11ULRR010..
This work also partly
supported by the Grant-in-aid for Scientific
Research from JSPS, Nos 22860076.
References[1] A. Fasoli, et al.: Nucl. Fusion 47 (2007) S264.
[2] M. Osakabe, et al.,: Rev. Sci. Instrum. 79 (2008)
10E519.
[3] W. W. Heidbrink, et al.: Plasma Phys. Control.
Fusion 46 (2004) 1855.
[4] K. Toi et al.: Nucl. Fusion 40 (2000) 1349.
[5] T. Ito et al.: Rev. Sci. Instrum. 81 (2010) 10D327.