Proceedings of ITC/ISHW2007

Experimental conditions for electron Bernstein wave heating by useof EC waves injected from high-field side in CHS
Y. Yoshimura, K. Nagasakia, S. Ferrando-Margalet, T. Akiyama, M. Isobe, A. Shimizu, C. Suzuki, C.
Takahashi, K. Nagaoka, S. Nishimura, T. Minami, K. Matsuoka, S. Okamura, CHS group,
S. Kubo, T. Shimozuma, H. Igami, H. Takahashi, and T. Mutoh
National Institute for Fusion Science,322-6 Oroshi-cho, Toki 509-5292, Japana

Institute of Advanced Energy, Kyoto University, Uji 611-0011, Japan

In the Compact Helical System (CHS), electron heating by electron Bernstein waves was experimentallyinvestigated to study a technique for the high-density plasma heating over cutoff. The EBWs are excited throughmode conversion process from X-mode waves injected to plasmas from the high-field side. In the experiment,
within a range of oblique EC wave beam injection angle, evident heating effect was observed. Dependences of theheating effect on the wave's polarization and the toroidal injection angle show that the absorption of the modeconverted EBWs should be the cause of the plasma heating effect.
Keywords: electron Bernstein wave, EBW, slow X-B, high-density plasma heating, high-field side
injection, ECH, CHS

1. Inroduction
Electromagnetic (EM) plasma waves such as
ordinary (O) or extraordinary (X) mode waves suffer
cutoff in high-density plasmas, and the EM plasma waves
cannot contribute to electron heating over the cutoff
density. Electron Bernstein (B) waves, on the other hand,
have the advantages of an absence of density limit and astrong absorption even in low-temperature plasmas. Since
the B-waves are a kind of electrostatic wave in plasmas,
they have to be excited by means of mode conversion
processes from injected EM-waves. Three types of mode
conversion process are considered: the so-called fast X-B,
slow X-B, and O-X-B. Among them, the O-X-B mode
conversion technique [1] has been considered the mostpromising way to heat overdense plasmas because ECH
systems including steerable beam injection antennas fromthe low-field side are technically available on existing
tokamaks and helical systems [2-6].
Compared to the O-X-B process, the slow X-B
process can more simply and easily realize mode
conversion to B-waves, since the difficulty of achieving
high O-X mode conversion efficiency does not exist.
When injected through a fundamental resonance layerfrom the high-field side (so called the X-B accesswindow), the X-mode EC-waves propagate into theplasmas and are mode converted into the B-waves at theupper hybrid resonance (UHR) layer. The B-waves areabsorbed at the Doppler-shifted electron cyclotron
resonance, resulting in plasma heating. When injectedaway from the X-B access window, the X-mode ECauthor’s e-mail: yoshimura.yasuo@LHD.nifs.ac.jp

waves suffer right-hand cutoff and cannot heat theplasmas effectively.
So far in the WT-3 tokamak, using an O-X
polarization twister installed at the high-field side, Bwave heating was performed by injecting O-mode ECwaves to avoid the right-hand cutoff of X-mode waves[7]. Also in the COMPASS-D tokamak, electron heatingand current drive by the high-field side X-mode injection
were demonstrated [8]. However, generally speaking, ECwave injection from the high-field side (that is, from theinner side of the torus) is accompanied with difficulties
due to insufficient available space for installing an ECwave injection antenna system or a reflection mirror.
The Compact Helical System [9] provides a good
opportunity to investigate the slow X-B heating scenario
experimentally, since due to its two helical coils
generating plasma confining magnetic field, it has two XB access windows in the poloidal cross section. In thevertically elongated poloidal cross section, one window isat the inner side of the torus in a position similar totokamaks, while the other is at the outer side where awider space is available for installing an elaborate
structure such as the movable mirror for beam direction
scan, as seen in Fig. 1. Installing a mirror inside the CHS
vacuum vessel, the slow X-B experiments wereperformed [10]. In this paper, the dependences of theslow X-B heating effect on the wave's polarization andthe beam direction are presented as follows. The
experimental setup such as CHS and the EC wave
injection system are described in Sec. 2. The slow X-B
experimental results are presented in Sec. 3. Then Sec. 4

Proceedings of ITC/ISHW2007

LCFSRCUHR
inner FR0

innerhelicalcoil- 40
60

vacuumvessel wallBouter FRX
newmirror

outerhelical
coil

100

140

distance in radial direction (cm)Fig. 1 Experimental configuration for slow X-B experiment.

2. Experimental Setup
The CHS is a helical device with the toroidal period
number m = 8 and the polarity l = 2. The magnetic fieldstructure with the rotational transform for plasma
confinement is generated totally by the external coils
such as a couple of helical coils and three pairs ofpoloidal coils. The major radius of CHS plasma is about
1.0 m and the averaged minor radius is 0.2 m, so that theaspect ratio is 5. The magnetic field at the plasma axis
can be set up to about 2.0 T.
The ECH system on CHS furnishes two gyrotrons.
The operating frequency of one of them is 54.5 GHz andthat of another one is 106.4 GHz. The slow X-B
experiments described in this paper were performed withthe 54.5 GHz one. The transmission line for the 54.5 GHz
waves was constructed as a quasi-optical Gaussian beamtransmission system using 12 mirrors, three of theminside the CHS vacuum vessel and two of them l/4 and l/8
grating polarizers. The three inner-vessel mirrors are
installed on the top port and the final plane mirror can betilted 2-dimensionally so that the injected EC-wave beam
direction can be scanned in both the poloidal and toroidal
directions. The beam injections from the top port result in
the injections from the low-field side. The injected ECwave beams are circularly focused having the beam size
(1/e radius of the electric field amplitude) of 22 mm at theequatorial plane. Using the two polarizers, thepolarization of EC-wave beams can be varied arbitrarily.

1000800
600
40020003#233456NBIECH

600

2400

120005432100050

100150time (ms)radiation power (kW)40power (kW)from existing
final mirror

The total length between the gyrotron output window andthe CHS plasma center is about 17 m. The maximum
injection power and pulse length of the 54.5 GHz waves
to the CHS vacuum vessel are 415 kW and 100 ms,respectively.
In addition to the existing EC-wave power injection
system described above, a new plane mirror was installedinside the vacuum vessel between plasma and an outerhelical coil. By directing the EC-wave beam from theexisting antenna system at the top port to the new mirror,
an injection of 54.5 GHz EC-wave from the high-fieldside becomes possible. The beam is reflected 39 degrees
upward and can be steered in the toroidal direction bytilting this new mirror toroidally. The experimental
configuration is schematically drawn in Fig. 1.

line-ave. electron
plasma storeddensity (x 1019 m-3) energy (kJ)summarizes the contents of this paper.

200Fig. 2 Waveforms in the slow X-B discharge.

3. Dependences of heating effect on the EC-wave
beam direction and the polarization
Figure 2 shows a typical time evolution of adischarge of the slow X-B heating. An EC-wave power of275 kW was obliquely injected in three pulses during thedischarge at incidence angles of 20 degrees
counterclockwise in the toroidal direction and 39 degrees
upward. The wave polarization was set at nearly the Xmode. The first pulse was for plasma generation, and thesecond and the third ones were applied to the plasmas
sustained with 845 kW neutral beam injection (NBI). The
plasma stored energy significantly increased with thesecond and the third injections of ECH power. The
central electron temperature increased from about 1.0 to
1.5 keV by the second injection and from about 0.6 to 1.2
keV by the third injection, while the line-average electron

Proceedings of ITC/ISHW2007
density linearly increased during the plasma duration.
The increases in the plasma stored energy were thencaused by increases in the electron temperature. The
electron temperature profiles measured using Thomson
scattering measurement during and just before the third
ECH power injection are plotted in Fig. 3. It can beclearly seen that the electron heating occurred at theplasma core region, not at the peripheral region where theouter fundamental resonance layer exists. The heating
effect is considered to be a result of the B-wave heating,and the resultant improvement in NBI heating efficiency
and reduction in radiation power from the plasma.

just before ECH (100ms)
during ECH (110ms)#233456

10.58090
100 110 120
distance in radial direction (cm)Fig. 3 Electron temperature profiles just before EC-wave
injection and during injection measured using Thomson
scattering measurement.

At the third injection timing of this discharge, theline-average electron density reached close to the O-mode
cutoff density of 3.8x1019 m-3 for the 54.5 GHz EC-waves.
In other discharges, the heating effect was also observedfor plasmas even with the density over the O-mode cutoff.
The magnetic field on the plasma axis was set at 1.95 T,that is, the fundamental resonance magnetic field of the54.5 GHz EC-waves. Here, an essential point regarding
the magnetic field setting is not the on-axis resonancecondition but the presence of another fundamental
resonance layer in the plasmas (X-B access window) infront of the new mirror as seen in Fig. 1.
In the toroidal scan of the EC-wave beam direction,effective plasma generation and significant plasma
heating occurred only at the counterclockwise injection.
Otherwise, the plasmas could not overcome the radiation
barrier. This dependence can be understood as follows.
Due to the rather upward (39 deg) beam reflection fromthe new mirror, the beam path is beyond the range of theLCFS

0-10-20

mirror center

7010

plasma axis

020

direction of B (deg)electron temperature (keV)

1.5

X-B access window when the beams are injected with atoroidal incidence angle of around 0 degrees or clockwise.
However, because the helical coil winding of the CHS isleft-handed, that is, the vertically elongated poloidal crosssection rotates counterclockwise around the magnetic
axis with the toroidal angle, the X-B access window
moves upward and “opens” for beams injected
counterclockwise.
In the polarization scan of the injected EC-waves, it
was confirmed that the heating effect degraded when thepolarization was set at nearly O-mode. Figure 4 shows
the variation of the direction of the magnetic field alongan EC-wave beam path which aims at the magnetic axis
and is injected perpendicularly to the flux surfaces. The
beam path is on the equatorial plane. The definition of theangle is as follows. Looking from outside of the torus, thetoroidal direction in the right side is 0 deg, and the angle
increases counterclockwise. At the LCFS, the direction is-13 deg. In the perpendicular injection case, the O-mode
injection is realized by setting the linear polarization withthe electric field oscillation in the direction of -13 deg,
and the X-mode in the 77 deg.

-30
80 85 90 95 100 105 110 115 120
major radius (cm)Fig. 4 Variation of the magnetic field direction along a beam
path perpendicular to the flux surfaces.

The dependence of the heating effect on the linear
polarization direction is plotted in Fig. 5. The heating
effect is evaluated with the value of the plasma stored
energy during EC-wave injection divided by that justbefore injection. Only at the polarization direction of -10deg, the heating effect vanished. In the slow X-B
experiment the EC-waves were injected obliquely, thenthe linear polarization with the direction of -13 deg does
not mean the pure O-mode but the O-mode purity is
considered high. This plot shows that the X-mode
component is important for the heating, and that the mode
conversion to the B-wave would be the key for theplasma heating. Setting pure O- and X-mode polarization

Proceedings of ITC/ISHW2007

Wp during ECH / Wp before ECH

by the elliptical polarization would make the dependence
clearer.

1.6
1.5~ O-mode~ X-mode

1.4
1.3
1.2
1.1
1
0.9
0.8
-50

050
100
150
linear polarization direction (deg)Fig. 5 Dependence of the heating effect on the polarization of
the EC-waves.

The numerical calculations to confirm the realizationof slow X-B heating in the experimental configuration are
under investigation. Some of the results can be seen inthis conference [11].

4. Conclusions
In CHS, the slow X-B heating technique wasinvestigated by using an inner-vessel mirror whichenables the EC-wave beam injection to plasmas from thehigh-field side through the fundamental resonance layer.
As a result of the beam direction scanning, the increasesof the plasma stored energy and the electron temperature
were performed only when the EC-wave beams wereinjected through the X-B access window. The polarization
scanning experiment showed that nearly O-mode
polarization had no heating effect. Those results anddependences show that the slow X-B heating was realizedin CHS.

Acknowledgements
The authors would like to express their thanks to theCHS staff for performing the CHS experiments. This
work was carried out under the framework ofbidirectional collaborative research program between
Kyoto University (Heliotron-J group) and NIFS (Codenumber: NIFS04KUHL005).

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