J. Plasma Fusion Res. SERIES, Vol.1 (1998) 30-33

Overview of CHS Experiments
MATSUOKA Keisuke, OKAMURA Shoichi, KUBO Shin, IDEI Hiroshi, IGUCHI Harukazu,FUJISAWA Akihide, IDA Katsumi, MORITA Shigeru, TANAKA Kenji, MINAMI Takashi, TOI Kazuo,OSAKABE Masaki, YOSHIMURA Yasuo, AKIYAMA Ryuichi, TAKAHASHI Chihiro, SAKAMOTO Ryuichi,INOUE Noriyuki, OHDACHI Satoshi, EJIRI Akira, SASAO Mamiko, NOMURA Izumi, ISOBE Mitsutaka,PETERSON Byron, LEE Seishu, KURAMOTO Hideharu, MUTO Sadatsugu, OZAKI Tetsuo, GOTO Motoshi,
KADO Shinichiro, SAGARA Akio, NISHIMURA Shint, TAKAYAMA Sadatsugul, SHIRAI Yoshiakil,
PAVLICHENKO Rostislavt, KONDO Takashil, HASEGAWA Yasuhirol, TAKECHI Manabu2,
OHKUNI Kotaro2, MATSUNAGA Go2, TAKAGI Shoji2, SHIMIZU Akihiro2 and FUJIWARA Masami
National Institute for Fusion Science, Toki 509-5292, JapanlGraduate IJnivesrity
for Advanced Studies, Hayama 240-0193, Japan2Dep. of Energy Eng. Science, Nagoya IJniv., Nagoya 464-8603, Japan(Received: 30 Septembet L997/Accepted 1,2 January 1998)

Abstract
Recent experimental results from CHS are overviewed. Plasma parameters are extended to higher
electron temperature at higher densities in an ECH plasma by introducing 106 GHz gyrotron and tohigher ion temperature in an NBI plasma by realizing the so-called high Ti mode. Understanding of heliotron/torsatron plasma physics is deepened by profile measurements, especially by electric potentialmeasurements with HIBP. On the basis of CHS results obtained so far, it might be concluded that theenergy confinement improvement over the international stellarator scaling and elimination of the loss
cone for high energy ions are important issues to be addressed for the low aspect ratio heliotron/torsatron. Those are thought to be closely related to the helical ripple. The quasi-axisymmetric configuration
having essentially no helical ripple is proposed to overcome the ripple problem.

Keywords:
CHS, heliotron/torsatron, plasma-wall interaction, confinement improvement, high Ti mode, HIBP,
radial electric field, plasma rotation, helical ripple, quasi-axisymmetry

1. lntroduction

value of 5 o/o necessary for the reactor although the experimental verification of ballooning stability has remained. To study the high energy ion confinement, NB
injection with variable injection angles [7] and ICRF
heating were done, which showed poor confinement ofperpendicularly-accelerated high-energy ions, although
ICRF based on the electron heating was successful [8].
It is widely recognized that the improvement of theenergy confinement time by factor 2 from ISS95 scaling

CHS experiments [1] have been continued since1.988 to elucidate heliotron/torsatron plasma character-istics and to improve its performance. So far, the description of equilibrium of low aspect-ratio helicalplasma has been established [2] and contributions tothe ISS95 scaling law have been made [3]. The Hmode is realized by modifying the edge r profile withOH current [4], although the energy confinement timeis improved by about 20 o/". T\e ( B) value of 2.! o/"has been obtained [5, 6], which paved the road to theis necessarv for the reactor. The radial electric field
O1998 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchCorrespondinE author's e-mail: matsuoka@ nifs.ac.jp30

Matsuoka K. er a/., Overview of CHS Experiments

has been doubled and the Parameter of ECH plasma
has been extended; central electron temperature Te(0)of 2 keV at the average electron density n" of about
1x101e m-3, ?:(0) of 1 keV at n"of 3.5x101e m-3 whenshear andlor the plasma rotation shear are required toreduce the anomalous transport. In CHS the toroidal
plasma rotation driven by NB is suppressed because of
the magnetic pumping by its helical ripple [9]. Making
the ion pressure peaked is one of methods to produce
the radial electric field shear for the confinement improvement in such a machine as CHS. Here, recent experimental results from CHS are introduced by taking

R"*, is 0.921 m and the magnetic field strength B is
1.76 T (at R:1m, which corresponds to 1'9 T at R""'.)[12]. Low density plasmas are well inside the collisionless regime of which transport is important for helicalsystems. To make the transport analysis of ECH plasma
without any assumption the one-pass absorbed powerits future prospect into consideration.

should be obtained experimentally. It has been possible
by subtracting the transit power through the plasma

2. Experimental Results
2.1 Plasma performance improvement withfrom the incident power. The transit power is estimatedby measuring the temperature rise on the SiC plate andthis method can also be applied for estimating the incident power when there is no plasma. Indication of thecollisionless transport is expected to be studied in thecentral region due to the relative importance of theneoclassical effect to the anomalous one.
The central ion temperature of 800 eV which isthe highest value so far was obtained in the so-calledhigh Ti mode, where two neutral beams were balanceinjected with the total port-through power of 1.6 MWat R*i. of 0.92t m [13]. Here the electron density
profile is made to be peaked by neutral beam fueling,where no gas puffing is applied and the wall is conditioned with titanium gettering between shots. On theother hand, when the gas is puffed and the density
profile is flattened, the ion temperature is lower thanthat in the high Ti mode at the same average electron
density. The highest ion temperature is obtained at n"of about 1.5x10te m-3 and Ti decreases as the density
increases (at the same time the density profile gets flattened). The peaked ion pressure profile is favorable for
producing the radial electric field in CHS in which theplasma rotation is strongly damped by its large helicalripple. The confinement improvement for ion is foundto be not due to the poloidal plasma rotation shear but
due to the enhanced radial electric field shear from theforce balance analysis on bulk ions and impurity ions(carbon in this case). However, the electron tempera-plasma-wall interaction
The plasma-wall interaction is one of key elements
for the confinement improvement. Recently two newapproaches were applied for a coating method and fora divertor scenario. The plasma-touching surface wassuccessfully coated with boron by the real time boronization (RTB) method [10] in which decaborane vapor
was directly puffed into an NBI heated plasma. Thismethod was developed for the steady-state superconducting LHD where a helium glow discharge for boronization is not available. After one day operation ofRTB the oxygen content normalized by the electron
density is decreased by factor 2 or 3 and the stored energy increased by 20-30o/" as well as the density limit.
However, it might be difficult to make boronization on
shadow regions of in-vessel components with RTB.

The local island divertor (LID) [11] by utilizing
the m: l/n:L island produced with the perturbationcoils was successful in controlling the edge recycling
and in improving the confinement, where the magnetic
axis position R*6 is 0.995 m. Total 16 coils were placedon the top and bottom of CHS at the vertically elongated plasma cross-section (the toroidal field period is
8 in CHS). The exhaust efficiency of less than 17o wasdemonstrated with LID and the leading edge problemof the pump limiter was mitigated by inserting it into
the island. Under the same gas puff condition the average electron density decreases roughly by half with theLID operation and at the same electron density thestored energy increases by 20-30o/o. This improvement
results from an increase in the electron temperature in
the core region, which might be caused by the low recycling. It should be noted that this divertor configuration
owes to the l profile inherent in CHS where the edge

ture in the high Ti mode is lower than that in theL-mode, of which reason has not been understood. Theelectron temperature is comparable to the ion temperature in the high Ti mode. This means that the ion confinement is significantly improved, because NBI, theunique heating source, delivers its energy preferentiallyt/2nis about L.

to electrons under the experimental condition. The global energy confinement time of high Ti mode is almost
comparable to that of the L-mode in CHS. Figure Lshows the parameter range of CHS plasmas. The larger

2.2 Extention of plasma parameter range
By introducing a new 106 GHz gyrotron the total

input power (53 GHz*106 GHz) for electron heating
31Matsuoka K. et al., Ovewiew of CHS Experiments

1x10te m-3. FNA (fast neutral analyzer) has shown
preliminary data on the energy spectrum; the dip at theenergy of around 2 keV, which exists at the NB heatingphase, disappears when ECH is applied[16]. This may
be due to the fact that the resonant loss-cone loss exist-200 0ooo^1500o:;1000<)q)t-ing at the negative potential is eliminated by the change
in the potential to the positive direction.
Electric potential pulsation which continues during
the discharge, like ELMs, has been found in an ECH *o-o^oo oo oooooooto^o^

NB plasma with n of about Lx10re m-3, B of 0.9 T andR^* of 0.92I m. There are two types of the pulsating
potential. The global type, where the whole potential
profile pulsates with the pivot point at r/a of abont O.5,is seen at a lower electron density and with a higher
ECH power, and the local type, where only central partof the potential pulsates, at a higher electron density
and with a lower ECH power. The potential bifurcationof both types may be explained with the neoclassicaltheory based on the nonlinear relation between the^lI5'""o-tp". oq,q"^,o^,o'dt"oo-"500rrr6(o) ixro"m'')1oFig. 1 Central electron (circles) and ion (squares) tempera-tures as a function of central electron density inECH and NBI plasmas. Range of electron temperature extends to higher densities due to 106 GHz gyrotron and high Ti mode has realized higher ion
temperatures (refer to larger symbols).

radial current and the radial electric field [17].

circles and larger squares represent recent results of the

2.4 Configuration improvement by inward shift
In heliotron/torsatron, an inward shift of the magnetic surface makes the magnetic configuration more

central electron and ion temperatures, respectively.

2.3 New findings with HIBP

omnigenous. This is because the equi-B surface deter-

HIBP measures potential fluctuations accompaniedby the burst-like magnetic fluctuations, of which amplitude is the largest in CHS plasmas and which are observed in a low p (less than 1 %) NBI plasma with coinjection and at Rdis of O.921 m. The mode numbers of
the burst mode are m:2/ n:1.. Electric potential fluctuations are found to be accompanied with the magnetic fluctuations. At the growing phase of the burst
mode the maximum amplitude of the potential fluctuation is about 40 V and it is located at /2n:I/2 surface [14]. Identification of the burst mode is under discussion on the basis of the picture of interchange

mined mainly by the helical coils is shifted inward
relative to the magnetic surfaces. In CHS where thepositive pitch modulation of 0.3 is adopted in the helical coil for the purpose of good accessibility, it is necessary to move the magnetic surface significantly for om-nigeneity. Strongly inward shifted configuration(R-i,:0.888 m) finally becomes to have o:*1 characteristics [18], where the bottom of the ripple has thesame magnetic field strength. This might be the uniquemethod to improve the CHS magnetic field configura-tion from the neoclassical point of view. ECH plasmas
with low collisionality in this configuration are expected
to show better confinement because the role of neoclassical transport becomes important as the collisionality decreases. However, no sigrificant improvement
has been seen up to now in the global energy confinement time in comparison with the LHD scaling. Detailed measurements on electron temperature profiles

modes.

mode the maximum amplitude of the potential fluctuation is about 40 V and it is located at t/2n:L/2 surface [14]. Identification of the burst mode is under dis-cussion on the basis of the picture of interchange
modes.

In general, the potential is positive when the density is low (order of 1018 m-3) and tends to be negative
at higher densities. The potential profiles are essential
for estimating the resonant loss of NB ions deeply
trapped in the helical ripple during their slowing down[15]. To investigate the effect of the potential on theare needed especially in the core region where the neoclassical transport manifests itself against the anomalousone.

3. Summary and Discussion
A steady progress is seen in CHS experiments andion energy spectrum, ECH is applied to NB heatedplasmas having the negative potential with n" of about

the plasma parameter range is extended according tothe increase in the heating power and to the confinement
32

Matsuoka K. et a/., Ovewiew of CHS Experiments

improvement; the central electron temperature of 2keV and the ion temperature of 800 eV have been obtained. By measuring the electric potential with HIBP

References[1] K. Matsuoka et al., Plasma Physics and Contolled
Nuclear Fusion Research 1988 (Proc. 12th Int.
Conf Nice, 1988) Vol. 2, IAEA, Vienna, (1989)

our knowledge on potential-related physics is increased,for example, its effect on the resonant loss of deeply
trapped ions. New findings on electric potential pulsation will also give insight to physics understanding of
toroidal plasmas. On the basis of experimental results
on several issues obtained so far (confinement timescaling, access to high p, high energy ion orbit, divertor
configuration), it might be concluded that most criticalissues are the confinement improvement and the loss
cone of ion orbits, which are closely related to the future prospect of low aspect ratio heliotron/torsatron
configuration. From the experiment of o:*1 configuration the effort of reducing the neoclassical transport
might not be enough to improve the confinement. HighTi mode shows importance of the radial electric field
shear. The helical ripple inherent in heliotron,/torsatron

4lr.

[2] H.Yamada e/a/., Nucl. Fusion 32,25 (L992).
[3] U.Strouth e/a/., Nucl. Fusion 36, 1063 (1996).
[4] K.Toi et al., Plasma Phys. Contrl. Fusion 38, 1289
(1ee6).

[5] K. Matsuoka e/ a/., Fusion Eng. Design 26, L35(1ees).

[6] S. Okamtra et al., Nucl. Fusion 35,283 (1995).
[7] S. Okamura et al., Plasma Physics and Controlled

Nuclear Fusion Research 1992 (Proc. 14th Int.
Conf. Wurzburg, 1992) Yol. 2, IAEA, Vienna,(19e3) p.507.
[8] S. Masuda et el., Nucl. Fusion 37,53 (1997).

[9] K. lda et al., Phys. Rev. I-ott.67,58 (1991).
[10] A. Sagara et al., J. Nucl. Mater. 241'243, 972(ree7).

prevents spontaneous or externally-driven plasma rotations because of the magnetic pumping. If the rotation
shear and,/or electric field shear is essential to the suppression of anomalous transports, experiments are recommended in a helical machine where the viscosity
against the plasma rotation is low. One of such configurations is now under consideration, which is quasi-axi-[11] A. Komori et al., J. Nucl. Mater. 241-243, 967(r9e7').
S.
[12] Kubo et al.,in these Proceedings, p.118 (1998).
proceedings, p.239 (1998).
[13] K. lda et al., in these[14] S. Ire et al.,in these Proceeditgs, p.273 (1998).
[15] A. Fujisawa et al.,Phys. Plasmas 4,1357 (1997).
[16] S. Okamtra, private cornrnunication[17] A. Fujisawa et al., in these Proceedings, p.84

symmetric having essentially no helical ripple IL9, 201.
On the other hand, ease of divertor configuration is
also very important for the future machine. Configura-(1ee8).

tion optimization which is compatible with divertor[18]H. Mynick etal.,Phys. Rev. Lett. 4E,322 (1982).

[19] K. Matsuoka et al., Plasma Physics Reports 23,

functions will be needed in the research of helical plas-s42 (ree7).

mas.

[20] S. Okarnura et al., in these Proceedings, p.164(1ee8).

Acknowledgements
The authors would like to acknowledge Messrs. K.

Motoki, S. Ohtsuka, N. Kijima and S. Hattori for operating NBI and diagnostics, and also to express theirgratitude to Director-general A. Iiyoshi for his continuing encouragement.

33