23P102-P
Divertor experiments in ohmic discharges of QUESTQUESTのオーミック放電におけるダイバータ生成実験
MITARAI Osamu (1), NAKAMURA Kazuo (2), HASEGAWA Makoto(2), ISHIGURO Masaki(2), et al.,
御手洗 修(1), 中村一男(2)，長谷川 真(2)，石黒正貴(2)，出射 浩(2)，藤澤 彰英(2)，花田和明(2)，図子 秀樹(2)，H. Q. Liu(2),東島 亜紀(2)，中島 寿年(2)，川崎 昌二(2)，QUEST グループ(2)，福山 淳(3)(1) Tokai University, 9-1-1 Toroku, Kumamoto 862-8652, (2) RIAM, Kyushu Univ.
(1)東海大 〒862-8652 熊本市渡鹿９−１−１, (2)九大応力研,(3)京大工(3) Kyoto Univ.

The plasma current start-up and ramp-up during two types of the divertor operations are experimentallystudied in the limited central solenoid (CS) flux in QUEST. Depending on the inner and outer divertor
coil configurations, two types of the divertor operations have been conducted and compared to have ahigher plasma current. Initial coil current setting for breakdown is carefully planned for both operations.
At present, the inner divertor operation provides larger plasma current because the reverse induction issmaller than others.

1. Introduction
Although the toroidal field is small in a sphericaltokamak, the safety factor can be large due to the low
aspect ratio. Therefore a large plasma current can beflown stably. However, as the flux of CS is small, it
is difficult to induce the large plasma current.
Moreover it is difficult to induce a large plasma current
during the divertor operation because the divertor coilcurrent induces the plasma current oppositely.
As the divertor coil current flows in the same
direction of the plasma current, it reduces the verticalfield for equilibrium. Therefore, careful operational
plan is needed to increase the plasma current during thedivertor operation with a limited flux of CS.
In this work we have conducted the divertor operation
in QUEST with a small CS flux of 80 mVsec. In
QUEST two divertor coils are installed. They are theinner and outer divertor coils. Therefore, essentially
two types of divertor operation are possible. We
present these two type discharges and compared theircharacteristics.

1. Inner and outer divertor operation[1] Determination of the breakdown condition
To minimize the reverse induction by the divertor coileffect on the plasma current, the time constant divertor coilcurrent is initially imposed. As CS has a cancellation coils,
the vertical field at the plasma center is given byequilibrium code. Coil currents of IPF17= 0.9 kA andIPF26=1.15 kA were set with IPF35-12= -1.9 kA and ICS=0 kA
for βp=0.8 to have a divertor configuration. This is shownin Fig. 2-(Left).
PF coil currents
from the breakdown
condition to the final
operating point arelinearly increased.
The temporal
evolution of theBreakdown
plasma current andNo Breakdown
divert coil currentsetc are shown in Fig.
Fig. 1 Relationship between initial
3. The initial
PF17 and series-connected divertor
divertor coil current
coil current for vertical stray field.
is IPF35-12=-1 kA andPF17 coil current is
IPF17= 0.6 kA as shown in Fig. 1. Therefore, the initial
vertical field is almost 5.4 Gauss. CS flux is fully
utilized for the plasma current ramp-up, and the plasmaPF35-1 PF35-2

PF26
PF17

BV [0.68m]= - 4.6166 × 10 IPF35-12 [A]+8.600 × 10 IPF17 [A] (1)-6-6

where IPF35-12 is the inner and outer divertor coil current
connected in series, and IPF17 is the vertical field coilcurrent. To apply the divertor coil current, the verticalfield coil is also applied to reduce the stray field forbreakdown. This relationship is shown in Fig. 1. Bv=0
means that the vertical field is cancelled by PF17 coil. In
experiments breakdown took place at the open circle.
[2] Determination of the divertor configuration
Final equilibrium is determined by the Hasegawa

Fig. 2. (Left): Magnetic surface by Hasegawa equilibrium
code. (Right): Magnetic surface at t=1.549s obtained by
EFIT at ICS=0 after experiments (#11631).

current reaches 40 kA. (We note that in the limiter
discharge the plasma current is ramped–up to 100 kA by
the same CS flux (80mVs) but by the larger vertical field.
The plasma is placed at the inboard side.) It is seen thatthe divertor coil current induces the loop voltage to aslightly positive value, which means the reverse induction.
EFIT code shows the magnetic surface as shown inFig. 2-(Right). Almost divertor like cross section has
been obtained at ICS=0 (t=1.54s). As the CS coil current
cannot be reversed in the present system, the duration of
the divertor configuration is very short. To have alonger divertor discharge, double swing operation of CS
should be introduced.

11631-SP-HF

1000-1(d)-3＃11631400
PlasmaRposition_val(mm)200(i)

0600-200

observed in these experiments that inner divertor
operation is producing more plasma current for the same
CS flux.
EFIT code shows the divertor like magnetic flux
surface as shown in Fig. 4-(Right) at ICS=0 (t=1.44s).

-400
1.60Fig.3.
Temporal evolution of the inner and outer
series-connected divertor discharge in QUEST. (a) Plasma
current, (b) CS current (c) PF17 and PF26 coil current (d)PF35-12 coil current connected in series (e) RF power (f)loop voltages (g) magnetic flux loop signals (h) Oxygen
impurity and (i) plasma position. (#11631).

2. Inner divertor operation[1] Determination of the breakdown condition
When the inner divertor PF31 coil alone is used, thevertical field at the plasma center is given by
BV [0.68m]= - 0.5433 × 10 -6 IPF31 [A]+8.600 × 10 -6 IPF17 [A]-40 (a)-60-800-2-4(b)-6-8-10210.5
0012712cs_current

SP_CUR_SIG(kA)
SP_CUR_VAL(kA)
HF_CUR_SIG(kA)
HF_CUR_VAL(kA)

1.5(c)

21 (f)
0-1-2-3
12712-Vloop-4
0.2
flux00(Vsec)
0.15
flux02(Vsec)
0.1
flux04(Vsec)
0.05
0 (g)
-0.05
-0.1
108 12712-Oxygen
6 (h)
420
1200

12712-SP-HF-21000-4 (d)-6127121-PF35-1-current-8100
80 12712-RFCD-PC
60 (e)
40200
1.2
1.3
1.4
Time(sec)vloop00(V)vloop02(V)vloop04(V)12712-Flux

800＃11631400
PlasmaRposition_val(mm)

(i)

2000

600

-200

Z[mm]

Ip(kA)

1.6

IC S(kA)

1.5

12712plasmaCurrent-20

Vloop(V)'RFnet(kW)'

Flux (Vs)

1.5

Oxygen'RFCD(kW)'

1.4Time(sec)

R [mm]

1.3

IPF26 (kA)

400
1.2

IPF17 (kA)

PRF (kW)11631-Oxygen(h)PlasmaZposition_val(mm)11631-PF35-12-current-4100
80 11631-RFCD-PC
60 (e)
40
20
0
1.2
1.3
1.4
Time(sec)

800

11631-Flux(g)IPF35-1 (kA)-2

flux00(Vsec)
flux02(Vsec)
flux04(Vsec)

Fig. 4. (Left): Magnetic surface by Hasegawa equilibrium
code. (Right): Magnetic surface at t=1.44s obtained by
EFIT at ICS=0 (#12712).

vloop00(V)vloop02(V)vloop04(V)11631-Vloop

PRF (kW)(c)

R [mm]IPF35-12 (kA)

00(f)

Z[mm]

SP_CUR_SIG(kA)
SP_CUR_VAL(kA)
HF_CUR_SIG(kA)
HF_CUR_VAL(kA)

10.5

Flux (Vs)11631cs_current

Oxygen(b)1.5

210-1-2-3-4
0.2
0.15
0.1
0.05
0-0.05-0.1
10864201200

Vloop(V)11631plasmaCurrent(a)

IPF26 (kA)

IPF17 (kA)IC S(kA)

Ip(kA)

0-10-20-30-40-50
0-2-4-6-8-10
2current is induced up to 60 kA by full CS flux. It is

PlasmaZposition_val(mm)400
1.2'RFnet(kW)'

1.3

1.4Time(sec)

1.5-400
1.6'RFnet(kW)'

1.5

1.6(2)

where IPF35-1 is the inner divertor coil current. Larger innerdivertor coil current IPF31 can be applied because it
produces a small opposite vertical field to the equilibrium.
[2] Determination of the divertor configuration
Using the Hasegawa equilibrium code, PF coil currentsof IPF17= 0.95 kA and IPF26=0.8 kA were set with the innerdivertor current of IPF35-1= -5 kA and ICS=0 kA for βp=0.8
to have a divertor configuration. This is shown in Fig.
4-(left).
The temporal evolution of the plasma current anddivertor coil current etc are shown in Fig. 5. As the time
constant divertor coil current of 5 kA is imposed, noreverse induction effect exists. As initial vertical field is24 Gauss, it is larger than the previous case. The plasma

Fig.5. Temporal evolution of the inner divertor discharge inQUEST. (a) to (i) are the same as in Fig. 3. (#12712).

This result may show that an inner divertor with asnowflake divertor may be a better choice for a ST tohave a larger plasma current. As series-connected innerand outer divertor corresponds to the Super X divertor(SXD), more studies are needed from the viewpoint oframping-up the large plasma current with SXD.
References[1] MITARAI Osamu, NAKAMURA Kazuo, et al., (2010)
Plasma and Fusion Research: Volume 6, 1402003(2011)[2]MITARAI Osamu, NAKAMURA Kazuo, et al. J.
Plasma Fusion Res. SERIES, Vol. 9 (2010) p100-105.