J. Plasma Fusion Res. SERIES, Vol. 8 (2009)

Non-Inductive Formation of Spherical Tokamak Plasmasby ECH on CPD
Tomokazu YOSHINAGA, Kazuaki HANADA1) , Kohnosuke SATO1) , Hideki ZUSHI1) ,Kazuo NAKAMURA1) , Hiroshi IDEI1) , Mizuki SAKAMOTO1) , Yousuke NAKASHIMA2) ,Makoto HASEGAWA1) , Yuta HIGASHIZONO1) , Shoji KAWASAKI1) ,Hisatoshi NAKASHIMA1) , Aki HIGASHIJIMA1) , Rajendraprasad BHATTACHARYAY3) ,Kosuke DONO3) , Hiroshi HONMA3) , Masaki ISHIGURO3) , Takashi SAKIMURA3) ,Tomofumi RYOKAI3) and Toshimasa MIYAZAKI3)
National Institute for Fusion Science, Gifu 509-5292, Japan
Research Institute for Applied Mechanics, Kyushu University, Fukuoka 816-0811, Japan
2)
Plasma Research Center, University of Tsukuba, Ibaraki 305-8577, Japan
3)
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Fukuoka 816-0811, Japan
1)(Received: 1 September 2008 / Accepted: 22 January 2009)

Non-inductive plasma current generation and the resultant formation of closed magnetic surfaces
have been achieved by electron-cyclotron-heating (ECH) in the compact plasma-wall-interaction experimental device (CPD). The rapid increase of plasma current called “current jump”, which connects
externally applied open-ﬁeld conﬁguration with closed-ﬁeld equilibrium, has been observed under relatively high vertical ﬁelds. By comparing normal X-mode injection and co-directional O-mode injection,
the critical currents, at which current jump starts and stops, are found to be independent of the incident
wave mode. However, under open-ﬁeld conﬁguration before the current jump, co-directional O-mode ismore eﬀective than normal X-mode in generating plasma currents.
Keywords: spherical torus, non-inductive current drive, electron cyclotron heatinga pressure-driven current, which is a short-circuit current along the tilted magnetic ﬁeld lines to cancel thevertical electric ﬁeld generated by the toroidal drifts[3, 5]. Since this current decreases with the verticalmagnetic ﬁeld (Bv ), the pressure-driven mechanism is
insuﬃcient to form the nested magnetic ﬂux surfaces
by itself especially under high Bv ﬁelds. Recently,
however, it was found that this initial current generation phase is followed by the “current jump” phase,
which is the rapid current rise resulting in the ﬁeld
transition from open conﬁguration to closed ﬂux surface conﬁguration [5, 6]. Moreover, additional current
ramp-up by increasing Bv has been achieved after current jump [7, 8].
The current jump and the succeeding current
ramp-up have been demonstrated most successfully in
the LATE device, which has a slim center post evenamong the STs (the 0.114 m diameter center post in
the 1 m diameter vacuum vessel) [8]. On the otherhand, the other similar scale STs with CSs such as
CPD, TST-2 [9] and CDX-U [3], have the center posts
which are twice as large as LATE (∼ 0.25 m) in similar size vacuum vessels (� 1.4 m). Wider center post
limits the aspect ratio of the plasma loop to a higher
value. Since no current jump has been observed in theconventional tokamaks, it is not certain whether theECH-based current start-up scenario via current jump
can be applicable to such STs with somewhat higher
aspect ratios. Recently, the current jump phenomena

1. Introduction
In tokamak systems, toroidal plasma currents areindispensable in the formation of nested magnetic surfaces for plasma conﬁnement. These plasma currents
are usually generated by ohmic heating (OH), whichuses toroidal electric ﬁelds (ET ) induced by the central
solenoids (CSs). In fusion devices, however, ET will besmall and might be insuﬃcient to start-up plasma currents because those coils should be superconducting
coils (e.g. � 0.3 V/m in ITER [1]). Recently, variousalternative current initiation methods are being studied, especially in spherical tokamak (ST) devices since
the central space of STs are tightly limited. Moreover, if non-inductive current start-up scenarios canbe fully established, ST reactors without CSs, whichis advantageous to realize high β, high bootstrap current fraction, compact shape and low cost, might beconstructed.
Electron cyclotron heating and current drive(ECH/ECCD), which will be used to assist OH startup for reliable avalanche breakdown in ITER [1], isone of the possible candidates for the current initiation method. In many devices, including conventional
tokamaks, generation of toroidal plasma current has
been observed in EC-heated plasmas during the breakdown phase [2, 3, 4]. The non-inductive current generated in the open ﬁeld conﬁgurations is understood as
author’s e-mail: yoshinaga.tomokazu@LHD.nifs.ac.jp

100©2009 by The Japan Society of Plasma
Science and Nuclear Fusion Research

T. Yoshinaga et al., Non-Inductive Formation of Spherical Tokamak Plasmas by ECH on CPD

ment. Microwave at 8.2 GHz from 8 klystrons is transmitted separately to the eight rectangular waveguide
antennas and injected from the weak ﬁeld side. Fourantennas are placed for the injection of the wave vector normal to the ﬁeld line and of the electric ﬁeld
inclined at 22.5◦ to the vertical (Z) axis (we call thismode “X-mode” for simplicity). The remaining fourantennas are used for the injection of the “O-mode”,
which has the electric ﬁeld orthogonal to the X-mode.
The waves in O-mode can be injected tangentially by
the movable mirrors. The working gas is hydrogen.
Plasma current is measured by the Rogowski coil,and magnetic ﬂux proﬁles are estimated from the45 ﬂux loop signals by using a similar technique tothat mentioned in [10]. In addition, a lithium (Li)
sheet beam imaging technique for density proﬁle measurement [11, 12], a ﬁltered high-speed (1 ms) camera,
and an ultra-high-speed (25 μs) camera are used tomonitor the plasma shapes.

3. Experimental Results
Figure 2 (a)-(b) shows time evolutions of a typicalcurrent jump discharge under a weakly mirror-shaped
steady vertical ﬁeld at Bv = 39 G and n (decay index) = 0.1 (both at the vessel center, R = 0.33 m)
as shown in Fig. 2 (d). During the weak microwave(Prf ∼ 1 kW) injection before t = 0.12 s under the ﬁlling gas pressure at ∼ 10−3 Pa, cylindrical plasma is
generated on the fundamental EC resonance (ECR)
layer for a 8.2 GHz microwave (B = 0.29 T) at
R = 0.19 m. As Prf increases after t = 0.12 s[Fig. 2 (a)], plasma current (Ip ) also increases gradually with Prf and reaches 1.2 kA under Prf = 50 kW
at t = 0.154 s, as shown in Fig. 2 (c). Then, thecurrent jump phase is turned on and Ip rapidly increases up to 2 kA in ∼ 3 ms, and keeps this value
until Prf starts to decrease at t ∼ 0.32 s. Magnetic
analysis shows that the externally applied open magnetic ﬁeld shown in Fig. 2 (d) changes into the upward
shifted closed ﬂux conﬁguration as shown in Fig. 2 (e)in this current jump phase. The aspect ratio of thelast closed ﬂux surface (LCFS) is R0 /a ∼ 2, while
the major radius (R0 ) and the minor radius (a) are
R0 ∼ 0.2 m and a ∼ 0.1 m, respectively. This transition of ﬁeld topology via current jump can be veriﬁed
by both high-speed camera images and sheet Li beam
images [12]. Moreover, sheet Li beam diagnostics suggests that the electron density exceeds the cutoﬀ density for a 8.2 GHz EC-wave (8.3 × 1017 m−3 ) after
current jump [12]. This suggests that the electronBernstein wave heating might be taking place in thisphase.
The result that the current jump has been observed in CPD as well as in LATE and in TST-2
demonstrates that this phenomenon can occur com-Fig. 1 The CPD device.

has been observed under small Bv (� 10 G) and low
microwave power (Prf � 5 kW) injection in TST-2[6]. However, it seems still too early to conclude that
the ECH-based scenario is well reliable, since the Bv
is limited to a very small range. In order to conﬁrm
this, ECH has been applied to CPD under higher Bv(� 50 G) by using higher Prf (� 60 kW). The experimental results showed that the current jump can occur
also in CPD and suggested that the current start-up
by ECH/ECCD may be applicable to some extent.
The dependencies of the current jump phenomena on
the microwave power, the injection modes, and thevertical ﬁelds are also investigated. This paper describes the experimental setup in section 2, experimental results in section 3, discussions in section 4,and summary in section 5.

2. Experimental Apparatus
Figure 1 shows the schematic of CPD. Both of
the inner diameter and the height of the stainless-steel
vacuum vessel are 1.2 m. The center post, whose outer
diameter is 0.22 m, encloses four turns of toroidal
ﬁeld coils and CS. Around the mid-plane, it is covered by four pieces of quarter round tungsten-coatedshield, the outer diameter and the height of which is∼ 0.255 m and 0.3 m, respectively. Three sets of outerpoloidal ﬁeld coils (PF17, PF26 and PF35), which canbe driven separately by the pre-programmed powersources, are used. The CS is not used in this experi-101

T. Yoshinaga et al., Non-Inductive Formation of Spherical Tokamak Plasmas by ECH on CPD

Fig. 3 Ip - Prf characteristics for co-directional O-mode
injection and normal X-mode injection. Bv = 39 G,
n = 0.1 at R = 0.33 m.

Prf . While the co-directional O-mode injection seems
to be slightly more eﬀective than the normal X-mode
injection, the current generation eﬃciencies of those
two injection modes are quite similar before current
jump. However, there is a clear diﬀerence between
these modes in the current jump phase. The threshold Prf at which the current jump starts is Prf ∼ 20 23 kW in co-directional O-mode, while it is ∼ 26 29 kW in normal X-mode. Since Ip before and after
current jump seems to be independent of the injection
modes as the TST-2 results [6], the essential parameter for the occurrence of the current jump should be Ip
itself, which determines the critical poloidal ﬁeld conﬁguration by the self ﬁeld. The diﬀerence between thetwo injection modes would be related to the ECH eﬃciency to raise the electron pressure and the pressuredriven Ip to some threshold level which is needed tostart the current jump.
In Fig. 4, Ip with and without current jump areplotted against Bv for various discharges with variousPrf � 60 kW. Clear current jumps can be observed
under relatively high Bv ﬁelds (� 30 G). Higher Ip(� 2 kA) can only be achieved via current jump under this Bv range, while Ip saturates at ∼ 1.7 kA
without the current jump. This shows that the current jump is essential to obtain the closed-ﬁeld equilibrium at higher Ip under higher Bv . In contrast, underlower Bv (� 30 G) ﬁelds, current jump is not observed
clearly. In such discharges Ip increases slowly with increase of Prf as shown in Fig. 5 (a) and (b), and reaches
to a certain level that is proportional to Bv . Since theratio of Ip to Bv without clear current jumps is similar
to that with current jumps (dIp /dBv ∼ 0.08 kA/G),
closed-ﬂux equilibria may be formed even without cur-Fig. 2 Typical discharge with current jump. (a) Prf , (b)Ip and Bv , (c) enlarged Ip around current jump, (d)external poloidal ﬁeld, (e) closed-ﬂux conﬁguration
after current jump (t ∼ 0.18 s).

monly in STs. A larger aspect ratio (R0 /a ∼ 2) than
the typical case in LATE (R0 /a ∼ 1.4) [8] can be allowed as well as the twice wider center post.
By the careful wall conditionings and the optimizations of the ﬁlling gas pressure, current jump canbe observed at a lower Prf than the case in Fig. 2,even under the same Bv condition. In Fig. 3, the transient values of Ip during the initial phase of such optimized discharges are plotted against the transient Prfin normal X-mode and co-directional O-mode. Since
the increasing rates of Prf (dPrf /dt � 1 kW/ms) areconsidered to be very small by considering the energy
conﬁnement time of CPD plasmas (� 10 μs), the relationships can be understood as a steady state dependencies. These characteristics do not depend on theincreasing rate of Prf . The values of Ip before current jump show the monotonic increases with Prf inboth modes. Since Ip in this phase is considered to be
the pressure driven current, the increase of Ip should
correspond to the increase of plasma pressure with102

T. Yoshinaga et al., Non-Inductive Formation of Spherical Tokamak Plasmas by ECH on CPD

4. Discussion
To hold a toroidal plasma loop in equilibrium thefollowing relationship should be satisﬁed:
μ0 Ip
Bv =4πR��
8R0li
3ln+ − + βp,eq ,a2 2(1)

where li is the internal inductance and βp,eq is thepoloidal beta for equilibrium of the plasma loop. The
linear increase of Ip with Bv in the rate of dIp /dBv ∼0.08 kA/G shown in Fig. 4 suggests that a certain constant βp,eq ∼ 0.7 on the assumption of R0 � 0.2 m,a � 0.1 m and li ∼ 1 for plasma loop. By assuming thebulk electron density ne ∼ 1 × 1018 m−3 and the bulk
electron temperature Te ∼ 10 eV for typical parameters in CPD, the bulk electrons’ βp scales as βp,bulk= 8π 2 a2 ne Te /μ0 Ip2 ∼ 1 × Ip−2 (Ip in kA), and becomes
1 when Ip = 1 kA. In order that the equilibrium condition under higher Bv may be satisﬁed only by the bulk
electrons, the relationship that βp,bulk = βp,eq ∼ 0.7
must be satisﬁed. This means that the bulk electronpressure should rise with the square of Bv as well as
Ip , which is unlikely in CPD plasmas with the poor
conﬁnement times (� 10 μs). The contribution fromthe tail electrons (βp,tail ), which are considered as thecurrent carriers during and after current jump phase[5, 7], is therefore suggested to exist to keep the constant βp,eq especially under higher Bv ﬁelds in placeof βp,bulk , which would decrease with Bv . This is alsosuggested by the directional Langmuir probe experiments in CPD, in which the existence of high-energy
electrons is suggested only in the direction of positive
Ip generation.

Fig. 4 Ip as a function of Bv under Prf � 60 kW.

Fig. 5 Discharge without current jump under lower Bv .
(a) Prf , (b) Ip and Bv , and (c) poloidal ﬂux structure estimated at the ﬂattop. Bv = 28 G withn = 0.34 (both at R = 0.33 m) is applied.

5. Summary
Non-inductive current generation and the formation of the closed-ﬂux equilibrium have been achieved
via current jump by using a 8.2 GHz EC-wave on
CPD. The current jump can be obtained under higher
Bv (� 30 G), while Ip slowly increases with Prf under lower Bv (� 30 G). The values of Ip before andafter current jump does not depend on the injectionmode, while the threshold Prf at which the current
jump starts is lower in co-directional O-mode than in
the normal X-mode. These results suggest that thecritical parameter for current jump is Ip itself, andtherefore the poloidal ﬁeld structure. The injection
modes are related to the current generation eﬃciency
in the open-ﬁeld conﬁguration.
Since the aspect ratio of the Q-shu University with
Steady State Spherical Tokamak (QUEST) [13], whichis the next step larger ST device, is quite similar tothe aspect ratio of CPD, it can be expected that thenon-inductive current start-up via current jump would
be applied to the QUEST operation.

rent jump as well as in the current jump discharges.
However, this type of discharge is less eﬀective in Prf .
In the discharge shown in Fig. 5 Prf ∼ 40 kW must
be injected to achieve Ip ∼ 2 kA even under the optimized circumstances, while in the current jump discharge the same Ip can be achieved by Prf � 30 kWeven within the less eﬀective normal X-mode.
There is an oﬀset of Bv ∼ 5 G in the Ip - Bv
characteristics as shown in Fig. 4. Related to this, theupward shift of the LCFS structure is seen as shownin Fig.2 (d), while it is suppressed under the Bv ﬁeld
with higher n as shown in Fig.5 (c). These mightsuggest the existence of some error ﬁelds in verticaldirection.

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6. Acknowledgment
This work was performed with the support andunder the auspices of the NIFS Collaboration Research Program (NIFS05KUTR007).
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