23P109-P
Dynamo magnetic fluctuation driven by CHI and its influence on HIST
equilibrium configurations
CHIダイナモ駆動磁場搖動とHIST平衡配位への影響
Keisuke Matsumoto1, Takafumi Hanao1, Kengo Ito1, Mitsuru Ishihara1, Yusuke Kikuchi1
Naoyuki Fukumoto1, Masayosi Nagata1, and Takashi Kanki2
松本圭祐1, 花尾隆史1，伊藤兼吾1，石原充1，菊池祐介1, 福本直之1, 永田正義1, 神吉隆司21

Graduate School of Engineering, University of Hyogo, 2167 Syosya, Himeji City, Hyogo 671-2280, Japan2
Japan Coast Guard Academy, 5-1 Wakabacho, Cre City, Hiroshima 737-8512, Japan1
兵庫県立大学 大学院工学研究科 〒671-2280 兵庫県姫路市書写2167
2海上保安大学校 〒737-8512 広島県呉市若葉町５-１

We have observed magnetic field flucutations during the amplification and sustainment of the sphericaltorus (ST) plasmas in the HIST device. To identify the detail mechanisms regarding a helicity transport, wehave investigated the characteristics of the observed magnetic fluctuations in double pulsing CHI
experiments. Fluctuation has been found to propagate from the open flux column region toward the coreregion.

1. Introduction
Flux amplification and current drive by dynamo
effect is one of the most interesting physicalphenomena in astrophysical and laboratory plasmas.
The coaxial helicity injection (CHI) pulse produces
effectively fluctuating flows and magnetic fields
which are considered to play a role in the dynamo
activity needed for driving a current in the closed
flux regions. We have identified propagation
characteristics of the magnetic flucutations during
the driven and decaying process in the double
pulsing operations on HIST.
2. Experimental setup
The HIST device is a low aspect ratio torus withmajor radius R = 0.30 m, minor radius a = 0.24 m,and aspect ratio A=1.25. The capacitor banks(Vmax=10 kV, C=0.6-2.6 mF) are used for ST
formation. The two sustainment banks (Vmax=900 V,
C=195 mF and 335 mF) have been prepared for thedouble gun pulsing experiment.
Figure 1 shows the main elements of the HIST
device, indicating the flux conserver (FC),magnetized coaxial plasma gun (MCPG), thetoroidal field (TF) coil, and diagnostics. The HIST
device’s diagnostics are 16-channel surface poloidal
pick-up coil, a current density probe, internal
magnetic probing arrays, Ion Doppler Spectrometer(IDS), a three-axis MHD dynamo probe (Machprobe with magnetic pick-up coils), a three-axis
Hall dynamo probe and a CO2 laser interferometer.
We have used a 6-channel probe incorporating both
small size Rogowski loops and flux loops with arectangular cross section (2.4×10-3 m2) to measure

the toroidal current density profile and theforce-free parameter λ ≡ µ 0 jt B t on the FC
midplane. We have measured ion saturation current
from a double probe of the Mach probe.

Fig.1. Schematic diagram of the HIST device anddiagnostics.

3. Experimental Results
Figure 2 displays (a) temporal behavior ofcontour plots of the radial profile of the magnetic
fluctuation, (b) short-time Fourier transforms
spectra at R=0.075 m, R=0.25 m and R=0.3 m on
the midplane. We have found that the magnetic
fluctuation propagates from the open flux column(OFC) region toward to the core region. Here, theseparatrix locates in R~0.15 m. Fluctuation
frequency of 80 kHz can be seen in the both regions.
In the core region only, however, the fluctuations
with 20-40 kHz become dominant. There is a goodcorrelation between the magnetic field and the flow
velocity fluctuation at 80 kHz. Therefore, this may
suggest that the magnetic fields fluctuation with 80
kHz contributes to generate the dynamo electricfield.

second CHI pulse all over three axial locations.
Magnetic fluctuation propagates from the gun to FC.
Fluctuation velocity is identified to be Alfven
velocity as shown by Fig. 4.

(a)

(a)
(b)
Fig.3. (a) Time evolution of axial profile of the magnetic
fluctuation amplitude observed at the inboard, (b) FFTanalysis at each axial position.

(b)
Fig.2. (a) Time evolution of radial profile of themagnetic fluctuation amplitude observed on themidplane, (b) FFT analysis at each radial position.

Figure 3 shows (a) temporal behavior of themagnetic fluctuation in the axial direction, (b) FFTspectra at z=0.5 m, z=0.642 m and z=0.784 m. The
fluctuation with 80 kHz is clearly excited by theFig.4. Propagation velocity versus Alfven velocity

3. Summary
The propagation of magnetic fluctuations with 80
kHz suggests that helicity may be transported fromthe plasma gun source to the closed flux surfaces
through the OFC.