J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

Initial experimental results and magnetic surface analysis of tokamak-helicalhybrid plasma confinement device "TOKASTAR-2"
Tetsutarou OISHI, Kozo YAMAZAKI, Koki OKANO, Hideki ARIMOTO,
Kazuhisa BABA, Makoto HASEGAWA and Tatsuo SHOJI
Nagoya University, Chikusa-ku, Nagoya 464-8603 Japan(Received: 20 November 2009 / Accepted: 19 February 2010)

New torus plasma confinement device having tokamak-helical hybrid magnetic configuration “TOKASTAR-2”
was constructed. Purposes of study are investigation on the effect of outer helical field on the tokamak plasma andconsideration on formation of vacuum magnetic surface applying helical field. We confirmed that pre-ionizedplasma of the tokamak operation is produced using the electron cyclotron resonance heating. Calculation ofmagnetic field line has revealed that magnetic surface can be formed using additional outer helical coils, even if
plasma current is not induced.
Keywords: tokastar, tokamak, helical, equilibrium, hybrid confinement, compact torus, magnetic surface

1. Introduction

In the research of confinement of torus plasma using
magnetic fields, a concept of superposition of the external
helical magnetic field to tokamak plasma has been widelyproposed. One of the objectives of this concept is toimprove the stability of plasmas, for example, to suppress
plasma current disruption [1, 2].
A tokamak-helical hybrid plasma confinement device"TOKASTAR-2" has recently been constructed toinvestigate the effect of helical field to tokamak plasma.
The name of TOKASTAR means the abbreviation of
tokamak and stellarator [3]. A first plasma was justproduced in June, 2009. Purposes of the study using
TOKASTAR-2 are as follows:- investigation on the effect of outer helical fields on
tokamak plasmas.
- consideration on formation of vacuum magnetic
surface applying outer helical fields.
In this paper, characteristics of newly constructed
TOKASTAR-2 device and present states of the study forthe purpose described above. In chapter 2, overview of
TOKASTAR-2 device and calculation of toroidal magnetic
field are described. Thereafter, experiment to produce
pre-ionized plasma of tokamak operation and trial for
current induction using ohmic heating coil are shown inchapter 3. At last, consideration on the formation ofvacuum magnetic surface without plasma current bymodifying the shape of coils is discussed.

Fig.1

Photograph of the experimental setup of
TOKASTAR-2 device. Left: source of 2.45
GHz radio frequency wave, right: vacuumvessel.

Fig.2

Coil configuration of TOKASTAR-2.

2. TOKASTAR-2 Device

Figure 1 is the photograph of experimental setup ofTOKASTAR-2 device. Radio frequency (RF) wave

author’s e-mail:t-oishi@mail.nucl.nagoya-u.ac.jp69©2010 by The Japan Society of Plasma
Science and Nuclear Fusion Research

T. Oishi et al., Initial Experimental Results and Magnetic Surface Analysis of Tokamak-Helical Hybrid Plasma Confinement Device “TOKASTAR-2”

layers in the central part of OH coil, 22 turns in upper andbottom part of OH coil, and 98 turns in HF coil. Several
200 µF capacitors are utilized to energize pulsed current toeach coil. The major radius Rp and the averaged minor
radius <ap> of the toroidal plasma with nearly circularcross-section is expected to be approximately 0.1 m and0.04 m, respectively. Plasma parameters measured using
Langmuir probes and Rogowski coil are electron density ne~ 1016 m-3, electron temperature Te ~ 10 eV, and plasma
current Ip ~ 10 A.
The toroidal magnetic field produced by the TF coils
was calculated using Biot-Savart law. Figure 3 shows
contour plot of toroidal field produced by toroidal fieldcoils. In this case, the current in a TF coil ITF is 80 A ×50 turns = 4000 AT. ECR layer (Bt = 0.0875 T for the RFfrequency = 2.45 GHz) appears in the high field side.
However, radial profiles of the toroidal field differ between
at toroidal angle φ of 0 rad (just inside TF coil) and π/8 rad(between TF coils). Figure 4(a) is the comparison of theradial profile of the toroidal field in the midplane at φ = 0
rad and φ = π/8 rad shown together with analytical solutionof averaged toroidal field Bt = µ0NITF/2πR, where N is thenumber of toroidal period (= 8). Difference of thetoroidal fields between these two toroidal angle increases
near the TF coils, namely, toroidal ripple δ defined by δ(R,z) = (Bmax(R, z) – Bmin(R, z))/(Bmax(R, z) + Bmin(R, z)) [4]as a function of major radius and vertical position increases
near the TF coils as shown in Fig. 4(b). This calculation
has revealed that the minimum value of the toroidal ripple

having the frequency of 2.45 GHz is used to produce
plasma by the electron cyclotron resonance (ECR) heating.
The injected power of RF wave is 2.0 kW at maximum.
Base pressure in the vacuum vessel is in the order of 10-3
Pa. He or H2 gas is introduced in the vessel as theworking gas. Present experiment region of gas pressure is0.4 ~ 7 Pa. Figure 2 shows the coil configuration ofTOKASTAR-2. Tokamak plasma is produced using theeight toroidal field (TF) coils, ohmic heating (OH) coils
separated to three parts (top, central, bottom), and a pair ofvertical field (VF) coils. The toroidal field at the center of
the TF coil (R = 0.12 m) is 0.1 T at maximum. External
helical field can be applied using the outer helical field(HF) coils of the toroidal period n = 2 or 1. TF, VF, OHcoils are located inside vessel. Each coil consists ofcupper conductor with a diameter of 3.2 mm. There are
50 turns in TF coil, 100 turns in VF coil, 42 turns × 2Fig.3Fig.4 (a) Radial profile of the toroidal field in themidplane at φ = 0 rad and φ = π/8 rad shown
together with analytical solution of Bt =µ0NITF/2πR. (b) Toroidal ripple defined by(Bmax – Bmin)/(Bmax + Bmin).

Contour plot of toroidal field produced bytoroidal field coils. (a) Top view at themidplane. (b)(c) R-z plane at toroidal angleof 0 rad (just inside TF coil) and π/8 rad(between TF coils), respectively.

70T. Oishi et al., Initial Experimental Results and Magnetic Surface Analysis of Tokamak-Helical Hybrid Plasma Confinement Device “TOKASTAR-2”

is about 0.3 %, which is evaluated at around R = 0.085 m.
In this figure, the profile of toroidal ripple obtained fromthe analytical solution of the toroidal field is shown
together. It is defined by the position of fifty coilfilaments located in the wall of TF coil as shown in Fig.
3(a) and following equations,

ion saturation current measured using Langmuir double
probe. Both values are measured or calculated at major
radius R = 0.07 m and height from the midplane z =0.045 m. Three cases with different voltage charged fora current discharge to TF coils are compared. RF was
injected continuously during the period shown in thefigures. Time that the toroidal magnetic field Bt reaches
to the ECR field can be varied with scanning a charged
voltage to the capacitor for the discharge to TF coils. Onecan see that the onset of the probe signals corresponds tothe time that Bt reaches to the ECR field. After the onsetof discharge, the probe signal vanishes because the peak ofdensity profile moves to the low field side. At the last ofdischarge, probe signals appear again at the time thattoroidal field become close to the ECR field. These facts
suggest that plasma was produced by ECR. Investigationon the pre-ionized plasma has another purpose as apreliminary examination for the operation using themagnetic surfaces without inductive plasma current. In
this case, it is desirable to adjust the position of initiationof plasma to the position of the magnetic surface.
Formation of the magnetic surfaces without plasma
current will be described in the next section.
Now we are trying to induce the plasma current in theECR pre-ionized plasma described above. Figure 6
shows the typical waveform of tokamak operation.
Toroidal field, current in OH coil, power of RF, plasma
current intensity of optical emission are shown together.
RF having the power of 0.25 kW was injected
continuously. Current in the VF coil was 2000 AT.
The plasma current increases after the onset of the current
in the OH coil. It reaches up to 25 A in this shot.
However, it has to be much improved to generate thesufficient rotational transform of the magnetic field. Weroughly estimated available toroidal current from the loop

B t (R , φ ) = µ 0 NI TF / 2πR (1 + f 1 + f 2 )()f = ∑ ε / ((R / R ) − 1)cos (N (φ + ∆φ )), (1)f 1 = ∑ ε 1 j / (R / R1 j ) − 1 cos (N (φ + ∆φ1 j ))Nj2j2j2jN2jwhere suffix 1 and 2 indicates the inner and outer TF coil,respectively, suffix j (= 1~50) corresponds each coilfilament, ε is a weight of summation (= 1/50), ∆φ is atoroidal angle of each coil filament. As shown in Fig.
4(b), the minimum value of the toroidal ripple and thelocation having minimum toroidal ripple have good
agreement between calculated and analytical value.

3. Tokamak operation and its pre-ionized phase

One of the purposes of study using TOKASTAR-2 isthe investigation of the effect of outer helical field totokamak plasma. We have a plan to induce plasma
current in plasmas produced using the ECR heating. This
ECR plasma is called "pre-ionized phase" of the tokamak
operation. However, it is not trivial that plasmas are
initiated by ECR because the RF wave might be reflected
randomly in a small and non-torus vessel like this device
and there is a possibility that it is not absorbed correctlyat ECR layer. To confirm whether pre-ionized ECR
plasma can be produced or not, plasma parameters were
measured at the position where ECR layer appears.
Figure 5 shows temporal evolution of toroidal field andFig.5

Temporal evolution of toroidal field and ion saturation current measured using Langmuir
double probe. Both values are measured at major radius R = 0.07m and height from themidplane z = 0.045 m. Three cases with different voltage charged for a current discharge
to TF coils are compared.

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T. Oishi et al., Initial Experimental Results and Magnetic Surface Analysis of Tokamak-Helical Hybrid Plasma Confinement Device “TOKASTAR-2”

voltage and Spitzer resistivity deduced from electron
temperature. Te measured using the Langmuir probe isabout 10 eV. Then, Spitzer resistivity η = (N(Z) Z) 1.65× 10-9 lnΛ / Te3/2 = 4.8 × 10-5 Ωm, where ion charge Z= 2 and coefficient N(Z) = 0.85 for helium plasmas, andCoulomb logarithm lnΛ = 17. If we assume the shape oftorus plasma having the major radius R = 0.1 m andminor radius a = 0.04 m, the resistance Rp = η × 2πR /πa2 = 6.0 × 10-3 Ω. The maximum loop voltage Vloop
in this device is about 8 V. Therefore, expected plasma
current Ip = Vloop / Rp = 1.3 kA. It is about fifty times
larger than the value obtained at present. Now we haveplans to increase the loop voltage by upgrading power
supply circuit and optimize the vertical field byequilibrium calculation to increase Ip. If Ip can beincreased by a hundred times compared than that in thepresent status, the safety factor q = aBt / RBp will beapproximately 3 for Bt = 0.1 T.

Fig.7 Additional helical coil for the formation ofvacuum magnetic surface without plasma
current.

4. Formation of magnetic flux surface by applying
outer helical fieldAnother objective of the study is consideration onprobability of formation of magnetic surface by applying
helical fields. Therefore, magnetic field tracing code
HSD (helical system design) [5] is used to investigate
whether vacuum magnetic surfaces can be formed or not in
TOKASTAR-2. Magnet field lines can be calculated inthis code by defining coil configuration and currents ineach coil as input parameters.
This calculation of tracing magnetic line has revealed
that the vacuum magnetic surface can be formed by using
additional helical coil. Figure 7 shows the configurationof the additional helical coils for the formation of vacuum
magnetic surface without plasma current. Last closed
Fig.8

Fig.6

Waveform of tokamak operation.
(a)Toroidal field, (b) current in OH coil andloop voltage, (c) plasma current and (d)intensity of optical emission.

Last closed flux surface formed using theadditional helical coils obtained from thecalculation of magnetic field line tracing.
ITF = 6250 AT, IVF = 15218 AT, IHF = 6250
AT, IH'F = 7500 AT.

Fig.9 Averaged plasma minor radius as a functionof a ratio IHF/IVF in several cases of IHF/ITF.

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T. Oishi et al., Initial Experimental Results and Magnetic Surface Analysis of Tokamak-Helical Hybrid Plasma Confinement Device “TOKASTAR-2”

flux surface (LCFS) having vertically elongated shapeformed using the additional helical coils are shown in Fig.
8. In this case, current in TF coil (ITF), VF coil (IVF), HF
coil (IHF), and additional helical coil (IH'F) was 6250 AT,
15218 AT, 6250 AT, and 7500 AT, respectively. Note that
all the Poincare-plot points are projected in the projectedside view (R-z) and it does not mean that the flux surface
has finite width.
The shape of the magnetic surface varies with theratio between ITF, IVF, IHF, and IH'F. In this paper, we
regard the averaged minor radius <r> defined by <r> = (a× b)1/2, where a and b is short and long radius of LCFS,
respectively, as an indicator of the size of LCFS. Figure 9
show a dependency of <r> on the ratio IHF / IVF. In thisfigure, results in several cases of IHF / ITF are plotted
together. The ratio IH'F / IHF was fixed to be 1.2 in allcases. It has been revealed that IHF / IVF = 0.3 ~ 0.35 is an
optimized ratio to enlarge the cross section.
As a next step, detailed analysis on the characteristicsof the magnetic field structure, such as rotational transformor magnetic well, will be needed to evaluate the propertyof plasma confinement.

5. Summary

New torus plasma confinement device having
tokamak-helicalhybridmagnetic
configuration“TOKASTAR-2” was constructed. It can impose outerhelical field to tokamak plasma by using a pair of helicalcoils located outside toroidal coils and along the toroidal
direction. We confirmed that pre-ionized plasma of thetokamak operation is produced using the electron cyclotron
resonance heating. Ohmic heating was applied to induce
plasma current. However, it has to be further improved torealize stable tokamak confinement.
Calculation ofmagnetic field line tracing has revealed that magnetic
surface can be formed using modified outer helical coils.
Characteristics of the magnetic surface for plasma
confinement should be investigated in the future.

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