Proceedings of ITC/ISHW2007

ECCD Experiments in Heliotron J, TJ-II, CHS and LHD
K. Nagasakia), G. Motojimab), A. Fernándezc), Á. Cappac), J. M. Fontdecabac), Y. Yoshimurad), T.
Notaked), S. Kubod), T. Shimozumad), H. Igamid), K. Idad), M. Yoshinumad), T. Kobuchid),
Heliotron J Teama) b), TJ-II Teamc), CHS Teamd) and LHD Teamd)a) Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto, Japanb) Graduate School of Energy Science, Kyoto University, Gokasho, Uji, Kyoto Japanc) Laboratorio Nacional de Fusión, EURATOM-CIEMAT, Madrid, Spaind) National Institute for Fusion Science, Oroshi-cho, Toki, Gifu, Japan
Electron cyclotron current drive (ECCD) experiments have been made in stellarator/heliotron (S/H) devices
such as Heliotron J, TJ-II, CHS and LHD. The experimental results show that ECCD can be controlled by thepower injection angle, absorption position and magnetic field structure. The current drive efficiency is similar, γ =neIECR/PEC=8-16×1016 A/Wm2, ζ=32.7neIECR/PWTe =0.03-0.05. The reversal of driven current direction is observed
depending on the magnetic field ripple structure, indicating that the amplitude and direction of EC current isdetermined by the balance between the Fisch-Boozer effect and the Ohkawa effect, and that the Ohkawa effect isstrong in S/H devices compared with tokamaks. Control of net toroidal current by using ECCD is demonstrated; net
zero current state is attained by cancelling the bootstrap current.
Keywords: electron cyclotron current drive, stellarator, heliotron, Heliotron J, TJ-II, CHS, LHD, current drive
efficiency, Fisch-Boozer effect, Ohkawa effect

1. Introduction
Non-inductive current has an important role on therealization of high performance plasmas and thesustainment of steady state plasmas in toroidal fusion
devices. In S/H systems, no Ohmic current is required forequilibrium since the confinement magnetic field isgenerated by external coils. However, it is known that
finite plasma pressure drives bootstrap current as well as
in tokamaks, which modifies rotational transform profile,resulting that the equilibrium and stability is affected. For
example, the bootstrap current increases the rotationaltransform in LHD, moving the rational surface of ι/2π=1
to low shear region, giving rise to the confinement
degradation due to the formation of magnetic island [1].
Electron cyclotron current drive (ECCD) is
recognized as a useful scheme for stabilizing MHD
instabilities and analyzing heat and particle transport[2][3]. For example, in large tokamaks such as JT-60U,
neoclassical tearing mode has been stabilized by localized
ECCD, leading to the improvement of normalized beta
[4]. In S/H systems, on the other hand, the ECCD is
expected to avoid dangerous rational surface bycancelling the bootstrap current particularly in low shear
S/H devices. From the viewpoint of diagnostics, the S/Hsystems have the advantage of precise measurement of
the EC current. The estimate of EC current is not so
author’s e-mail: nagasaki@iae.kyoto-u.ac.jp

simple in tokamaks since a large amount of Ohmic
current flows, and the effects of toroidal electric field andplasma resistivity have to be taken into account. In the
S/H systems, we are able to measure the EC current withthe accuracy of the order of less than 1 kA by using a
conventional Rogowskii coil because of no Ohmic
current.
A systematic research on ECCD in S/H systems was
performed experimentally and theoretically in W7-AS for
the first time [5] [6]. Several results such as thedependence on injection angle and electron density andthe effect of trapped particles were discussed. Recently
international collaboration research on ECCD has been
conducted in Heliotron J (Kyoto Univ.) [7], TJ-II(CIEMAT) [8], CHS (NIFS) [9][10] and LHD (NIFS)[11] in order to understand the ECCD physics and toinvestigate the applicability of ECCD to the control ofplasma equilibrium and stability. This paper summarizes
these recent experimental results on ECCD. The
comparison of experimental results from these four
devices will give us common understanding of the current
drive physics.
This paper is organized as follows. The experimental
set up including the ECH/ECCD system is described inSec. 2. The experimental results, especially thedependence on injection angle, electron density andmagnetic field structure are shown in Sec. 3. The trappedProceedings of ITC/ISHW2007(a) Heliotron J

particle effect generated by the magnetic field ripple isdiscussed, which strongly affects the current drive
efficiency. Summary is given in Sec. 4.
B/B(ϕ=45deg)

1.1
1.0
0.9

ECH injection port
(for fixed N||)
(for N|| scan)straight section

0.8
0.7

0

15

30

45

60

75

90

Toroidal Angle (deg)1.05(b) TJ-II
ECH
injection port

B/Bmax

1.00

0.95

L1
0.90

0

15

L2

30

45

60

75

90

Toroidal Angle (deg)1.05(c) CHS

1.00

B/B0

Heliotron J, TJ-II, CHS and LHD are S/H fusion
devices, which have the device parameters of major
radius, R=1.2 m, minor radius, a=0.1-0.2 m, magnetic
field, B=1.5 T (Heliotron J), R =1.5 m, a =0.1-0.2 m, B
=1.0 T (TJ-II), R =1.0 m, a =0.2 m, B =1.9 T (CHS), andR =3.75 m, a =0.6 m, B =2.8 T (LHD). The plasma
parameters discussed in this paper ranges as
ne=0.2-1.5×1019 m-3, Te=0.3-2 keV and Ti=0.1-2 keV. The
plasmas are produced and heated by ECH only, and theupper density limit is determined by second harmonic
X-mode cut-off, nec = 1.75×1019 m-3 for 53.2 GHz (TJ-II
and CHS), nec = 3.0×1019 m-3 for 70GHz (Heliotron J), nec= 4.4-×1019 m-3 for 84 GHz (LHD) and nec = 17.5×1019
m-3 for 168 GHz (LHD),
The magnetic field structure is different among four
devices since the field spectrum is different. Figure 1
illustrates the magnetic field profiles along the magnetic
axis. Heliotron J has a capability of controlling the coilcurrent independently, making it possible to change themagnetic ripple structure at the launching position. The
magnetic field has a structure from local maximum(ripple top structure) to local minimum (ripple bottom
structure) at the ECH power deposition. In TJ-II, the EC
power is injected near the local minimum position. In
CHS and LHD, the magnetic field ripple is small along
the magnetic axis at the magnetic axis location of R=0.95
m and R=3.75 m, respectively, and the ripple structureappears when the magnetic axis is shifted by verticalcoils.
The ECH/ECCD system is routinely used for plasma
production and heating in S/H systems. Since the S/Hsystem has 3-D magnetic field structure, wide injection
angle range and precise polarization control are required
to change the absorption profile and parallel refractive
index. Recent progress on high power millimeter wave
technology gives us focused Gaussian beam injection
system with injection angle and polarization wellcontrolled. Table 1 shows the main features of
ECH/ECCD system in Heliotron J, TJ-II, CHS and LHD(see the references for details). The focused Gaussian
beam is injected with a wide range of both toroidal andpoloidal injection angle by using a steering mirror, whichenables us to have enough oblique launch angle for large
refractive index. In Heliotron J, a focused Gaussian beamis launched from the corner section (the toroidal angle,
ϕ=0 deg) for N|| scan, and non-focused Gaussian beam islaunched from the straight section (ϕ=45 deg) for theother scans. In TJ-II, two unit 53.2 GHz ECH/ECCD

Configuration
5:1
6:1
5:2
5:3
5:4 a
5:4 b

1.2

0.95

Raxis
92.1 cm
94.9 cm
97.4 cm

Vertically elongatedcrossection
ECH injection port
0.90

0

15

30

45

Toroidal Angle (deg)(d) LHD1.05
R=3.53m
R=3.60m
R=3.75m
1.00

B/Bmax2. Experimental Setup

1.3

0.95

Horizontally elongatedflux surface

0.90

0.85

ECH injection port
1.5L
0

5

10

15

ECH injection port
2-O
20

25

30

35

Toroidal Angle (deg)Fig. 1 Magnetic field profile along magnetic axis
in (a) Heliotron J, (b) TJ-II, (c) CHS and(d) LHD.

Proceedings of ITC/ISHW2007
1.5(a) Heliotron J
19-3

ne=0.5x10 m

1.0

PEC=300kW

0.5

Ip (kA)systems are used, in which a steering mirror is installed ineach injection system. The injection position issymmetric with respect to the toroidal angle, leading that
the EC current can be widely controlled by scanning theinjection angle with different N||. In CHS, the EC beam is
injected from the top of torus, and the refractive index is0.16 in vacuum at the magnetic axis of R=0.95 m for thetoroidal angle of 7 deg. In LHD, the toroidal angle is 8.3
deg at vertical port (1.5L port), 9.9 deg at horizontal port
for co-injection, and it is -5.9 deg at vertical port, -9.9 deg
for counter-injection.

0.0
-0.5
-1.0
-1.5
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

N||
Heliotron J

TJ-II

CHS

LHD

3

(b) TJ-II#14945-14987
QTL2

=0

QTL1

=0

QTL1 N||

Frequency

70GHz

53.2GHz

53.2GHz

84GHz

Maximum
injection
power

0.4MW

0.3MWx2

0.3MW

1.3MW

Maximum
pulse length

0.2sec0.5sec0.1sec

3sec

Injection
mode

Focused/nonfocused
Gaussian

FocusedGaussian

FocusedGaussian

FocusedGaussian

Injection
angle

Controllable/
fixedControllable

Controllable

Controllable

Polarization

Controllable

Controllable

Controllable

Controllable

2nd X

1st
O/2nd X2

QTL2 N||Ip (kA)

1
0
-1
19

-2
-3

2nd X

Reference

[12]

2nd X
[13]

[14]

[15]

Table 1 Heating systems for ECCD experiment in
Heliotron J, TJ-II, CHS and LHD

Ip (kA)

Injectionmode

8

Plasmas are produced and heated by the second
harmonic X-mode ECH. A rough estimation ofdevelopment time of total toroidal current is given by
Lp/Rp where Lp and Rp are the plasma inductance andresistance. Lp/Rp is estimated from 0.1 to a few sec byassuming neoclassical conductivity. The measured toroidal
current is continued to increase during discharge at low
densities so that the current is somewhat underestimated in
the low-density regime. Finite current may modify therotational transform, affecting global confinement. However,neither strong confinement degradation nor MHD

instabilities were observed in the experiment reported
here. Details of ECCD experiment in each device aredescribed in Refs. [7-11]
As predicted from the ECCD theory, the amplitude
and driven direction depends on the parallel refractive-3

PEC=200kW/200kW
-0.4

-0.2

0.0
N||

0.2

0.4

#113284-113340(c) CHS6

R=94.9cm
19 -3
ne=0.5x10 m

4

Pinj=250kW

2
0
-2
-4
-6

3. Experimental Results

ne=0.4x10 m

-0.2

-0.1

0.0

0.1

0.2

N||

Fig.2 Dependence of toroidal current on EC injection
angle. The magnetic axis in CHS is 94.9cm.
index, N||. Figure 2 shows the dependence of the toroidal
angle on the EC injection angle. It can be seen that Ip
increases with increasing N||, and saturates at a certain N||.
ECCD is a main contribution to the total current since thebootstrap current is small due to the low pressure at ne =0.5×1019 m-3. The flow direction is the one expected fromthe Fisch-Boozer effect opposite to N|| [16]. The sametendency is observed in LHD when the injection angle ischanged from the clockwise to counter-clockwise
direction. The EC current amplitude is the same order, a
few kA, up to now in all the devices, although themagnetic field structure is different. As discussed later,
the toroidal direction of EC current strongly depends on

Proceedings of ITC/ISHW2007|ω /ω|=0.488
2.0
1.0

N =0.44

0||

Bootstrap current
ECCD
STD Configuration
P =320kW

I (kA)

EC

0.0
-1.0
-2.0
0.0

0.5

1.0

1.5

19

2.0

-3

n (10 m )e

Fig.3 Separation of bootstrap current and EC current
in Heliotron J.
2

N||~0.4

1
0

IEC (kA)

the magnetic ripple structure. Under the condition fortoroidal injection scan, the Fisch-Boozer effect may bestronger than the Ohkawa effect. In the injection angle
scan with the magnetic field fixed, the electron cyclotron
resonance is Doppler-shifted due to finite N||, resultingthat the decrease in electron temperature and/or thechange in ripple structure possibly affects the EC current.
The N|| dependence considering the Doppler shift effect isleft for future.
The measured non-inductive current in ECH
plasmas is composed of bootstrap current and EC current.
The exclusion of bootstrap current is required for
accurate estimation of the EC current. One method is toestimate the bootstrap current by EC beam launch in the
direction normal to the magnetic field where thebootstrap current should be dominant. Another way is to
use their different dependence on the magnetic field
direction. The bootstrap current, which is proportional to
B×∇B drift, changes the flowing direction whenreversing the magnetic field, while that of the EC current
associated with the B strength does not change its flowing
direction. Magnetic field reversal experiments have been
conducted in Heliotron J in order to separate the EC
current from the bootstrap current. We confirmed that the
global plasma parameters such as stored energy and Te
did not change when reversing the magnetic field
direction. Figure 3 shows the density dependence of
estimated bootstrap current and EC current in Heliotron J.
The bootstrap current increases with increasing plasma
density, and saturate at ne >1.0×1019 m-3. This amplitude
of bootstrap current agrees with a neoclassical prediction[17]. The bootstrap current is less than 0.5 kA at low
density, which is smaller than the EC current, so that thetoroidal current is mainly driven by ECCD.
In Heliotron J, the magnetic field ripple structure at
power deposition position can be widely changed bycontrolling current in each coil. As illustrated in Fig. 1,
the power deposition varies from the ripple top to theripple bottom mainly by changing the bumpinesscomponent in field spectrum with the magnetic fieldstrength fixed. Figure 4 shows the density dependence ofEC current for three field configurations. Here thecontribution of bootstrap current is eliminated by the fieldreversal experiment. The maximum EC current of 4.6 kA
is attained at the ripple top heating (the ratio of themagnetic field at the straight section to that at the cornersection, h=1.06). The EC current flows in the opposite
direction at the ripple bottom heating (h=0.82), and its
amplitude is one-third as low as the ripple top heating.
One reason for current reversal is that velocity space
effects are responsible for the ECCD. The Fisch-Boozer
effect considers the perpendicular excursion in thevelocity of a group of electrons with positive v||.

-1
-2
h
0.82
0.95
1.06

-3
-4
-5
0.0

0.5

1.0
19

1.5

2.0

-3

ne (10 m )Fig.4 Density dependence of EC current in
Heliotron J. The circle, triangle and squaresymbols denote the EC current at the ripple
bottom (h=0.82), flat (h=0.95) and ripple
top (h=1.06) heating, respectively.
Acceleration of these electrons causes an excess ofelectrons with counter-clockwise v||, resulting in a current
in the clockwise toroidal direction. On the other hand, theOhkawa effect drives current in the opposite direction to
the Fisch-Boozer current [18]. Asymmetry in v|| is lost
due to the bounce in the magnetic ripple, and a deficit invelocity space generates an electrical current in the
counter-clockwise toroidal direction. In the highbumpiness configuration, the electrons are accelerated in
the valley of the ripple, and they tend to become trapped,
thus enhancing the Ohkawa effect. These qualitative
predictions are consistent with the experimentallymeasured ECCD direction. The transition from the
Fisch-Boozer current drive to the Ohkawa current has
been demonstrated also in Ref. [5]. These suggest that the
Ohkawa effect has comparable strength to the
Fisch-Boozer effect in S/H systems, and the ECCD

Proceedings of ITC/ISHW2007
1

0.7

0.6

iota

IEC (kA)0-1

ϕ=-7deg

R=94.9cm
19 -3
ne=0.4x10 m#112243-112255
ϕ=+7deg

Pinj=215kW

-2
0.85

0.90

co-ECCD

0.5

0.4

ctr-ECCD

0.3
0.2

0.3

#112256-112267

0.95

1.00

1.05

0.4

Baxis (T)Fig.5 EC current as a function of magnetic fieldstrength in CHS
direction is determined by the difference between them.
The experimental results on magnetic field scan in
CHS are shown in Fig. 5. The injection angle is +7 deg
and -7 deg, and the magnetic axis is fixed; Raxis＝94.9 cm.
The Doppler shift of cyclotron resonance is estimated as(ω-2ωce)/ ω ∼ 1 % for Te=1 keV and N||=0.16, meaningthat the Doppler shift effect is weak. The EC current islargest when the resonance is located at magnetic axis.
The flowing direction is consistent with the Fisch-Boozer
effect. The reduction in current drive efficiency at
off-axis ECCD is observed also in Heliotron J. This may
be because the electron temperature is decreased and/or
the ripple effect is large.
In low shear devices, even a small amount of current
strongly affects rotational transform profile. According to
HINT2 code simulation results on a Heliotron J
configuration, a localized current of -5 kA changes thecentral rotational transform from 0.56 to 0.18, generatinghigh magnetic shear at the core region. The rotationaltransform profile has been measured by an MSE
diagnostic in LHD. Figure 6 shows the measurement
results for co-ECCD, no ECCD and ctr-ECCD. The
co-ECCD increases the central rotational transform, viceversa. This tendency qualitatively agrees with thedirection of poloidal magnetic field generated by themeasured EC current. The quantitative comparison withmeasured current will be done in the future experiment.
The ratio of driven current to injection power,
IEC/PEC, and the figure of merit,γ=

co 1.2s
co 1.8s
ctr 1.2s
ctr 1.8s

ne I EC R
PEC

0.6

0.7

0.8

Fig. 6 Measurement of rotational transform by
MSE diagnostic in LHD.
proposed in the following form [3]

ς=

nI R
e3 ne I EC R
= 32.7 e EC2
ε 0 PECTePEC Te(2)where parameters have a unit of ne in 1020 m-3, IEC in A, R
in m, PEC in W, and Te in keV. This dimensionless figureof merit includes important parameters such as ne and Te.
If ζ changes under the same plasma conditions, it meansthat ζ reflects the effect of electron thermal velocity andtrapping. Table 2 summarizes the ECCD efficiency on
Heliotron J, TJ-II and CHS. No result from LHD is
included since it is not estimated yet due to short ECH
pulse length compared to the current evolution time. It
should be noted that these efficiencies are typical values
ever obtained, not optimized ones. Although the magnetic
field structure is different among the devices, the EC
current amount is a few kA in all the devices, and the
ECCD efficiency is similar within a factor of 2. Rather
low efficiency compared to tokamaks may be due to thestrong Ohkawa effect enhanced by the magnetic ripple.
The ray tracing calculation code is under development,
which would clarify the role of trapped electrons bycomparing between experiment and theory.
Heliotron J

TJ-II

CHS

Maximum IEC

4.6 kA

2 kA

6 kA

η=IEC/PEC

14 A/kW

10-15 A/kW

35 A/kW

γ=neIECR/PEC∼ 8x1016
A/Wm2∼ 9x1016
A/Wm2∼16x1016
A/Wm2

ζ=32.7n20IARm/
PWTkeV

∼0.05

∼0.03

∼0.04

(1)

are conventionally used for the estimation of ECCD
efficiency. The drawback of these functions is that they
have dimension, and they do not reflect the Te
dependence. A figure of merit describing dimensionless
ECCD efficiency including the Te dependence is

0.5
ρTable 2 ECCD efficiency in Heliotron J, TJ-II and CHS.

Proceedings of ITC/ISHW2007(a) Heliotron J#208542

19-3

ne (10 m )1
02

Wp (kJ)1
0
2

Ip (kA)
070GHz ECH

-2

200

250

300

Time (msec)(b) TJ-II

maximum current ever observed is 6 kA, and the ECCD
efficiency
is
γ=neIECR/PEC=8-16×1016
A/Wm2,
ζ=32.7neIECR/PECTe=0.03-0.05. In S/H systems, themagnetic ripple structure strongly affects the ECCD. The
direction of EC current is reversed in Heliotron J when the
power is deposited at the ripple bottom position, indicatingthat the electron trapping is a key factor to determine theEC current.
The EC current is comparable to the bootstrap
current, and the ECCD has a potential to control therotational transform profiles. The control of net current
has been successfully demonstrated by compensating thebootstrap current with the EC current. Since the bootstrap
current profile should be different from the EC current
profile, we will need to extend the controllability ofrotational transform profile connected to the suppressionof MHD instabilities.

Acknowledgements
The authors are grateful to the Heliotron J, TJ-II, CHS
and LHD staff for conducting the experiments. This work
was partly supported by NIFS/NINS under the NIFS
Collaborative Research Program (NIFS04KUHL005) andunder a project sponsored by the Formation ofInternational Network for Scientific Collaborations.

References[1]

Fig.7 Demonstration of control of toroidal current,(a) zero toroidal current state in Heliotron
J, (b) cancellation of EC current using twounit ECH systems in TJ-II
While the ECCD efficiency is not so high, the EC
current is comparable to the bootstrap current, meaningthat we are able to control the total toroidal current. Zeronet current state has been demonstrated in Heliotron J as
shown in Fig. 7. The total current is suppressed below 0.4
kA during the discharge by compensating the bootstrap
current of 1.5 kA with ECCD. Such a state has also been
demonstrated in CHS. In TJ-II, the EC current driven byeach launcher is cancelled and the low current of IBS=-0.5
kA is kept. The multi ECCD systems are useful for
extending the current control range.
4. Conclusion
The ECCD experiment has been performed in
Heliotron J, TJ-II, CHS and LHD. The driven current isaffected by plasma and field parameters such as N||,
collisionality, resonance position and magnetic fieldstructure. The injection angle scan experiment indicatesthat the control ability for EC current is high. The
estimated driven current and its efficiency are similar. The

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