J. Plasma Fusion Res. SERIES, Vol.5 (2002) 288-291

Recent Experiments in Heliotron J
OBIKI Tokuhiro, SANO Fumimichi, KONDO Karsumil, WAKATANI Masahiror, MIZUUCHI Tohru,
HANATANI Kiyoshi, NAKAMURA Yujit, NAGASAKI Kazunobu, OKADA Hiroyuki,
BESSHOU Sakaer, NAKASUGA Masahikor, TSURU Hirokir, MANABE Yoshitol, SHIDARA Hiroyukir,

ANG Wanlengr, TAKAMIYA Tashor, IRIGUCHI Masaor, TAKEDA Masafumir, KAWAZoME Hayator,
MAENo Shogor, ToMIYAMA Keishir, oHNo Yoshinorir, KUBo Hiroyasur, NISHIoKA Yusuker,
YUKIMOTO Hidetoshir, IJIRI Yoshiyuki, SENJU Tohru, YAGUCHI Keiji, SAKAMOTO Kinzou,TOHSHI Kiyoshi, SHIBANO Masashi, TABARES Francisco2 and TRIBALDOS Victol
Institute of Advanced Energy, Kyoto University, Uji 611-0011, JapanlGraduate School of Energy Science, Kyoto (Jniversity, Uji 611-0011 Japan,2l,aboratorio Nacional de Fusi6n, Asociaci6n EURATOM-CIEMAT, Madrid, Spain(Received: 21 December 2001 / Accepted: 4 Jwe 2002)

Abstract
Recent experiments in Heliotron J are reported with special regard to the ECH plasma confinement.

After the initial verification of its sound magnetic surfaces, the 53.2 GH4 400 kW ECH experiments in
Heliotron J were carried out, leading to a successful production of the collisionless hydrogen plasmas
with electron temperatures up to - 1 keV at low electron densities of (0.2 - 0.3) x 101e m-3 which allows
us to study the characteristics of the llv electron transport in the helical-axis heliotron line. It wasobserved that the core confinement properties were strongly dependent on the selected plasma
configuration, suggesting a possible link to the surface quantities such as its iota profile. In addition tothe core properties, the edge and scrape-off-layer (SOL) plasma behavior is discussed, and also the initial
results from the newly-installed 70-GHz ECH are presented.

Keywords:
Heliotron J, heliotron, helical-axis heliotron, bumpiness, magnetic well, helical and island divertors

1. lntroduction
In order to prepare the next generation of theof Heliotron J are the strongly modulated helicalhelical conductor, two kinds of the toroidal coils andthree kinds of vertical coils, as shown in Fig. I [2]. The
iota value varies as its coil parameters in the range of0.3 to 0.8 with a low magnetic shear under conditions
when the major plasma radius of the plasma (R) is 1.2 m
and the average minor radius of the plasma (ao) is 0.1 0.2 m. As for the plasma heating systems, ECH (0.5

heliotron line in the helical magnetic field configuration,
Heliotron J was constructed with the aim of obtaining
the experimental basis for the physics design principles

in the "helical-axis heliotron". The main characteristics
variation of its magnetic axis, the resultant reduction ofMW), NBI (1.5 MW) and ICRF (2 MW) are in

the neoclassical llv transport, the favorable MHDpreparation.

characteristics with its edge magnetic well and theoperational capability of studying the island divertor aswell as the helical divertor []. The magnetic field (B (1.5 T) is generated by the coil system of the Z = llM = 4

This paper discusses the recent results from the53.2 GHz and 70 GHz ECH experiments in order to get
an overview of the operational flexibility and the global

C o rre sp ondin g autho r' s e -mail : ob iki @ iae. lcy o to - u. ac.j pScience and Nuclear Fusion Research@20O2 by The Japan Society of Plasma288

Obiki T. et al., Recent ExDeriments in Heliotron J
Vacuum chamber

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B(0) (r)Fig. 2 Plasma internal energy Wo as a function of themagnetic field B(0) on the axis in the straight section under the condition of the 53.2 GHz, 400 kW
ECH for the STD configuration.

confinement l)erformance of Heliotron J before thedetailed transport analysis that will be carried out fromnow on.

2. Electron Beam Mapping

production and heating studies were carried out using

three 53.2 GHz gyrotrons. Two oblique injection

Heliotron J is a flexible, concept-exploration-level
facility to search for the optimized helical-axis heliotron
from the experimental viewpoints. As such a device, theelectron beam mapping study is essential to afford afoundation for its plasma confinement studies. Before
entering into the plasma experiment, the mapping
experiments al low magnetic fields (300 G) were madeto confirm the existence of the closed and nested
magnetic surfaces for the basic (BSC) and standard(STD) configu'rations, showing that the measured data
were in fund:rmental agreement with the field line

launchers were located at the corner section of Q- 129"and one perpendicular injection launcher was located at

the straight section of Q - 203", where @ is the toroidal
angle starting from the middle position of the straightsection selected as the origin. These microwaves of theTE62 mode were injected from the outside of the torus

through smoothed oversized waveguides. The total
injection power was about 400 kW and the pulse width
was 40-50 ms. As shown in Fig. 2, the optimal ECH
heating was found to be in general agreement with that

tracing calculations taking into account the earth

situation where the 2nd harmonic resonance layer could
locate in the straight section of the confinement torus at
B(0) = 9.95 T. In addition, the highly efficient ECH wasalso found to exist in the higher magnetic field region of
more than B(0) = 1.4 T, where no resonance layer exists
in the plasma core region. The fundamental resonancelayer was located only at the edge region (rlao 2 0.8) ina limited toroidal zone near the corner section, where
the mod-B structure is tokamak-like and the magnetic
field gradient is large. This heating mechanism remains
to be studied in relation to the strong inward heat pinchor the electron Bernstein wave heating that was modeconverted from the edge fundamental ECH.
At low electron densities of n. - (0.2 - 0.3) x 10te
m-3, the electron temperatures were measured by the soft
X-ray absorber foil method to be - 1 keV, indicating theproduction of the collisionless plasmas of v* << 0.1 inmagnetic field and the ambient magnetic field near thetorus. See ref . l2l. With regard to the configuration

effects around the BSC configuration that will be
discussed bekrw, the mapping experiments were also
made as a function of the auxiliary vertical field coilcurrent, Iay. The results at low fields revealed the island
formation and the shrinkage of the confinement region
due to the emergence of low-order rational surface suchas Llznt = ll2 (or 417), but the influence of this error
field (including the effect of the natural islands) on theplasma confincment at higher fields still needs the highfield mapping results to elucidate its origin.

3. ECH Plasma Production and Heating
After the experimental verification of the sound
BSC and STD magnetic flux surfaces, ECH plasma289

Obiki T. et al., Recent Experiments in Heliotron J<Rutt-> (m)ECH 53 GHz

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Fig. 3 Volume averaged beta <B> as a function of lineaveraged electron density F" under the conditionof the 53.2 GHz, 400 kW ECH at B(0) = 0.95 T for
the STD configuration.

100rAY (s)Fig. 4 Average minor radius of the confined plasma (ao),average minor radius of the last closed magnetic
surface without the vacuum chamber (a"). and theaverage major radius of the magnetic axis <r9"",">
around the BSC configuration as a function of
auxiliary vertical coil current /ou. Under the condition of ao < a", the confinement region is restrictedby the chamber wall. Under the condition of ao =a", it is determined by a magnetic separatrix. The
setting of /AV(100%) corresponds to Lv = 1/t4 kAT.

their core regions, where y* = rRoq/(v"T.) and y* = I
stands for the normalized plateau collisionality.
According to the DKES-code calculation [3], theneoclassical diffusion coefficient D in this collisionlessregime is expected to become comparable to that of theequivalent tokamak without the help of the electric field.
Thus, these plasmas are expected to provide good

opportunities to experimentally study the beneficial
effect of the bumpiness on the collisionless electron

BSC

transport in the helical-axis heliotron. On the other hand,at higher densities of n"> 1.0 x 10re m-3, the increase inintemal plasma energy was observed. The diamagnetic
measurement showed the maximum attainable internalenergy We - O.7 kJ or <B> - 0.2 Vo at fle - 1.8 x l0re
m-3 which is near the cutoff density at B(0) = 0.95 T for
53.2 GHz ECH, as shown in Fig. 3. Recently, a7O GHz,
400 kW well-focused 2nd harmonic ECH experimenthas been started, which has provided a plasma of <P> 0.3 Vo at B = 1.25 T.

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4. Gonfiguration Effects-100-50100r^v (%)

Magnetic configuration effects of Heliotron J werestudied with special regard to the magnetic surface
quantities by changing the coil current such as theauxiliary vertical coil current (1ay) under conditions

Fig. 5 Diamagnetic internal energy Wodi'as a function ofthe auxiliary vertical coil current /ou under the condition of 53.2 GHz,400 kW ECH at B(0) = O.gS t.

where the electron density was controlled with hydrogengas puff. Because the change of 14y shifts the averaget(aoll2tr and t(Oll2tt show the central rotationaltransform and the edge rotational transform, respectively.

minor radius of the confined plasma as well as theaverage major radius of the magnetic axis (<R*i.>), theiota (rl27t), etc. with reference to those of the basic
configuration (BSC) in this experimental scan as shownin Figs. 4 and 5, the magnetic field strength B(0) on theaxis was maintained to be 0.95 T (Znd harmonic290Obiki T. et al., Recent Experiments in Heliotron J

resonance fiekl) in order to attain the central 53.2 GH4

result of the poloidal and toroidal distribution of the ion
saturation current was found to be in basic agreement

400 kW ECI{ heating. As shown in Fig. 5, theexperiment revealed that (i) the configurations of theoutward axis shift (tl2n > 0.53) introduced a gradualwith the expectations from the calculated divertor
footprints. In Heliotron J, a part of the whisker-fieldlines outside the confinement region in the STD (ordecrease in the attainable plasma energy content, (ii) theconfigurations of the inward shift (tl2rc < 0.53)
introduced a rapid decrease in the energy content, andBSC) configuration crosses the inner vacuum chamber

wall, thus forming the divertor legs attached to the wall.
The divertor legs have localized distributions on thewall with the dependence on the tracing direction. It was
found that a strong up-down asymmetry was present in
the profiles of density and floating potential on thegeometrically symmetric upper and lower divertor legs.
This asymmetry was reversed as the main magnetic fieldof the device was reversed. A possible explanation isthat (iii) the maximum energy content 0.7 kJ in this scan
was obtained at the BSC (t12fi = 0.53). These results
also revealed that the decrease in Wrlaf; (the plasmaenergy content normalized with the corresponding
confinement volume) at t(0)l2n < 0.53 was moremarked than that of t(0)l2n > 0.53. Physics
interpretations of these results should be made on thebasis of the confinement dependences not only on theparticle orbits in relation to the axis shift, but also on thekey surface quantities such as the iota, magnetic shear,magnetic well, etc. From this viewpoint, the finding thatboth the energy content and the normalized energy
content decreased under conditions when the low-orderrational surface such as tl2n= ll2 was introduced in theconfinement region raised an important question as tothat this asymmetry is due to the grad-B drift [4].

6. Summary
The recent plasma experiments in Heliotron J havepresented the following results:

For the BSC/STD configurations,(1) the low-field mapping data were in basic agreementwith the expectations,(2) it was found that 53.2 GHz and 70 GHz Znd

the experimental soundness of the magnetic surface at
the rational tlZrc value. With regard to the shift of theiota value, the plasma current effect is also a future issue
to be measured and studied. In addition, the plasma-wall

harmonic ECH could provide collisionless plasmas
in the core region, thus allowing us to examine thedetailed l/v electron transport in the helical-axis
heliotron. The core electron temperature reached I keV at low densities of (0.2 - 0.3) x 10re m-3.
The diamagnetic <B> reached about 0.3 Vo for 70-

interactions should be clarified under the axis-shiftconditions since the axis shift can easily cause a partial
contact of the confining plasma with the stainless steel

wall of Heliotron J. Thus, further experiments are

GHz ECH,(3) the obvious configuration effects of plasma
confinement were observed in relation to theneeded to clear up these questions, but it was found thatthe initial plasmas in Heliotron J addressed a sensitiveenergy content dependence on the selected magnetic
configuration, then encouraging future expectations toget experimental insights into its optimization.

variation of the surface quantities such as the iota,

but their definite interpretation still needs moreexperimental information such as the mapping data
at high fields,(4) the experimental understanding of the edge andSOL region is increasing.

5. Edge and SOL Properties
The detailed study of the edge and SOL properties
is important to investigate the improvement of the coreconfinement as well as to study the divertor scenario inReferences[]Heliotron J. The edge and SOL properties for ECH
plasmas were measured using stationary or movable

F. Sano et al., J. Plasma Fusion Res. SERIES 3, 26(2000).
t2l T. Obiki er al., Nucl. Fusion 41,833 (2001).
t3l M. Wakatani et al.,Nu'cl. Fusion 40,569 (2000).

electro-static probes, and the typical parameters near thelast closed surface in the SOL region were Z" = 40 - 50
eV and ne = 0.1 - 0.2 x 10re m-3. The charged particle
fluxes in the divertor region were also measured, and thel4l W. ANG er al., in this conference.

291