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

Observation of separated plasma structure
in non-inductive discharge in QUEST
Yuta HIGASHIZONO, Mizuki SAKAMOTO, Yousuke NAKASHIMA1) , Ryou YONENAGA1) ,Hideki ZUSHI, Kazuaki HANADA, Masaki ISHIGURO, Kazuo NAKAMURA, Hiroshi IDEI,
Makoto HASEGAWA, Kohnosuke SATO, Itaru GOUDA2) , Shoutarou TSURU2) ,Shoji KAWASAKI, Hisatoshi NAKASHIMA, Aki HIGASHIJIMA, Nobuhiro NISHINO3) ,Osamu MITARAI4) , Takashi MAEKAWA5) , Yasuaki KISHIMOTO5) , Yuichi TAKASE6) andQUEST Group
Advanced Fusion Research Center, Research Institute for Applied Mechanics, Kyushu University, Kasuga,
Fukuoka 816-8580, Japan1)
Plasma Research Center, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan
2)
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580,
Japan
3)
Department of Mechanical System Engineering, Graduate School of Engineering, Hiroshima, Japan
4)
Kyushu Tokai University, Kumamoto, Japan
5)
Department of Nuclear Engineering, Graduate School of Engineering, Kyoto University, Kyoto, Japan
6)
Graduate School of Frontier Science, The University of Tokyo, Tokyo, Japan(Received: 20 November 2009 / Accepted: 23 February 2010)

Non-inductive discharge using an 8.2 GHz electron cyclotron heating wave is performed in QUEST.
High-speed (20000 f ps) and medium-speed (500 f ps) cameras are used to observe the plasma behavior.
In an open magnetic ﬁeld, high-speed camera measurements show that a part of the slab plasma near
the center stack separates and moves radially outward. Medium-speed camera measurements show thatthe separated plasma stops around the second harmonic electron cyclotron region at R ∼ 0.6 − 0.8 m.
The plasma current, Ip, correspondingly increases during the plasma stagnation and remains higher
than at other times during stagnation. After the plasma moves to the outer region near the vacuum
chamber, Ip decreases. Hard X-ray measurements in an experiment with the same external magnetic
ﬁeld structure show that the hard X-ray signal increases along with increasing Ip. It suggests thathigh-energy electrons are generated by the stagnation of the plasma at R ∼ 0.6 − 0.8 m.
Keywords: spherical tokamak, non-inductive discharge, Hα image, CCD camera, high energy electron
visible light emission. The plasma behavior observed
by a high-speed camera also provides useful information on the plasma-wall interaction. A monitoring system combining the camera measurement with a software program that can replay captured images justafter the plasma discharge, together with the previous 10 images, has been developed. Therefore, thechange in Hα emission due to the plasma position andhydrogen recycling in each plasma discharge can beobserved.
In the compact spherical tokamak CPD, current
ramp up using 8.2 GHz ECH was demonstrated. A
CCD camera tangentially viewing a poloidal crosssection of the plasma was applied to CPD. Topologicalchanges in the plasma were observed before and after
the current jump. It was reported that the current
jump occurs due to slight diﬀerences in the operating
parameters such as the poloidal ﬁeld (PF) coil currents, ECH power, and so on [4]. During the current jump, a negative ﬂoating potential was observed
through directional Langmuir probe measurements.
These results suggest that high-energy electrons were

1. Introduction
Spherical torus (ST) plasma conﬁnement is an efﬁcient method for achieving high beta plasmas. Since
an ST has only a limited space at the center stack (CS)region to ensure a low aspect ratio, plasma initiation
and ramp-up without an ohmic solenoid is a critical issue. Electron cyclotron heating (ECH) has been used
to initiate the plasma and ramp-up the plasma current in many devices [1, 2, 3, 4]. Full non-inductive
plasma current; up to 20 kA, in a plasma start-up sequence on a conﬁguration, without a center solenoid
was achieved on LATE [3]. Since the physical mechanism of the initiation of plasma current by ECH remains unclear, detailed analysis is needed to clarify
the mechanism.
High-speed camera measurements have been usedin many magnetic plasma conﬁnement device [5, 6, 7,
8]. Observing the plasma behavior with a high-speed
camera is an eﬀective method for visualizing the behavior of edge plasmas as two-dimensional images ofauthor’s e-mail: yuta@triam.kyushu-u.ac.jp

7©2010 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Y. Higashizono et al., Observation of Separated Plasma Structure in Non-Inductive Discharge in QUEST

deeply involved with the current start-up. In QUEST,
non-inductive discharge using 8.2 GHz begun in 2008.
It is crucial to realize the current ramp-up without anohmic solenoid and clarify in detail the mechanism
associated with the current ramp-up.
In this paper, the experimental setup is describedin section 2. Experimental results and discussion arepresented in section 3. The paper is summarized in
section 4.

discharge described in this paper is sustained usinghydrogen gas puﬃng and ECH by 8.2 GHz klystrons.
The maximum ECH power is 200 kW . In a typicalexperiment, the plasma discharge is operated at ﬁveminute intervals. In Fig. 1 (b), the external magnetic
ﬁeld described in this paper is shown. Straight lines
at R ∼ 0.33 m and R ∼ 0.66 m correspond to the ﬁrst
and second harmonic regions of ECH, respectively inthe case of a TF current of ∼ 30 kA.
A high-speed camera (NAC K5) and a mediumspeed camera (DITECT HAS-220) are installed in
M H08 and M H15 ports of QUEST, respectively.
Both cameras view the poloidal cross-sections fromthe horizontal ports. In this paper, the frame speedof the high speed camera and medium-speed camera
is 20000 f ps and 500 f ps. An interference ﬁlter thattransmits 75% of light at wavelengths from 650 nm
to 690 nm is attached in front of a lens so that only
Hα -light (656.3 nm) can be observed in the plasma.
These cameras are connected to a PC in the QUEST
machine room. Two-dimensional images are captured
during the QUEST experiment just after the PC in
the machine room receives a trigger signal, which issynchronized with the start-up of the coil current.
Hard X-ray measurements were performed using
semiconductor detectors (CdTe). The detector measures the range of 3 − 200 keV at an eﬃciency of morethan > 0.3. Raw signals are recorded every 0.2 μs
to study the quick response of hard X-rays just after
the RF waves are injected. Fast electrons collide withions and bulk electrons and radiate in the hard X-ray
range of the spectrum.

2. Experimental Setup
Figure 1 shows toroidal (Fig. 1 (a)) and poloidal(Fig. 1 (b)) cross-sections of QUEST, a medium
sized spherical tokamak device whose chamber heightis 2.8 m and radius is 1.4 m. The diameter of the
CS is 0.4 m. The major (R) and minor radii of theplasma are 0.68 m and 0.4 m, respectively. In thispaper, the Z-axis and R-axis correspond to the vertical and radial directions, respectively, as shown in thepoloidal cross section. Eight toroidal ﬁeld (TF) coils
produce the toroidal magnetic ﬁeld, Bt ∼ 0.25 T at
R ∼ 0.64 m. PF coils are used to create the verticalﬁeld, Bv. The magnetic ﬁeld for plasma conﬁnement
is optimized by adjusting the TF and PF. The plasma

High Speed
Camera@MH08(a)

3. Experimental Results and Discussion
3.1 Bv dependence of plasma current

Midium Speed
Camera @MH15

A vertical ﬁeld scan experiment in an open magnetic ﬁeld structure was performed on non-inductive
discharge #3749. In this experiment, gas was puﬀed
at 0.15 s for 0.1 s. After gas injection, the base pressure increased from 2.5×10−7 T orr to 1.3×10−6 T orr.

CdTe X-ray
Detector@MH05

1st 2nd

(b)

PF2
R[m]
Z[m]

1.5

1
1.0

#3749
0.5

PF1
0.0

Ip [kA]

PF7
-0.5

-1.0

0.5

PF6
-1.5
0.4

0.8

1.2

1.6

0

Fig. 1 (a) Top view of QUEST chamber and installation positions of high-speed camera, medium-speed
camera and X-ray detector. (b) Poloidal crosssectional view of QUEST.

0

5

10

15

20

25

30

35

40

Bv [Gauss]

Fig. 2 Bv dependence of Ip in #3749.

8

Y. Higashizono et al., Observation of Separated Plasma Structure in Non-Inductive Discharge in QUEST

Plasma was produced by 8.2 GHz ECH at 0.4 s andterminated at 0.65 s. The RF power and TF current were kept almost constant at 85 kW and 30 kA,
respectively for the entire plasma duration. The currents of poloidal coils 1, 7 (P F 1, 7) and 2, 6 (P F 2, 6)
were increased at the same rate. Thus, the n-index(decay index) was kept constant during the discharge.
Figure 2 shows the Bv dependence of Ip. In thisshot, the vertical ﬁeld increased from 0 to 35 G at
R ∼ 0.64 m and Z ∼ 0.0 m. As shown in this ﬁgure,Ip was maximum at a Bv of 10 − 15 G.

(a)

1

#3732

Ip [kA]

0.8
0.6
0.4
0.2
0
0.4

0.45

0.5

0.55

0.52

0.53

0.6

0.65

(b)

1

(c)

0.8
0.6
0.4

3.2

Relationship between plasma current and plasma behavior

0.2
0
0.5

(d)

At shot number #3732, Bv was kept constantat 10 − 15 G; other parameters were the same as in#3749. In this magnetic conﬁguration, a distinctive
feature was found in the images captured by the highspeed and medium speed cameras. Figure 3 shows Hαimages captured by the medium-speed camera. Figure 3(a) shows the vertically extended plasma near
the CS at 0.398 s. The slab structure is attributed
to plasma production in the fundamental harmonic
region by 8.2 GHz ECH. In Figs. 3(b) at 0.406 s
and (c) at 0.414 s, the topology of these emission proﬁles changes and the emission peak shifts to the outerside. Tangential images captured by the mediumspeed camera enabled us to obtain the radial positionof the plasma in detail. Figures 4 (a) and (b) show
the time evolution of Ip and a contour plot of themedium-speed camera image data, which is extracted
in the radial direction of the one vertical point indicated by a dotted line in Fig. 3(b), respectively. Figures 4 (c) and (d) corresponds to the expanded timescale of Fig. 4 at 0.5 − 0.55 s. As these graph shows,the plasma current changed repeatedly, and the radial position of Hα emission correspondingly shiftedat the inner(R < 1.0 m) and outer(R > 1.0 m) regions. Interestingly, both cycles were completely thesame. When Hα emission had a peak in the inner region, Ip became higher than the Hα emitted in theouter position.
To examine the origin of the plasma behavior,high-speed camera images captured at high time resolution were used. Figure 5 shows the time evolution ofIp from Fig. 4 at an expanded time scale, 0.4−0.416 s.
Figure 6 shows the time evolutions of images at the

0.51

0.54

0.55

Fig. 4 Time evolution of Ip and contour plot in radial direction of Hα emission captured by medium-speed
camera.

beginning of the discharge #3732. Since the highspeed camera has a horizontal view, as shown in Fig.
1, the CS is identiﬁed at the center of the images. The
capture time of images (a)-(h) run from 0.4002 s to
0.4009 s at 0.0001 s intervals. In Fig. 6(a), vertically
expanded structure corresponding to the fundamental
harmonic region of 8.2 GHz ECH was observed. In
the upper part of Fig. 6(b), a cloud separate from theslab plasma was conﬁrmed. The separated structure
gradually moved away from the slab plasma as shownin Figs. 6 (d) to (f). The separated plasma stopped in
the radial direction and expanded in size, as shown in
Figs. 6 (f) to (h). After this sequence, Ip was considered to increase. Therefore, it was clariﬁed that theplasma separated before the increase in Ip.
Figure 7 shows the radial proﬁles of the image
data captured by the medium-speed camera. The vertical positions of the data are the same as in Fig. 3.
The separation of the plasma described previously occurred between 0.4 s and 0.410 s. After the plasma
separation, the separated structure showed a peak andheld at R ∼ 0.6 − 0.8 m, after which Ip increased
0.8

(b)

(c)

Ip [kA]

(a)

CS

0.4
0.2
0
0.4

R

#3732

0.6

0.402 0.404 0.406 0.408 0.41 0.412 0.414 0.416

Time [s]Fig. 3 Medium-speed camera pictures of #3732.

Fig. 5 Time evolution of Ip in expanded time scale.

9Y. Higashizono et al., Observation of Separated Plasma Structure in Non-Inductive Discharge in QUEST

(b)

(f)

(d)

(g)

Plasma Current

(h)

0.4

Fig. 6 High-speed camera pictures at #3732.

Intensity [a.u.]

250
200

(b)

250
0

Intensity [a.u.]

(a)

200

Intensity [a.u.]

(c)

0.399
0.401

0.403
0.405

0.407
0.409

HX

B

B

B

1.2
1
0.8
0.6
0.4
0.2
0
0.45

0.5

0.55
Time [s]

0.6

3
2.5
2
1.5
1
0.5
0

HX [a.u.]

(e)

(c)

Ip [kA](a)0.65

Fig. 8 Time evolution of Ip and hard X-ray spectrum.

Time [s]

#3732

150

1

100
50
0.413
0.415

0.417
0.419

Time [s]Ip [kA]

0.409
0.411

150

0.5

100
50
0
250

200

0.419
0.421

0.423
0.425

0.427 Time [s]
0.429

0

150

0

100

0.5

1

1.5

HX signal [a.u.]

50
0
0.20

0.40

0.60

0.80

1.0

1.2

Fig. 9 Ip as a function of hard X-ray spectrum.

1.4

R [m]

Fig. 7 Radial proﬁle of Hα emission captured by medium
speed camera.

than that of CPD. We have not achieved the RF power
up to 160 ∼ 240 kW in QUEST.
Hard X-ray measurements yielded useful information associated with the relationship between Ip andplasma separation. Figure 8 shows the time dependence of Ip and the raw output of the hard X-ray data.
In this experiment, the magnetic conﬁguration was thesame as that of #3732, but no information about theradial position of the plasma was available because
the camera was aligned diﬀerently. As shown in thisﬁgure, Ip increased when the hard X-ray signal increased and vice versa. Figure 9 shows Ip as a functionof hard X-ray signal. The plasma current is positively
correlated with the hard X-ray signal. This result suggests that high-energy electrons were generated by thestagnation of the plasma at R ∼ 0.6 − 0.8 m. Since
the second harmonic resonance heating region of electrons is at R ∼ 0.65 m in this magnetic conﬁguration,
electron heating produced the high-energy electrons,
which may be concerned with the increase of Ip.

rapidly. While the stagnation continued until 0.405 s,
Ip remained higher than that after 0.407 s. Therefore, the stagnation of the separated structure, which
was maintained at R ∼ 0.6 − 0.8 m, is believed tohave caused the increase in Ip. The peaked proﬁle ofthe stagnation plasma became ﬂat at 0.407 s, and Ip
simultaneously decreased. The ﬂat proﬁle persisted
until 0.413 s. After that, the Hα emission shifted tothe outer region and showed a peak near the QUEST
chamber at R ∼ 1.2−1.3 m. Then, Ip correspondingly
decreased to a greater degree. The peaked proﬁles in
the outer region terminated at 0.423 s. High-speed
camera measurements showed the slab plasma without
the separated structure after 0.423 s, and the separation of the plasma occurred again between 0.423 s and0.425 s. Thus, the sequence in the plasma behavior
described above was repeated in this discharge. Although Ip increased with the repetitive change of Hαsignal in this experiment, the sheet-shaped plasma did
not change into the tokamak equilibrium state when
Ip increased. One of the reasons may be RF power. As
shown in reference [4], current jump deeply depends
on the RF power. In the CPD experiment, threshold
power of RF for current jump is 20 ∼ 30kW . According to the CPD results, 160 ∼ 240 kW is considered
to be needed for current jump in QUEST because thevolume of the QUEST chamber is about 8 times larger

4. Summary
Non-inductive discharge using 8.2 GHz ECH was
performed in QUEST. In an open magnetic ﬁeld structure, Ip depended on Bv when Bv at R ∼ 0.64 m andZ ∼ 0.0 m changed from 0 to 35 G. The n-index(decay index) was constant. Ip had a maximum at
Bv ∼ 10 − 15 G. When Bv was kept constant at
10−15 G, it was conﬁrmed that part of the slab plasmanear the CS separated and moved radially outward.

10

Y. Higashizono et al., Observation of Separated Plasma Structure in Non-Inductive Discharge in QUEST

Ip was related to the radial position of Hα emission
captured by the medium-speed camera. When theseparated plasma stopped at R ∼ 0.6 − 0.8 m, theIp correspondingly increased. During plasma stagnation at R ∼ 0.6 − 0.8 m, Ip remained high. After the plasma shifted to the region near the vacuum
chamber, Ip decreased. Hard X-ray measurements inan experiment with the same external magnetic ﬁeld
conﬁguration, Ip increased when the hard X-ray signal increased and vice versa. This result suggests thathigh-energy electrons are generated by the eﬀect of theplasma stagnation at R ∼ 0.6 − 0.8 m.

Acknowledgments

This work is supported by the NIFS Collaboration
Research Program（NIFS05KUTR013). This work is
partially supported by a Grant-in-Aid for JSPS Fellows (No. 20-5343).
[1] M. Peng, et al., Phys. Plasmas, 7, 1681, (2000).
[2] Y. Takase, et al., Nucl. Fusion, 46 S598, (2006).
[3] T. Maekawa, et al., Plasma Sci. Technol. 8 95 (2006).
[4] T. Yoshinaga, et al., PFR series 8, 0100, (2009).
[5] Y. Nakashima et al., J. Nucl. Mater., 363-365, 616(2007).
[6] N. Nishino et al., Plasma Fusion Res., 1 035 (2006).
[7] Y. Higashizono et al., J. Plasma Fus. Res., 2 S1087(2007).
[8] D.P. Stotler et al., J. Nucl. Mater. 313-316, 1066
(2003).
[9] H. Zushi et al., J. Nucl. Mater. 363-365, 1429 (2007).

11