23P107-P
Experimental Study of Magnetic helicity Injection Mechanismof the UTST Spherical Tokamak Plasmas
UTST球状トカマクにおける磁気ヘリシティー入射機構の検証
Takenori G Watanabe, Shuji Kamio, Qinghong Cao, Hirotomo Itagaki,Naoto Suzuki, Koji Ishiguchi, Koichiro Takemura, Kotaro Yamazaki,Takuma Yamada, Michiaki Inomoto, Yasushi Ono
渡辺岳典, 神尾修治, 曹慶紅, 板垣寛朋,鈴木直人, 石口孝治, 竹村剛一郎, 山崎広太郎,
山田琢磨, 井通暁, 小野靖
Graduate School of Frontier Sciences,Univ. Tokyo
5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8561, Japan
東大新領域 〒277-8561 千葉県柏市柏の葉5-1-5
A spherical tokamak (ST) plasma is produced by double-null merging (DNM) method by using two pairsof external poloidal field coils in the University of Tokyo Spherical Tokamak (UTST) device. In the latephase of the DNM formation, a single ST is connected to the external poloidal coil flux, which permits
magnetic helicity injection from the helicity source (the coil flux) into the helicity sink(the ST plasma). Our
2-D magnetic field measurement indicates that the magnetic helicity injection is related with magneticreconnection.

1. Introduction
Magnetic helicity is a useful plasma parameterfor of initiating or ramping up plasma current
drive for various magnetized plasmas. It is now
widely used in ST, spheromak and reversed fieldpinch (RFP) experiments such as the coaxial
helicity source in NSTX [1] and the point-source
dc helicity gun in PEGASUS [2]. The helicity
injection is also observed around the connection
region between poloidal coils and ST plasma
during double-null merging (DNM) formation in
the University of Tokyo Spherical Tokamak(UTST) plasmas[3].
2. Experimental equipment
A ST plasma is produced by the DNM method byusing two pairs of external poloidal field coils. In
this scheme, the magnetic reconnection generally
produces a current sheet and plasmoids, which may
directly influence its helicity injection processes [46]. The magnetic helicity is considered to be
injected from a helicity source with a higheigenvalue into a helicity sink with a low
eigenvalue. However, the helicity injection requires
the helicity source, i.e. the coil flux, to be connectedwith the helicity sink through magnetic field lines.
The eigen value profiles of ST plasma and thatduring helicity injection have not been measured
yet. We measured for the first time 2-D eigenvalue
profile of the connecting region between the coilflux and the core plasma flux. Figure 1 shows theexperimental setup of the UTST device (a) andcolor contours of the measured 2-D poloidal
magnetic flux with toroidal current density jt (b),toroidal eigen-vlues λt (c) and poloidal eigenvalues
λp (d) at 760μs after PF coil discharge began.

Fig1. (a) Experimental setup of UTST device with twopairs of PF coils on each side. Color contours of themeasured 2-D poloidal magnetic flux with (b) toroidal
current density jt (c) toroidal eigen-vlues λt and(d) poloidal eigen-values λp

3. Results
We observed for the first time in plasma

experiments that a long thin high λ region producedby the current sheet bridges the helicity source (thecoil flux) and the helicity sink (the ST plasma).
Both the ST plasma and the coil flux are surrounded
by a common flux during helicity injection.
Magnetic reconnection transforms the common flux
into a private flux, forming a long thin current sheethaving high λ. Also, we observed a plasmoid having
high λt that formed in the current sheet carries
magnetic helicity intermittently from the coil flux tothe ST plasma. The plasmoid tends to have apeaked λt profile at the center of the closed flux. It
has a maximum value of λt ∼ 1.4 μH−1 at around z =0.53m until 840 μs and moves toward the STplasma. Then, the flux decreases, but the high λt
area spreads. The λt value at the entrance region of
the ST plasma increases from 0.6 to 1.15 μH−1
between 680 and 880 μs because of this helicity
injection and plasmoid motion. The plasma current
increases from 90 to 125 kA under the constantvacuum toroidal field because of helicity injection
into the ST plasma.
4. Summary
In this experiment, the relationship between
magnetic helicity injection and magnetic
reconnection was observed for the first time inreconnection experiments [7]. Our 2-D eigen-value
measurement indicates that helicity injection iscaused by the dynamics of a high-λt current sheet
arising from magnetic reconnection that bridges thehelicity source and the ST plasma. Plasmoids withhigh λt are observed to grow intermittently andmove toward the central ST plasma, carrying
magnetic helicity.
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[2] D.J. Battaglia et al., Phys. Rev. Lett. 102, 225003(2009).
[3] T. Yamada et al., Plasma Fusion Res. 5, S2100(2010).
[4] Buchner and J.-P. Kuska Ann, Geophysicae 17, 604(1999).
[5] A.Bhattacharjee and X.Wang, Astrophysical Journal
372, 371 (1991).
[6] Keisuke Nishida et al., ApJ 690, 748 (2009).
[7] T.G. Watanabe et al., Plasma Fusion Res. 6, 1202131(2011).