23P104-P
Doppler Spectroscopy Measurement of ST Heating by Plasma Merging in UTST
UTST装置における球状トカマク合体加熱のドップラー分光計測
Shuji Kamio, Koichiro Takemura, Qinghong Cao, Hirotomo Itagaki, Naoto Suzuki, Takenori GWatanabe, Kotaro Yamasaki, Koji Ishiguchi, Ryota Imazawa, Takuma Yamada, Michiaki Inomoto,
Yuichi Takase and Yasushi Ono
神尾修治，竹村剛一良，曹慶紅，板垣宏知，鈴木直人，渡辺岳典，山崎広太郎，石口孝治，今澤良太，山田琢磨，井通暁，高瀬雄一，小野靖
Graduate School of Frontier Sciences, The University of Tokyo
5-1-5, Kashiwanoha, Kashiwa 277-8561, Japan東京大学 〒277-8561 柏市柏の葉5-1-5
The merging start-up of spherical tokamak (ST) plasma has been accomplished in UTST device using
external poloidal field (PF) coils. The plasma current up to 180 kA has been attained with assistance of thecenter solenoid (CS) coil. In this merging start-up method, significant ion heating caused by magnetic
reconnection is expected to help the formation of high-beta equilibrium. We have developed a Doppler
spectroscopy system using an 8×8-channel photomultiplier tube (PMT) assembly to observe the temporal
evolutions of ion temperature and flow. The bulk plasma ion temperature measured by helium II line wasapproximately 10 - 15 eV after plasma merging.

1. Introduction
The plasma merging method is a novel startup
scheme to form a high-beta equilibrium in a shortperiod. Plasma merging between two initial torus
plasmas induces magnetic reconnection, which is a
rapid conversion from magnetic energy to plasma
kinetic or thermal energy. Thus, plasma merging
increases the plasma thermal energy and decreases
the magnetic energy, leading to a significant
increase of the beta value. The merging startup
method has been demonstrated in theSTART/MAST [1] (UKAEA) and TS-3/4 (TheUniversity of Tokyo) devices. In TS-3, two STs
were merged to form a single ST with a beta valueof up to 50 % [2]. The high-beta value obtained inthese experiments comes mainly from significant
ion heating by magnetic reconnection. In TS-3, ions
are heated from 10 eV to 40 - 60 eV in 10 µs, whichcorresponds to a heating power of as large as 4 - 6
MW [3]. However, in these previous experiments,
plasma merging startup was performed by using PF
coils inside the vacuum vessel, which cannot be
used in future fusion reactors. The University ofTokyo Spherical Tokamak (UTST) [4] wasconstructed to investigate the feasibility of merging
startup of high-beta STs under a morereactor-relevant condition with no internal PF coils.
To confirm the formation of a high-beta ST plasma
in the UTST, ion-temperature measurement isurgently required.

2. The UTST Plasma Merging Device
A high-beta ST plasma is formed by double nullmerging (DNM) in the UTST. In the DNM startup
method, two initial STs are generated at null points
created inside the device, and they are merged toform a ST as illustrated in Fig. 1.

Fig.1. Schematic view of high-beta ST startup by
DNM in the UTST.

During plasma merging, the tension force of thereconnected magnetic field lines accelerates theplasma to the Alfvén velocity. The accelerated ions
are thermalized in the downstream area [4] bydamping mechanisms such as viscosity or fastshock structure, resulting in the formation of ahigh-beta ST equilibrium. In the UTST device, all3. Spectroscopic measurement system
The Doppler shift Δλ of a spectral line emitted
from a single traveling particle is applied todiagnose the collective behavior of plasma ions.
The ion temperature
is calculated by the fullwidth at half maximum (FWHM) of the line
broadening andis calculated from the Doppler
shift of the line center peak − ;= 1.7 × 10 ×

FWHM

4. Experimental results
Fig. 3 (a) shows the spectral shapes of the helium
impurity emission (HeII 656.0 nm) from the eightlines of sight. Ion temperature evolutions of eightspatial locations are shown in Fig. 3 (b). We
successfully measured the time evolutions of ion
temperature in the range of 5 - 15 eV at eight spatiallocations and a temperature increase was confirmed
during plasma merging time of 0.8 - 0.9 ms.

50 Ch1040 Ch2
20080 Ch340(1)(2)

Fig. 2 shows the schematic overview of thedeveloped
multi-channel
spectroscopic
measurement system. Emissions from ST plasma in
the UTST device are collimated by convex lenses
and introduced to eight plastic optical fibers withcore diameter of 1 mm and numerical aperture of0.5. The other ends of the optical fibers are bundled
in one-dimensional vertical array and coupled withthe entrance slit of a monochromator with focal
length of 500 mm and grating of 2400 grooves/mm,
as shown in Fig. 2 (b).

Output Voltage (mV).

10.5eV25

where A is mass number of the ion,
= peak#110617006-015(a) 1.0ms

060 Ch430080 Ch540060 Ch630060 Ch730080 Ch8400-0.10

10.9eV

7.5eV

7.8eV

7.0eV

7.4eV

6.9eV

7.1eV

00.10
∆λ (nm)Ti (eV)the coils are located outside the vacuum vessel, andthe swing of the PF coil currents applies a parallel
electric field at the null points to initiate the torus
discharge and drive the plasma current. The plasma
current is as high as 180 kA, and the pulse width isas long as 1.2 ms with the assistance of the CS coil.

(b)
15
10
50
15
10
5015
10
50
15
10
5015
10
5015
10
5015
10
5015
10
500.8

Ch1Ch2Ch3Ch4Ch5Ch6Ch7Ch8
1.0 1.2 1.4 1.6Time (ms)Fig.3. (a) Measured spectral distributions of HeII line
emissions from eight spatial locations at = 1.0 ms
from the initiation of discharge, (b) time evolutions ofderived ion temperature at eight spatial locations.

Acknowledgments
This work was supported by JSPS KAKENHI(22246119 and 22686085) and the Global COE
Program “Secure-Life Electronics”, MEXT, Japan.
References[1] A. Sykes et al., Nucl. Fusion 41, 1423 (2001).
[2] Y. Ono et al., Phys. Plasmas 4, 1953 (1997).
[3] Y. Ono et al., Fusion energy 2008 (IAEA Vienna,2009, in DVD-ROM) EX/P9-4[4] T. Yamada et al., Plasma Fusion Res. 5, S2100(2010).
Fig.2. (a) Cross-sectional view of the UTST device
and arrangement of optical fibers, (b) schematic view ofthe spectroscopic system with a monochromator, lenses
and an 8×8 PMT assembly.