J. Plasma Fusion Res. SERIES, Vol. 5 (2002) 428432

Behavior of the Edge Plasma near the Anchor Conducting
Plates and Their Effect on the Plasma Gonfinementin the GAMMA 10 Tandem Mirror
NAKASHIMA Yousuke, NATORI Takayoshi, KAWASAKI Yoko, KOBAYASHI Shinji,
ISHIMOTO Yuki, KATO Tatsuya, ORITO Kousei, FUKASAWA Takahiro, KATANUMA Isao,
ICHIMURA Makoto, CHO Teruji and YATSU Kiyoshi
Plasma Research Center, University of Tsukuba, Tsukuba lbaraki 305-8577, Japan(Received:ll December 2001 / Accepted: 5 July 2002)

Abstract
This paper describes the behavior of the edge plasmas near conducting plates installed for improving
the plasma performance of GAMMA 10. The conducting plates are fixed closely to the plasma surface
where the cross section of the plasma is flatly elongated in the anchor transition region. In this region asignificant asymmetry of the edge plasma was observed in both probe currents and floating potentials.
The direction of the shift in the ion-saturation current corresponds to the gradient B drift for ions. It isalso found that the deposition areas observed on the conducting plates, which show a rotational symmetry
to the magnetic axis, are expanding with the progress of plasma experiments. Most probable mechanism
explaining the above-observed results is discussed based on the results of the magnetic field and ion orbit
calculations. Effects on the plasma parameters of the conducting plates are studied and it is found that thefloating condition of the plates leads to the increase of the plasma performance during potential
formation.

Keywords:
GAMMA 10, tandem mirror, edge plasma, Langmuir probe, gradient-B drift, ion orbit calculation

the axisymmetric field line of the central and plug/

1. lntroduction

barrier regions. In the region containing such acomplicated magnetic configuration, discrepancy
between the equi-potential surface and flux surface ofplasma may cause a large amount of distortion of theImprovement of plasma confinement by using
electrostatic potential is a key element for tandem mirror
devices. In the GAMMA l0 tandem mirror, extensive

studies have been made and the improvement ofconfinement due to the potential formation has been
proved [1-3]. However, the existence of radial loss inaxially confined plasmas was also revealed. As asuspicious region where the radial loss occurs, we havepointed out anchor transition region which has a nonaxisymmetric structure of magnetic field and has flatly

electric field, which leads to the radial transport of theelongated plasma cross section. This region alsosurface in both inner and outer transition regions in eachanchor-cell. Several series of experiments on the basicplasma.

In 1998 four sets of conducting plates wereinstalled in the anchor transition regions to improve theperformance of plasma confinement further. The
conducting plates are installed close to the plasma

contains strong gradient-B structure where the magnetic
filed line of the minimum-B resion is connected with
Corresponding author's e-mail: nakashma@ prc.tsukuba.ac.ip428@2002by The Japan Society of Plasma
Science and Nuclear Fusion ResearchNakashima Y. et al., Behaviot of the Edge Plasma near the Anchor Conducting PlatescharacteristicJ of the conducting plates in plasma
discharges wore performed [3-5]. The effect of theplates on thir plasma parameters during potential

whole plasma at the anchor-cell. Another wave (RF2) isformation alr;o have been investigated by using

potential produced with ECH by using gyrotrons
installed in each plug/barrier region confines escaping

used for heating the central-cell ions by ion cyclotron

damping near the mid plane of the central-cell. The plug

Langmuir protres and calorimeters installed in the outertransition region of the east anchor-cell [4-6].
Furthermore, the anchor conducting plates wereparticles along the magnetic field line.

modified in their shapes and sizes for investigating theeffect of the plates in the summer of 2001. In this paper
the behavior of the edge plasma near the anchor
conducting plates are reported and the effect of the2.2 Anchor conducting plates and diagnosticsystem
The schematic view of the conducting plates at theeast anchor-cell installed in 1998 (APl) is illustrated
together with the magnetic flux tube in Fig. l(b). The

plates on the plasma confinement is discussed.

main conducting plates, which are placed along thelonger axis of the elliptic plasma cross section, areseparated to 4 pieces in an axial direction. At both endsof the longer axis, side plates are installed facing each

2. Experimental Apparatus2.1 The GAMMA 10 device
GAMMA 10 consists of an axisymmetric centralmirror cell, anchor-cells with minimum-B magnetic

other. The potential of each plate is changed in floating
and grounded conditions individually, and also can befields, and plu.g/barrier cells with axisymmetric mirrors.
Figure l(a) shows the schematic view of the GAMMA
10 tandem mirror. The length of the central-cell is 6 m
and both ends of the central-cell are connected to theanchor-cells through the mirror throat regions. Injecting
the gun plasma from both ends produces initial plasma,
and then main plasma is built up with ICRF waves
together with gas puffing in the central-cell. One of theICRF waves (ltFl) is used for MHD stabilization of thebiased by using external power supplies. On the sideplates of the east outer-transition region, arrays ofcalorimeters and Langmuir probes are installed in order
to investigate the behavior of the edge plasmas. In thecourse of experiments using the anchor conducting
plates, the space between each facing plate wasexpanded in the spring of 1999 (AP2) and recently thestructure was considerably modified and reinstalled in
the summer of 2001 (AP3). In the present conducting
plates, the distance between the plates is set to be
narrower and is carefully adjusted along the magnetic
field line in this region.

(a)
Coil

*tH{'*ry"ffi.tP

N,"rff;t\onrta{{angenenl$agnetrc$uxtuoe

3. Experimental Results

tt

Figure 2 shows the spatial profile of ion saturation
current measured with the movable Langmuir probe at
the east outer-transition region in a standard hot-ion
mode plasma used in the present experiment. A clear
asymmetry in x (vertical) direction is observed in theECHaGP(b)Main conducting plate-a-- wlthoul ECRH-e- withECRH
co*/1'-'-ot\"-.9<,,^P\rroFartiet

Rcc=8cm/Fixed Langmuir probes and calorimeters

oFig. 1 Schematic illustration of the GAMMA 10 tandem
mirror device: (a) coil arrangement and shape ofthe magnetic flux tube, (b) the conducting plates
and diagnostic system at the east anchor-cell.

f-*--lt----ltltl.n1-nLJ.lJ.|.l.r"/.JJ.l'lfu:3-2-1 o

Rcc=16cm

fl r,.<ifl., :h,- | 1.,+*-r-......1(doRcc=13cm

1-3-2-f 0 1 2 3-3-2-'t 0 1 2 3x [cm]

x [cm]

Fig. 2 Spatial profile of the ion saturation current in thevertical direction at the outer-transition region ofthe east anchor-cell.
429

Nakashima Y. et al., Behavior of the Edge Plasma near the Anchor Conducting Plates

periphery region (Rcc > 13 cm) and it has a tendency to
be enhanced with the distance from the plasma axis. Inon the side of facing the plasma, which were taken after
being used in the experimental period of 3 years (1998

this figure Rcc represents the effective radius of the2001, 19000 plasma shots). There are some depositedareas of impurity observed on the plate and theirlocations correspond to the opposite place to the ion
drift side. This phenomenon shows a clear structure-probe position which is mapped on the central-cell mid
plane along the magnetic field line. It is found that theabove feature is independent of the potential formation
produced by the plug ECH.
Figure 3 shows the floating potentials of the newest

containing a rotational symmetry in 180 degrees aroundthe magnetic axis. The left photograph of each region inFig. 4 gives the result obtained after the first one-year
operation (1998-1999) and the right one corresponds toconducting plates (AP3) measured under a similar
experimental condition as shown in Fig. 2. The
illustration in Fig. 3 shows that AP3 is separated in theazimuthal direction of the plasma cross section. Spatial
profile of floating potentials in y (horizontal) direction
measured with arrays of fixed Langmuir probes installedon the conducting plates facing each other at the same

Outer Transition

lnner Transition..-1./1

Z.
.FcOr

region are shown in Fig. 3. It is found that positive
potentials are measured in all the plates and generallydc(!

higher potentials are observed in the plates of innertransition region that is connected to the central cell.
The floating potentials on the top plate array in the outertransition region increase with the positive y (horizontal)
direction and those on the bottom plate array present an
opposite dependence to the top plate case. Thisdependence agrees with the profile of floating potentialin y-direction measured with an array of Langmuirt!

?Xi--ottom olatel

AncnorMid PlaneI.Fco(Lprobes installed on the same plates. These experimental.E

6results support that the asymmetry of the edge plasmameasured in the outer-transition region is ascribed to theeffect of ion drift motion due to the strong gradient-B

1500

1000r
500

tr

{t/----'e15(oorI

Hfi'o

0 "

00

iffi ffirl

i oUi

LJ /I;00

n7TtI-NFig. 3 Spatial profileof the floating potential on the conducting plates at the outer-transition region of theeast anchor-cell.

structure in this region.
Figure 4 shows the photographs of the main plates

APlAP2- '99-'01'98-'99
Northtilii

Soutw$'$."i"FsF;"xfm:gono,tt

Bofrara,W;;
Fig.4 Photographs ofthe anchor conducting plates used in 3 years. The circles show the area oftypical deposited areaby impurity material. All the plates are shown in inner surface (facing the plasma).
430

Nakashima Y. et al., Behavior of the Edge Plasma near the Anchor Conducting Plates

that after 3-years operation (1998-2001). A significantexpansion of the deposited area is recognized in every
region due to the latter 2-years operation. Theseobservations tre explained by such mechanism thatplasma ions hitting on the plate sputter impurities fromthe plate due to the drift effect and the sputteredimpurities are redeposited on the opposite plate.

downward in the south side) is observed and the passing
ions have a considerable possibility of collision with theconducting plates. A similar observation is confirmed atthe outer transition region (Fig. 5(b)). From the above

calculation, observed asymmetry in the probe
measurements and impurity deposit areas on the plates

is mainly ascribed to the gradient-B drift of ionsoriginated from the anchor-cell mid plane.

4. Discussion
4.1 lon orbit calculations

4.2 Ettect on the plasma parameters

To investigate the mechanism of the asymmetry inmeasured data, a numerical calculation for tracking thepassing-ion orbit has been carried out considering theeffect of gradient-B drift in the anchor transition region.
The edge regic,n at the anchor-cell mid plane is selected
as a launching position of test particles and the tracking

Figure 6 shows an example indicating improvementof plasma confinement by adjusting the electricalconnection in the conducting plates. The time behaviorof plasma line-density and diamagnetism in the central-cell are compared between the case in the floating mode
(Fig. 6(a)) and that in the grounded mode (Fig. 6(b)). In
the floating mode, a remarkable increase in both theline-density and the diamagnetism is observed
accompanied by the ECH pulse for potential formation.
In the case of the grounded mode, on the other hand, thedensity increase is small and a slight decrease of
diamagnetism is observed during the plug ECH. Thisshows that the floating mode is advantageous for
improving plasma confinement by potential formation.
A typical time variation of plasma parameters measured
under the condition that the space between the plates isof the orbit is started toward both inner (central-cell)and outer (plug/barrier cell) directions. The initial valueof test particles; is given by assuming the conservation ofmagnetic moment and energy of ions coming from thecentral-cell. The obtained results are shown in Fig. 5.
Symbols in the: figure represent the locations of guiding
center of passing ions launched at the initial positions in
Rcc = 20 cm, 26 cm and 30 cm, respectively. In theinner transition region (Fig. 5(a)), a shift of the orbit in
the direction of gradient-B (upward in the north side and(a)(b)

West lnnerTransition

West Outer Transition^ ^10nelso_o--oA,,o(z=400 cm)

6>?

80

4040

2go^ ^.10.EX^
X''-2B8o-o-40-4-80 -40 0 40 80
Y (cm)-80

E=2-4-2024

0Y (cm). RcC=20 cm . Rcc=26 Cm ^ Rcc=30 cm) Deposited area :f----)

OA-'10'E3 8r'It

Magnetic flux
surfaceAF.

qF
r
8= 2ilAnchorconducting plate

EOCUFig. 5 Results of the orbit calculations. Filled symbols
represent the guiding-center trace of ions. Cross

100

150

200

250

Time ( ms )section of magnetic flux surface is plotted corre-Fig. 6 Time behavior of plasma line-density and diamag-

sponding to the launching positions of test particles (Rcc=20 cm, 26 cm and 30 cm). Locations ofnetism measured at the central-cell. (a) in thefloating mode of the anchor conducting plates, (b)in the grounded mode, (c) in the expanded con-the anchor conducting plates and impurity-deposited areas are also indicated.

ducting plates.
431

Nakashima Y. et al., Behavior of the Edge Plasma near the Anchor Conducting Plates

region. Further investigation is needed to clarify theinfluence of this drift on the radial loss of core plasma
based on the more detailed experimental results. The
effect of the conducting plates on the plasma
performance is investigated and it is confirmed that thefloating mode is advantageous for improving plasma
confinement by potential formation. However, it isrecognized that expansion of the space between theexpanded by 20Vo is shown in Fig. 6(c). As shown in thefigure, it is found that the increase of line density as
well as diamagnetism during potential formation
become less even in the floating mode and their
behavior is similar to the case with the grounded mode.
These results clearly indicate that installation of theconducting plates near the plasma surface in a suitabledistance has a certain effect on improving the radial loss

during potential confinement. These observations can beexplained qualitatively by the following interpretation:plates facing each other reduces the above effect.

The floating condition of narrower conducting plates
prevent drift ions from escaping to the radial direction in
the transition region due to the positive sheath potential
produced in front of each main conducting plate. In thecase ofexpanded plate, however, the suppression effecton radial loss may be reduced since the location of
repelled ions becomes close to the vacuum chamber

Acknowledgement
The authors would like to acknowledge themembers of the GAMMA l0 group, University ofTsukuba for their collaboration in the experiments.

Referencestll
5. Summary

K. Yatsti, et al., Proc. 17th IAEA Fusion Energy
Conference, Yokohama, 1998, IAEA-CN-69/EX4I

Edge plasmas in the anchor outer-transition region

6.

l2l K. Yatsu, et al., Trans. of Fusion Technol. 35, No.

were investigated by using Langmuir probe array and amovable probe. A significant asymmetry in the ion

rT,52 (1999).

saturation current and the floating potential arerecognized, which corresponds to the direction ofgradient-B drift for ions. Impurity deposited areas
observed on the conducting plates showed the same
asymmetry and the structure of rotational symmetry in
180 degrees. The results of orbit calculation taking theeffect of gradient-B drift into consideration indicatedthat the drift effect is a dominant factor to cause theasymmetry of ions observed at the outer transition

t3l Y. Nakashima, et al., J. Plasma Fusion Res. 75, No.
r0, r2rr (1999).
K.
t4l Md. Islam, et al., J. Phys. Soc. Jpn. 69 No. 8,2493 (2000.
t5l Y. Nakashima, et al., Trans. Fusion Technol. 39,
No. lT, 139 (2001).

t6l Y. Nakashima, et al., J. Nucl. Mater. 290-293, 683(2001).

432