Evidence of O-X-B Heating in CHS
Yasuo Yoshimura1), Kazunobu Nagasaki2), Tsuyoshi Akiyama1),
Mitsutaka Isobe1), Akihiro Shimizu1), Chihiro Suzuki1),
Chihiro Takahashi1), Kenichi Nagaoka1), Shin Nishimura1),
Takashi Minami1), Keisuke Matsuoka1), Shoichi Okamura1)
and CHS group
1)
Shin Kubo , Takashi Shimozuma1), Hiroe Igami1), Takashi Notake1)
and Takashi Mutoh1)
1)
2)
National Institute for Fusion Science, Toki, Gifu, 509-5292, Japan
Institute of Advanced Energy, Kyoto University, Uji, Kyoto, 611-0011, Japan
Abstract. An evident increase in the plasma stored energy by applying 54.5 GHz electron
cyclotron (EC) waves has been observed in plasmas sustained by neutral beam injection (NBI)
with line-averaged electron density of above 3.8x1019 /m3, that is, the cut-off density of the 54.5
GHz waves, in the Compact Helical System (CHS). The heating effect can be seen even for
high-density over 8x1019 /m3: more than twice the cut-off density. The 54.5 GHz EC-wave beam
is obliquely injected to high-density plasmas. The scanning of the EC-wave beam direction gives
the evident increase in the stored energy at the beam direction where the leakage EC-wave
power from the CHS vacuum vessel becomes almost the minimum. The most possible cause for
this heating mechanism is an electron Bernstein wave heating via an Ordinary – eXtraordinary –
Bernstein (O-X-B) mode conversion process.
Email of Y. Yoshimura: yoshimu@ms.nifs.ac.jp
1. INTRODUCTION
Electromagnetic (EM) plasma waves such as ordinary (O) or extraordinary (X)
mode waves suffer cut-off in high-density plasmas, and EM plasma waves can not
contribute to electron heating over the cut-off density. Electron Bernstein (B) waves,
on the other hand, have advantages of no density limit and strong absorption even in
low temperature plasmas. Since the B-waves are a kind of electrostatic waves in
plasmas, they have to be excited through mode conversion processes from injected
EM-waves. Three types of mode conversion processes are considered, so called fast
X-B, slow X-B and O-X-B. Among them, the O-X-B mode conversion technique is
the most promising way to heat overdense plasmas by using existing ECH systems in
tokamaks and helical systems considering the required power sources and steerable
beam injection systems from the low field side. If the injection angle of the O-mode
wave satisfies a critical condition [1], the O-mode wave is mode-converted into the
slow X-mode wave at the cut-off layer, and then the slow X-mode wave is modeconverted into the B-wave at the upper hybrid resonance layer through kinetic
processes. However, despite many theoretical and experimental investigations, so far
the phenomena related to the O-X-B mode conversion were clearly demonstrated only
in W7-AS [2-4] and TCV [5] due to the narrow mode conversion window.
2. O-X-B EXPERIMENTS IN CHS
ECH
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In this paper recent experimental results are presented, which show an evidence of
O-X-B heating in the Compact Helical System [6]. A 54.5 GHz ECH system in CHS
is equipped with a 2-D steerable beam injection mirror antenna, which enables us to
find the optimum injection condition experimentally. Using this antenna system, ECwave power of 415 kW was obliquely injected in the toroidal direction twice during a
discharge. The first one was for plasma generation, and the second one was applied to
high-density plasmas sustained with 860 kW neutral beam injection (NBI). The
magnetic field on the magnetic axis was set at 1.9 T, slightly below the resonant
magnetic field 1.95 T. Figure 1 shows a typical time evolution of O-X-B heating.
200
FIGURE 1. Time traces of a discharge of O-X-B heating. A significant increase in the stored energy
was obtained by application of EC-waves to a high-density plasma over the cut-off. A horizontal thin
line on the bottom column denotes the cut-off electron density for the waves of frequency 54.5 GHz.
leakage EC-wave power (a.u.)
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Here, the O-mode cut-off density for the 54.5 GHz waves is 3.8x1019 /m3. A clear
increase in the plasma stored energy of more than 30 % was observed by the second
EC-wave power, and the stored energy was kept high until the end of the EC-wave
application. On these discharges, intensive gas-puffing was continuously applied,
resulting in the monotonous increases of the density. At the end of the second ECwave application, the density reached over 8x1019 /m3. Without the EC-wave
application after 112 ms, the NBI power could not sustain this high-density plasma,
and the radiation collapse occurred. During the second EC-wave application, the
radiation power from the plasma was reduced probably due to the suppression of
radiation losses at the peripheral region by slightly increasing or keeping the electron
temperature all over the plasmas, and/or a reduction of accumulation of impurities.
By scanning the EC-wave beam direction, the leakage EC-wave power from the
CHS vacuum vessel varied systematically as seen in Fig. 2. The leakage power level is
considered as a measure of mode conversion and/or power absorption efficiency. The
toroidal angle tor scan was performed by keeping the radial beam position at the
equatorial plane fixed at R = 92.1 cm where the magnetic axis was set. Here both the
beam steering mirror and the detector for leakage power measurement were set at the
poloidal cross section of tor = 0 deg. Beam direction scanning in the radial direction
also resulted in the variation of leakage power having a minimum. Around the beam
direction where the leakage power reached the minimum, the stored energy showed
evident increases. The increments of the stored energy are also plotted in Fig. 2.
Among the 4 variations (left-hand and right-hand circular, two orthogonal directions
of linear) in polarization with the beam direction optimized, the left-hand circular
polarization which is supposed to contain a high fraction of O-mode showed the best
performance. The discharge shown in Fig. 1 was performed with the optimized beam
direction (tor = -9 deg. and R = 92.1 cm) and the left-hand circular polarization.
FIGURE 2. Variations of leakage power of EC-waves from the CHS vacuum vessel and the maximum
increment of plasma stored energy during the second EC-wave application as functions of the EC-wave
beam injection direction. The leakage power was averaged over the period of the second EC-wave
application.
Most of plasma volume is enclosed with plasma cut-off layer due to high density in
O-X-B experiments as seen in Fig. 3. No heating effect was observed for normal beam
injections (k// = 0 with tor = 0) with O-mode aiming at the fundamental resonance
heating layer at the plasma peripheral region where the density was less than the cutoff. Therefore, the significant increase in the stored energy observed with oblique
incidences should be attributed to the O-X-B heating at the core plasma region and the
resultant decrease in the radiation losses, not to the cyclotron resonance heating at the
peripheral region.
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increment of stored energy (kJ)
FIGURE 3. Experimental configuration in typical O-X-B experiment. Poloidal cross sections at tor =
0, -11.25 and –22.5 degrees are plotted. Magnetic field on axis is 1.9T and line-averaged electron
density is 7x1019/m3. Co-axial ellipses in black denote the magnetic surfaces, and ellipses in blue:
plasma cut-off layers, red curves: fundamental resonance layers, black curves: mod-B surfaces.
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FIGURE 4. Dependence of O-X-B heating effect on magnetic field setting. The increment of plasma
stored energy is plotted as a function of magnetic field on axis.
Dependence of O-X-B heating effect on magnetic field setting was investigated by
magnetic field scanning keeping other experimental conditions such as ECH beam
direction, polarization, ECH power, NBI power and gas-puffing rate. As seen in Fig.
4, the increment of plasma stored energy increases with magnetic field. Here, on-axis
resonance field for 54.5 GHz waves is 1.95T. The dependence will be explained by the
change of power deposition of electron Bernstein waves from edge to core region with
the increase of magnetic field. By an ECH injection power scanning experiment with
the magnetic field on axis of 1.9 T, it was found that the increment of plasma stored
energy increases with ECH injection power. Theoretical investigations on power
deposition of the O-X-B heating under the experimental conditions are now underway.
3. CONCLUSIONS
Remarkable heating effects for overdense plasmas sustained with NBI by obliquely
injected 54.5 GHz EC-waves in CHS, which should be attributed to O-X-B heating
scenario, were successfully demonstrated. Optimization of EC-wave beam direction
shows that efficient O-X-B heating corresponds to reduction of leakage EC-wave
power. Left-hand circular polarization which is supposed to have high fraction of Omode is most effective for O-X-B heating. Increment of plasma stored energy by O-XB heating increases with magnetic field and ECH power.
ACKNOWLEDGMENTS
The authors would like to express their thanks to CHS staff for performing the CHS
experiments. This work was carried out under the framework of bidirectional
collaborative research program between Heliotron-J, Kyoto Univ. and NIFS
(NIFS04KUHL005).
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[5] A. Mueck et al., this conference ID55
[6] K. Matsuoka et al., Plasma. Phys. Control. Fusion 42, 1145 (2000).