J. Plasma Fusion Res. SERIES, Vol.5 (2002) 292-296

Profiles of Divertor Plasma in Heliotron J
ANG Wanleng. MIZUUCHI Tohrur. NAKASUGA Masahikor, SANo Fumimichi'. KONDO Katsumi,' NAGASAKI Kazunobur, OKADA Hiroyukil, BESSHOU Sakae, MANABE Yoshito,SHIDARA Hiroyuki, KAWAZOME Hayato, MAENO Shogo, TAKEDA Masafumi,
TAKAMIYA Tasho, TOMIYAMA Keishi, TSURU Hiroki, OHNO Yoshinori, KUBO Hiroyasu,NISHIOKA Yusuke, YUKIMOTO Hidetoshi, IRIGUCHI Masao and OBIKI Tokuhiror
Graduate School of Energy Science, Kyoto University, Gokasho, Uji 611-0011, Japan1
Institute of Advanced Energy, Kyoto (Jniversity, Gokasho, Uji 611-0011, Japan(Received: 11 December 2001 / Accepted: 5 July 2002)

Abstract
To investigate the property and structure of the diverted plasmas heated by 53.2GHz 2nd harmonic
ECH in Heliotron J, two sets of Langmuir probe arrays with two dimensional spatial resolution wereinstalled at the divertor legs which lie up-down symmetrically in the topology of the magnetic field. A
clear up-down asymmetry was observed in the profiles of the density and the floating potential. Theasymmetric profiles were turned over when the direction of the confinement field was reversed. As oneof the candidates for the mechanism causing the asymmetry, the effect of VB drift on the particle
transport has been investigated qualitatively and numerically by the particle orbirguiding center tracing
calculation. In the numerical results, an asymmetric profile and its reversal are observed.

Keywords:
Heliotron J, edge field structure, divertor, divertor probe array, divertor flux asymmetry, VB drift

1. Introduction

Experimental and numerical studies of divertorHandling of the heat and particle flux coming fromthe core plasma to the first wall is a fatal problem forhave been performed for Heliotron J, which is a helical-axis heliotron [5] device newly constructed in Kyoto
University. One of the main purposes is to find theconfiguration suitable for divertor in the helical-axis
heliotron. Besides, it is of interest and important toascertain if there is any asymmetry in the divertor flow
as seen in other heliotron/stellarator devices and tounderstand the mechanism causing the asymmetry sothat a scenario for the controllable divertor can besupplied. As the first step of experimental study, thestandard configuration with an intrinsic helical divertor

fusion reactors. The concept of a divertor has been
proposed as an essential and effective way to overcome

this problem since the dawn of fusion research.

In the diverted tokamaks, asymmetries between the

inner and outer (in-out asymmetry) divertor flux for
both single-null and double-null discharges have been
noticed. In addition to the in-out asymmetry, the updown asymmetries have been observed in double-nulldischarges [1]. As the crucial characteristic of theasymmetry, the dependencies of the asymmetry on thedirection of the confinement field have been shown[1,2]. Also, in heliotron/stellarator devices, which have
intrinsic diverted field lines, the asymmetry of divertor
flow and its field dependence have been observed [3,4].

structure has been chosen and the divertor plasmas havebeen measured using two sets of Langmuir probe array.

In this paper, the profiles of divertor plasmas, theobserved asymmetry in these profiles and an ion orbit-Corre sponding author' s e -mail: ang _wl@ center.iae.lcyoto-u.ac j p
292@2002 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchAngW. et al., Profiles of Divertor Plasma in Heliotron J

guiding center tracing calculation are discussed.

wall of the device, forming four main divertor legs in
one toroidal period of magnetic field (B-field), two in
the high field side and others in the low field side [6,7].

2. Experimental Arrangement
Heliotron J is a helical axis heliotron device withDivertor legs corresponding to this configuration havelocalized distribution on the chamber wall and thedistribution has dependence on the direction of the Bfield.
Figure I shows the experimental set-up of heating
equipment and diagnostics in the Heliotron J device. In
this study, the current-less plasmas were initiated andheated by 53.2 GHz second harmonic ECH with themaximum total power of -400 kW from three gyrotrons

an I = llm = 4 continuous helical coil. The major radius

is 1.2 m and the minor radius is 0.1--0.2 m. There are six
sets of coils which can produce various configurationswith various edge field structures by changing thecurrent ratio. In the standard configuration, the whiskerstructures of fir:ld lines covering the LCFS are observed[6]. A part of these whisker-structures crosses the inner(G2, G4, Gs).

To investigate the property and the structure ofdiverted plasmas, two sets of divertor probe arrays

Top DPA(DPA) were installed at the up-down symmetricpositions, Q=67.5" (top) and the Q=112.5" (bottom),corresponding to the low field side divertor legs in thestandard configuration. The installation positions of theDPA are illustrated in Fig. I with the Poincare-plots offfii f1

the edge field lines.

The each DPA consists of 28 dome-shaped
Langmuir probes. Figure 2(a) shows a schematic of the\s erprobe pin arrangement viewing from the plasma side.

Bottom DPA

The probe pins are arranged in a 4x7 matrix, in thenearly toroidal direction Xp and the nearly major radiusdirection Rp. Figure 2(b) shows the contour of theconnection length (L") of the divertor field lines crossingIECH (G2, G5)

Fig. 1 Experimental set-up of heating equipments anddiagnostics in the Heliotron J device.

the DPA.

3. Experimental Results(\-zIn a typical ECH discharge where the density isoooo oo oooo o,//----\)r//@low (<n">11n"-101e m-3, 7"-500 ev), the electrono @@@@ @o@@o @ l-'temperature 2", the divertor plasma density no and thefloating potential V1 deduced from the Langmuir probe

I-V characteristic were found to be in the range ofT"-lo4o eV, na-1xl0r8-4x1018 m-3, V--60-10 V, depending on the position of the probe tip. Figure 3 show
the contour plots of 7", n7 and Vi observed at the top andbottom DPA, respectively. As shown in this figure, thedistribution of the plasma on the both DPAs seems toshift to the smaller R as increasing Xo (the dashed linesk(m)2721,>l@//t

1OO,,,, ..90#*{ 80ffi* zn

I50
I40
r30
r10-20roX

are added to the contour of maximum n6 for the sake ofmaking a clear image). This characteristic of overall
plasma distribution is basically consistent with the angleof the Z"-contours as shown in Fig. 2(b). Note that thecontour of the connection length is the same for botht.428 1,.435 1.M2 1.449 7.456 7.463 r.470
R ('n)

DPAs due to the up-down symmetry of the fieldFig. 2 (a) Arrangement of the DPA, (b) profile of connec-structure. However, quite different characteristics of theradial (R) plasma profile were observed, especially in n6tion length on the region surrounding the probepins.
293

AngW. et al., Profiles of Divertor Plasma in Heliotron J20(a)0** 303-zoai

7 -20t;X--40Vf Xo=l-60
20(b)

6rfil

1

7.-20

3

Xt.4?31.435

1.442 1.449 1.456 1.463

g.i,-40

lr-60

f ry *^=t
1.42 t.43

1.470

1.44 r.45 r.46
R[-]

t.47

Rlml

Fig. 4 Profiles of the floating potential at (a) the top DPAand (b) the bottom DPA.
#30
ff26

3

X-bottom DPA, %(R) is close to zero but has a sharp
negative dip near R=1.465 m. These features of thedivertor plasma profile observed for each DPA werecompletely turned over when the direction of theconfinement field was reversed, the profile of top DPAli:3

as shown in Fig. 2(a) come to appear at the bottom DPA
as the direction of the confinement field was reversed,

ltii

vice versa.

5r"*r _5

4. Discussion

Iis

The dependence of the divertor plasma asymmetryon the direction of the confinement field assured us that

l.t
t.4B

1.435

t.M2

this asymmetry was not a measuring error but was

1.49

caused through some physical mechanism related to theRlhl

magnetic field such as the VB drift. In order tounderstand this mechanism, it is necessary to consider

Fig. 3 Profiles of electron temperature, density and floating potential at (a) the top DPA and (b) the bottom
DPA. Dashed and solid lines are added to indicate

the plasma transport not only in the edge region but also

the density peaks and the approximate peak L"
contour while arrows indicate the VB drift direc-

in the core region and the resulting plasma distribution
on the LCFS. However, as the first step of theinvestigation, the VB effect on the particle orbit in thetion of ions.

edge region is discussed here.

and V1 profiles. The peak positions of the n6 profile for

The VB drift direction of ions is qualitatively

the DPA differ from each other; for the top DPA it

consistent with the observed shift of the density peaks

seems to be shifted to the right side (larger R side) of thepeak Z"-contour while that for the bottom one is shifted

described in the previous section, at the top DPA it
points to the right side (larger R side) and at the top to
the left side. The region corresponding to VB drift
direction of ion would suffer an excess of ion flux, andthe floating potential in this region can have a relatively
positive value since the polarity of the local potential
may be defined by the ratio of the ion to electron flux.

to the left. Also, the Vi profiles show some distinct
structures. For example, in the cross section with Xo=1
cm, as show in Fig. 4, in the top DPA, % has an almost
constant negative value for R<l.455 m and changes topositive value for R=1.47O m. On the other hand, in the294

AngW. et al., Profiles of Divertor Plasma in Heliotron J

From these viewpoints, it can be qualitatively

in the polarity. Nevertheless, a negative but morepositive value (than that observed at the top one) of theunderstood that, at the bottom DPA, the region forR<1.45 m (for Xo=l cm), which corresponds to theVB drift direction of ion, has a more positive value
while the region for R>1.46 m has a more negativevalue while the opposite is true at the top DpA. The
polarity of the floating potential near the vicinity of theLCFS was negative from the measurement of themovable probe. At both DPAs, the floating potential of
the regions corresponding to the SOL may be negativebn

floating potential at the bottom DPA for the region
R<I.45m as mentioned above is supposed to be a resultsof the condition that more incident and hence excess ofions occurred due to the drift effect for this region.
The effect of the VB drift on the ion orbit in theedge region is investigated by using an orbit-guiding
center tracing code. This calculation can reproduce anasymmetric distribution and its reversal depending on
the field direction. Figure 5 shows one example of thecalculated results showing the distribution of "footprint"of the ion and the field line on the DPA. Here, 10,000
ions with 20 eV energy and random pitch angles weretraced starting at a random position on the LCFS
(assuming an uniform plasma distribution). The ions on
the bottom DPA tends to spread in the VB drift direction
wider than that of the top DPA, forming an asymmetry
between the two DPA. This is consistent with thequalitative expectation discussed above. On the otherhand, the electron "footprint" is known to be similar tothe symmetric distribution of footprint of field lines. The
existance of different regions with excess of ion andregions with lack of ion is believed to be one of thereason for the formation of the complicated Vl profile.

65Q)

Ed)
hoc(tt55

GI:o 45
oo-21 22

toro i da

I

However, the distribution of test particles in thenumerical calculation is much wider than the measuring
range of existing DPAs. Therefore, a new DPA with

23

24
angle (deg. )wider measuring range is needed to make further'bnq)

Ecomparison with the calculation result. Also, to obtainthe correct understanding about the effect of the VB
drift, it is necessary to carry out further experimental
investigation on the distribution upstream of the diverted
plasma and numerical studies on the effects of electricfield and collisions.

5. Summary
The divertor plasma distribution in the standard
configuration of Heliotron J has been investigated at thegeometrically symmetric (up-down) positions. For ECH
plasmas, the overall distribution is basically consistent;:ooo-with that expected from the field calculation. Thedetailed analysis of the n6 and Vs profiles showed,
however, the existence of a clear up-down asymmetry.

65 66 67 68 69 10

In order to understand this observation, the VB drift

toroidal angle (deg.;

effect on the ion orbit in the edge region was discussed.

The asymmetric spread of ion due to the VB effect is
supposed to be account for the formation of asymmetricFig. 5 lons hitting points on the DPA surfaces corresponding to (a) the top DPA, (b) the bottom DpA.
Distribution of footprint of field lines and the positions of probe pin are also shown for comparison.

experimental profiles.

295

AngW . et al., Profiles of Divertor Plasma in Heliotron J

t2l D.N. Hill et al., Fusion Technol. 21,1256 (1992).
t3l T.Mizuuchi et al.,J. Nucl. Mater. 266-269, ll39

Acknowledgement
The authors are deeply grateful to the member ofHeliotron
J group for their collaboration in thethe(1999\.

experiments and discussions. Fruitful discussions withDr. F. Tabares (EUTATOM-CIEMAT) are also greatly

t4l Y. Feng et al., Plasma Phys. Control. Fusion 40,

acknowledged.

t5l M.Wakatani er al., Nucl. Fusion 40,569 (2000).
[6] T. Mizuuchi et al., Proc. l2th Int. Stellarator Work-371 (1998).

shop (Madison, USA, 1999).

References

tll

t7l T. Mizuuchi et al., Proc. l0th Int. Toki Conf.

X. Bonnin and W.L. Rowan, Nucl. Fusion 39, 1009
ft999).

(Toki, Japan,2000).

296