23P051-P
Tokamak discharges and magnetic field configuration in TOKASTAR-2 device
TOKASTAR-2装置におけるトカマク放電と磁場配位
Makoto Hasegawa, Kozo Yamazaki, Hideki Arimoto, Tetsutarou Oishi,Reiya Nishimura and Tatsuo Shoji
長谷川誠, 山﨑耕造，有本英樹，大石鉄太郎，西村怜哉，庄司多津男
Department of Energy Engineering and Science, Graduate School of Engineering, Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-8093, Japan
名古屋大学工学研究科エネルギー理工学専攻 〒464-8039 名古屋市千種区不老町
The small device named TOKASTAR-2 is operated to generate tokamak and helical configurations
independently with tokamak-helical hybrid coils. Presently, we plan to install additional field coils togenerate vacuum magnetic surface without plasma current. By the magnetic field line tracing analysis, thesetup angle of additional field coil was decided. In tokamak experiments, the vertical displacement ofplasma was observed. To suppress this displacement, a conductive shell was installed. Plasma current
increased slightly. However, measured plasma current was still far smaller than design value of the device.
Then we plan to construct a new pair of pulsed vertical field coils installed inside the vacuum chamber.

1. Introduction
Tokamak and helical systems have been widely
investigated as efficient toroidal magnetic plasma
confinement devices. Both systems have uniquemerits and demerits. TOKASTAR configuration [1]is one of compact tokamak-helical hybrid confinement
systems. A new small device (R~10cm) named
TOKASTAR-2 is designed to generate tokamak
and helical configurations independently bydifferent type of coils.
One of main purposes of this device is toinvestigate the effects of outer helical application
on tokamak plasmas, especially, to suppress plasma
current disruption. Other purpose is to generatecurrent-less helical plasma, and confirm that plasma
current effects in the helical system when the plasma
current is induced.
Figure 1 shows the coil configuration of
TOKASTAR-2 device. Present TOKASTAR-2 iscomposed of four kinds of coil systems; eight toroidal
field (TF) coils, three ohmic heating (OH) coils, a pairof vertical field (VF) coils and a pair of helical field(HF) coils. All coil systems except VF coils are
installed inside the vacuum chamber.

2. Helical configuration
Present coil systems of TOKASTAR-2 without
additional helical field (AHF) coils cannot form thevacuum magnetic surface. We plan to install two pairof AHF coils to generate vacuum magnetic surfaces[2].
The AHF coils are located on the upper or lower
side of the arcs of HF coil as shown in Fig.1.
However, if particular setup angle is assumed in∆φ

Fig.2. Setup angle of AHF coil ∆φ.

TF coil-3

460mm

averaged radius <r> [x10 m]

AHF coil

HF coil
VF coilOH coil

920mm

Fig.1. Coil configuration of TOKASTAR-2

70605040-10No.1No.2010203040

setup angle of AHF coil ∆φ [deg]Fig.3. Averaged radius vs. ∆φ.

50

toroidal direction, the AHF coil interferes withmeasurement probes.
Here, we estimated the effect of the setup angleof AHF coils. The setup angle ∆φ is defined that theangle by which AHF coils are rotated in toroidal
direction as shown in Fig.2. Magnetic confinement
field line, TF coil, HF coil and AHF coil areindicated by red, black, yellow and green lines,respectively.
The simulation was carried out using two coilcurrent ratio patterns shown in Table I. Averaged
radiusTable I. Coil current ratios used in simulations of Fig.3.

No.1
No.2

IAHF/IHF
1.4
1.5

ITF/IHF
0.86
0.62

IVF/IHF
0.14
0.12

(a)

(b)

Z [cm]
(c)

1(d)

0-1
68

R [cm]

Fig.4. Successive photographs of tokamak discharge
taken by fast camera. Time interval is 24.7µs.
Photograph (c) corresponds to the plasma currentmaximum.
140120Plasma Current [A]as a function of setup angle ∆φ is shown in Fig.3.
The shadow area in Fig.3 indicates the toroidal
angle where the AHF coil interferes measurement
probes. Averaged radii in the region where ∆φ is15~40 deg are greater than these in the region
where ∆φ ~0deg. From this result and considerationof setup supporting structures for AHF coils, we
decided to construct the AHF coils at ∆φ=22.5deg.

without shellwith shell

100
80
60
40
20
00.00

0.02

0.04

0.06

0.08

0.10

0.12

Helium Gas Pressure [Pa]3. Trial of tokamak discharge
We have been trying to induce the plasma current
using OH coil and static VF coil systems.
Pre-ionized plasma was generated by electron
cyclotron resonance (toroidal field: 0.0785T at
R~10cm and radio frequency wave: 2.45GHz).
About 4V loop voltage and static 15G vertical field
were applied to the pre-ionized plasma. However,the plasma current was obtained only 90A.
Additionally, a vertical displacement was observed
by a fast camera (20500fps). Figures 4(a)~(d) show
successive photographs of tokamak discharge takenby the fast camera. Each photograph displays thearea between 6 and 8cm in R direction and between-1 and 1cm in Z direction. OH plasma moved
vertically upward shown in Fig.4. It is supposedthat the displacement is due to up-down unbalanceof eddy currents induced in vacuum chamber andapplied static vertical field. And the error field
made by OH coils keeps N value negative, whichmight be another reasons of the displacement.
Then we installed a conductive shell to suppress
the vertical displacement and to increase the plasma
current. Figure 5 shows the dependence of plasma
current on helium gas pressure PHe with/without theconductive shell. Plasma current with the shellincreased by about 40A compared with the case

Fig.5. Plasma current vs. helium gas pressure
with/without conductive shell.

without the shell at PHe~0.02Pa. The effect ofsuppressing the vertical movement will be
confirmed experimentally in the near future.
Although plasma current increased by installingthe conductive shell, it was not a drastic increase.
We plan to construct a new pair of VF coils whichis installed inside the vacuum chamber and has
appropriate inductance for pulse discharge system.
References[1] K.Yamazaki and Y.Abe, Nagoya Reseach Report
IPPJ-718 (1985).
[2] T. Oishi, et al.: Proc. 23th IAEA Fusion Energy
Conference, Daejeon,2010 ICC/P5-04