23P108-P
HIST up-grade scenario towards high performance using multi-pulsing CHIHIST装置におけるマルチパルスCHIによる高性能化シナリオ
Takafumi Hanao, Mitsuru Ishihara, Kengo Ito, Keisuke Matsumoto, Yusuke Kikuchi,Naoyuki Fukumoto, Masayoshi Nagata
花尾隆史, 石原 充, 伊藤兼吾, 松本圭祐, 菊池祐介, 福本直之, 永田正義
Graduate School of Engineering, University of Hyogo
2167, Syosya,Himeji671-2201, Japan
兵庫県立大学・院工 〒671-2280 兵庫県姫路市書写2167
The HIST device has been developed towards high-βand quasi-steady-state sustainment of high-q andlow-q ST plasmas by Multi-pulsing Coaxial Helicity Injection (CHI) method. We have successfully
demonstrated the flux/current amplification and sustainment of the plasmas in the double gun pulse
experiment. We have investigated the characteristics of the double CHI driven ST plasmas.

1. Introduction
Coaxial helicity injection (CHI) was so far
used to sustain spheromak and ST plasmas. A
critical issue for CHI is achieving good energy
confinement and sustainment simultaneously,
since the stochastic magnetic file lines destroy
significantly the magnetic flux surfaces.
Multi-pulsing CHI or repetitive transient CHI hasbeen proposed to resolve this key issue. Themulti-pulsing operation is that after the plasma
partially decays, a new CHI pulse is applied andthe repeated driven and decay process yields aquasi-steady-state plasma. This multi-pulsing
CHI experiment was successfully demonstrated in
the SSPX spheromak device at Lawrence
Livermore National Laboratory [1]. In the HIST
experiments, this new CHI method has been
examined for the high-q ST configurations.
2. Experimental device
The structures, sizes, capabilities, diagnostics,and operating conditions of HIST (R = 0.30 m, a =0.24 m, A = 1.25) are described in detail in Ref. [2],but some diagnostic and additional capacitor banksystems for M-CHI operations are also describedbriefly here. The capacitor banks (Vmax = 10 kV, C= 0.6-2.6 mF) are used for ST formation. The twosustainment banks (Vmax = 900 V, 336 mF and C =280 mF) have been prepared for the double gun
pulsing experiment. The HIST device has surface
poloidal pick-up coils, a current density probe,
internal magnetic probing arrays, Ion DopplerSpectrometer (IDS), a three-axis MHD dynamoprobe (Mach probe with magnetic pick-up coils), athree-axis Hall dynamo probe and a CO2 laserinterferometer.
Figure 1 illustrates the HIST device. The HIST

device can form and sustain the ST (high-q: q>1 andlow-q and spheromaks: q<1) by utilizing thevariation of the external toroidal field (TF) coilcurrent Itf. By operating the magnetized coaxial
plasma gun (MCPG) with the condition of λgun>λFC,
here λgun =µ0Igun./Ψbias and λFC is the eigen-value (λFC= 8.53) of the flux conserver (FC), required during
the driven phase, the magnetic helicity is transferredfrom the MCPG to the plasma.

Fig.1. HIST device

3. Results from double pulsing CHI experiment
Figure 2 illustrates temporal evolutions of thetoroidal plasma current It, the line averaged electron
density <ne> in the core, the edge poloidal magnetic
field Bp.in, Bp.out at the inboard side (R=0.15 m) andthe outboard side, respectively, λOFC =µ0It./Ψt in the
OFC region (at R~0.1 m) and λcore in the core region

(at R~0.25 m) in the comparison of single and double
pulsed discharges. The injection gun current is Igun
~30 kA in the both operations and the gun bias flux
Ψbias is adjusted properly. In this shot, the MCPG has
been secondly pulsed at t ~ 1.5 ms and then we haveobserved that the toroidal current It is effectively
amplified against the partially resistive decay. Inaddition, the life time tlife has increased up to 7 ms
which is longer than that in the single CHI case (tlife ~4 ms). The line averaged electron density <ne> andthe outer edge field Bp.out last between t = 1.5 ms andt = 5 ms in a steady-state manner as shown by Fig.1(b), (d). The toroidal current in the core has been
driven until t ~ 5 ms and after then started to decay
exponentially.

pulse should be applied while the plasma decays
sufficiently.
4. Summary
We have successfully demonstrated theflux/current amplification and sustainment of theplasmas in the double CHI pulse experiment. We
have investigated the characteristics of the double
CHI driven ST plasmas.
This work was supported by Grant-in-Aid for
Scientific Research (B) (22360391).

Life time [ms]15105t = 2.5 ms
t = 1.5 ms

00

0.5

11.5
Current gain

22.5

Fig.3. Comparison of the current gain and the life time
between the second pulse timing at t ~ 2.5 ms and t ~ 1.5ms

References
Fig.2.Time evolution of (a) It, (b) <ne>, (c) Bp.in, (d) Bp.out,(e) λOFC, (f) λcore, in the comparison of single (#14162)
and double pulsed discharges (#14160)

Figure 3 illustrates a relationship between aplasma life (sustained and decay) time and a current
gain (amplification ratio) and also a comparison
between the second gun pulse timing of 1.5 ms(near the driven phase) and 2.5 ms (near thedecaying phase). The current gain is defined by thefraction of the toroidal current It at the second pulse
peak to just before the second pulse. It was foundthat the second pulse at t=2.5 ms has a larger
current gain which means more effective current
amplification. In the case of t=2.5 ms, the plasma
current can be sustained during longer time thant=1.5 ms. It seems to be important for theMulti-pulsing CHI method that the repetitive gun[1] S. Woodruff, et al., Phys. Rev. Lett. 90, 205002-1(2004).
[2] M. Nagata, et al., Phys. Plasmas 10, 2932 (2003).