can check the current page or check for previous versions at the Internet Archive. Yahoo! is not affiliated with the authors of this page or responsible for its content.
Control of Non-Inductive Current in Heliotron J EX/P6-14
1
Control of Non-Inductive Current in Heliotron J

K. Nagasaki
1)
, G. Motojima
2)
, M. Nosaku
2)
, H. Okada
1)
, T. Mizuuchi
1)
, S. Kobayashi
1)
,
K. Sakamoto
1)
, K. Kondo
2)
, Y. Nakamura
2)
, H. Arimoto
2)
, S. Watanabe
2)
, S. Matsuoka
2)
,
T. Tomokiyo
2)
, K. Y. Watanabe
3)
, A. Cappa
4)
, F. Sano
1)

1) Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan
2) Graduate School of Energy Science, Kyoto University, Uji, Kyoto 611-0011, Japan
3) National Institute for Fusion Science, Toki, Gifu 509-5292, Japan
4) Laboratorio Nacional de Fusión, EURATOM-CIEMAT, Madrid, Spain

e-mail contact of main author: nagasaki@iae.kyoto-u.ac.jp

Abstract. Non-inductive current of electron cyclotron heated (ECH) plasmas has been examined in the helical
axis heliotron device, Heliotron J. The bootstrap and EC currents are separated by comparing the experiments
with positive and negative magnetic field. The estimated bootstrap current is found to be affected by the
magnetic field configuration. It increases with an increase in the bumpy component of the magnetic field
spectrum, which agrees well with a neoclassical prediction using the SPBSC code. The EC current driven by
oblique launch with respect to the magnetic field strongly depends on the field configuration and the EC power
deposition location. The EC current is enhanced when the EC power is deposited on the magnetic axis. The
maximum EC current and current drive efficiency are I
EC
= -4.6 kA,
= n
e
RI
p
/P
EC
=8.4
×10
16
A/Wm
2
,
respectively. The EC current changes its flowing direction depending on the magnetic field ripple structure
where the EC power is deposited.

1.

Introduction

Control of non-inductive toroidal current is one of key issues to realize high performance
plasmas in toroidal fusion devices. In helical systems, the toroidal current such as Ohmic
current is not required for plasma equilibrium since the confinement magnetic field is
generated by external coils. However, finite plasma pressure inherently drives non-inductive
current, so called bootstrap current, which affects the equilibrium and stability due to the
change in rotational transform. The bootstrap current has been experimentally studied in CHS
[1], LHD [2] and W7-AS [3] with regards to transport and MHD stability. Theories predict
that the bootstrap current can be suppressed by optimizing the magnetic field spectrum [4][5].
From the diagnostic point of view, helical systems are advantageous to precise measurement
of the total plasma current because of no inductive current. Small current of less than 1 kA is
possible to measure by using conventional Rogowski coils.

Electron cyclotron current drive (ECCD) is recognized as a useful scheme for controlling
rotational transform and magnetic shear related to the heat/particle transport, equilibrium and
stability. In helical systems, ECCD is considered to suppress the bootstrap current in order to
tailor the current density profile. Furthermore, the detailed study on ECCD in helical system
deepens our understanding of the ECCD physics in toroidal devices. ECCD in helical systems
was measured first in W7-AS [6]. Although the sophisticated investigation was performed,
the EC current was estimated by applying some theoretical results, that is, the EC current was
obtained by substituting the calculated inductive and bootstrap currents from total current
experimentally measured. Net plasma current was maintained to be zero in order to avoid the
effect of low-order rational values at the plasma edge degrading the confinement properties. EX/P6-14
2
In Heliotron J, we have experimentally estimated the EC current without using any numerical
calculation. No significant degradation of plasma confinement has been observed in the ECH
plasmas reported in this paper.

The objective of this paper is to study experimentally the properties of the non-inductive
toroidal current in ECH plasmas. A helical-axis heliotron device, Heliotron J, has high
flexibility to control the magnetic field configuration, making it possible to investigate the
properties of the bootstrap and EC currents in a wide range of magnetic field configurations.
The organization of this paper is as follows. The experimental setup is described in Sec. 2.
The experimental results on non-inductive current are shown in Sec. 3. The dependence of the
bootstrap and EC currents on the magnetic configuration and the EC power deposition is
discussed. Conclusions are given in Sec. 4.

2.

Experimental setup

Heliotron J is a medium sized plasma experimental device [7]. The device parameters are as
follows; the plasma major radius, R = 1.2 m, the averaged minor radius, a = 0.1-0.2 m, the
rotational transform,
/2 = 0.3-0.8, and the maximum magnetic field strength on the
magnetic axis, B
B
0
= 1.5 T. The coil system is composed of an L=1, M=4 helical coil, two types
of toroidal coils A and B, and three pairs of vertical coils. The Heliotron J device provides a
wide variety of magnetic configuration by changing the current ratios in each coil. In
particular, the bumpiness component, which is introduced as a third knob to control
neoclassical transport, is changed by controlling the currents in toroidal coils A and B. Three
configurations are chosen, that is, b
=B
04
B
/B
B
00
at
= 0.67 as 0.01 (low bumpiness), 0.06
(medium bumpiness) and 0.15 (high bumpiness) with keeping the toroidicity, helicity,
rotational transform and plasma volume almost fixed. Here B
mn
B
is the Fourier component of
the magnetic field strength in the Boozer coordinates.

The total toroidal current is measured by Rogowski coils wound on the inner wall of the
poloidal cross-sections at two different toroidal angles, that is, the corner and the straight
sections. We confirmed that the toroidal current by two Rogowski coils was almost the same.
The resistive current diffusion time is about 100 msec in the Heliotron J plasma parameters.
In the experiment reported here, the measured toroidal current is saturated within the pulse
length at n
e
> 0.5×10
19
m
-3
, but it continues to rise up during the discharge at lower density so
that the current is underestimated in low density regime.

A 70 GHz ECH system [8] is used for studying the non-inductive current in Heliotron J.
Plasmas are produced and heated by the 70GHz second harmonic X-mode ECH for which the
cut-off density is 3.0×10
19
m
-3
. The injected power is up to 350 kW, and the maximum pulse
length is 120 msec in the experiment reported here. The unfocused Gaussian beam is
launched from the top of the torus at the straight section where the flux surfaces are
bean-shaped, and the B-contour forms a saddle-type profile. Although the wave beam is
injected perpendicularly with respect to the equatorial plane, it crosses the resonance layer
obliquely because of the 3-D magnetic field structure, resulting that the finite parallel
refractive index, N
||
= 0.44, drives the EC current. The polarization can be controlled from the
X-mode to the O-mode by a linear polarizer installed in a miter bend. In the ECH experiment, EX/P6-14
3
the central electron and ion temperatures range 0.3-1.0 keV and 0.15-0.2 keV, respectively.

A poloidal cross-section in medium bumpiness configuration is shown in Fig. 1, from which
the EC beam is launched. The magnetic field, B = 1.25 T, is located at the magnetic axis at

0
/
=0.50. Here,
0
and
are the electron cyclotron frequency on the axis in the straight
section and the injected wave frequency, respectively. A ray tracing calculation using the
TRECE code [9] shows that the EC power is deposited at off-axis of = 0.2 due to the
Doppler shift resonance,
-k
||
v
||
= 2 ce
. When the magnetic field strength is set lower as 0
/

= 0.49
, the resonance layer moves
toward the helical coil so that the
EC power can be deposited at
on-axis. Electron cyclotron
emission diagnostic using a
multi-channel radiometer confirms
that centrally peaked T
e
profile
was formed at 0
/
= 0.49.
Transmitted wave measurements
[10] shows that the single pass
absorption rate is estimated about
90 % consistent with the ray
tracing results, indicating the
single pass absorption has main
contribution to plasma heating.
(b) 0
/
=0.49
(a) 0
/
=0.50
FIG. 1. Poloidal cross-sections of ECH injection port at (a) 0
/ = 0.50 and (b) 0
/

= 0.49. The solid line
corresponds to the magnetic field strength, B = 1.25 T.

3.

Experimental results
3.1

Separation of bootstrap current and EC current

Figure 2 shows the dependence of the measured toroidal current on the line averaged electron
density at 0
/
= +0.50 and
0
/
= -0.50 at medium bumpiness configuration. Here the sign
denotes the clockwise and counter-clockwise direction of magnetic field looking from the top
of the torus.