28aA04
NBIによるTS-4 FRCプラズマの安定性と閉じ込めの改善	 
Stability and Confinement Improvement of FRC Plasmas
in the TS-4 NBI Experiment
伊井 亨1、井 通暁2、魏 啓為2、梅澤 俊之2、小野 靖2
Toru Ii1, Michiaki Inomoto2, Keii Gi2, Toshiyuki Umezawa2, Yasushi Ono2
東大工1、東大新領域2
Grad. Sch. of Engineering, Univ. Tokyo1, Grad. Sch. of Frontier Sciences, Univ. Tokyo2
taneously driven by the modified pressure profile in
the FRC with NBI. Though these modifications
were localized outside the magnetic axis, the flux
decay was suppressed in a wide area of the FRC, toprovide significant extension of gross decay time.
These results indicate that NBI improves significantly the FRC confinement probably by active
control of pressure and current profiles as well as
electron heating.
References[1] T. Ii et al., Proc. 24th IAEA Fusion Energy Conference (IAEA, San Diego, 2012), ICC/P3-01.
[2] T. Ii et al., Rev. Sci. Instrum. 83, 083504(2012).
[3] E. Kawamori et al., Phys. Rev. Lett. 95, 085003(2005).
[4] E. V. Belova et al., Phys. Plasmas 13, 056115(2006).
This work is supported by a Grant-in-Aid for JSPS
Fellows 23-2462, Scientific Research 22246119
and 22686085.
P0 (kPa) -dE total /dt Ψ (mWb)(MW)15
τφ ~ 200 μs
10 τφ ~ 100τφ ~μs
30μs
5 w/o NBIwith
NBI
015105PNBI
00.80.4

η 0,eff (mΩ m)

The first investigation of tangential neutral beam
injection (NBI) to oblate field-reversed configurations (FRCs) has been made in the TS-4 plasma-merging experiment [1]. For this experiment,
we developed an economical high-power pulsed
NBI system by use of a washer-gun plasma source
and finally attained the maximum beam power of0.6 MW (15 kV, 40 A) for its pulse length of 0.25
ms, which is longer than the FRC lifetime in TS-4
[2].
Since low-n global mode is unstable in oblate
FRC in MHD regime, the oblate FRCs produced
from light gases often suffer from destructive tilting
or other low-n instabilities [3]. The co-NBI with
injection power of 0.6 MW largely extended themagnetic energy decay time of oblate FRCs from ~15 µs to ~ 30 µs, while no improvement is observedin the counter-NBI case. In the latter case, the fastions are not confined inside the separatrix. Theseresults indicate that the fast ions produced by theNB stabilized the low-n global modes and prolonged the discharge duration, as predicted in numerical work [4].
Oblate FRCs produced from heavier gases suchas argon and krypton reveal better stability against
the low-n modes due to kinetic or two-fluid effects.
Figure 1 shows time evolutions of trapped magnetic
flux, total loss power, thermal pressure, and effective resistivity at the magnetic axis of argon FRCs
with and without NBI. The NBI significantly extended the flux decay time from ~ 30 µs to ~ 200 µs
while the injection power is much smaller than themaximal loss power of 11 MW. The reduced totalloss power of less than 5 MW indicates that NBI
not only heats FRC plasma but also changes thestability properties.
With the NBI, the thermal pressure outside of themagnetic axis increases significantly from thatwithout NBI, as expected from the orbit calculationof fast ions produced by the NB. The NBI was observed to modify the current density profiles, suggesting that a diamagnetic plasma current is spon-00.4
0.2

(b)

(c)

(d)

η sp,⊥

0400(a)420
440
Time (μs)

460

Fig. 1. Time evolutions of (a) trapped magnetic flux, (b)total loss power, (c) thermal pressure, and (d) effective
resistivity at the magnetic axis of argon FRCs with andwithout NBI.