J. Plasma Fusiorr Res. SERIES, Vol. 5 (2002) 433436

Characteristic Plasma Behaviors in the Divertor Reversed
Field Pinch TPE-2M
HAYASE Kiyoshi, SATO Yasuhiro, KIYAMA Satoru and MAEJIMA Yoshiki
AIST Tsukuba Central, Tsukuba 305-8568, Japan(Received: 12 December 2OOl I Accepted; 24 May 2002)

Abstract
The behaviors of RFP plasma with a poloidal field divertor are studied in the TPE-2M Reversed
Field Pinch to establish and evaluate the divertor as a means of plasma-wall interaction controls in RFPs.
The divertor plasma is measured by Langmuir probe, visible spectroscopy and magnetic probe. The
characteristic behaviors in the RFP divertor are the plasma burst into the divertor region at the RFP field
setup stage and the strong magnetic fluctuation in the divertor region. The latter may cause theanomalous effusion or loss/scattering of particles through the X-point region and the additional helicity
loss which leads to the enhanced loop voltage.

Keywords:
RFP, nrversed field pinch, divertor, dynamo, plasma-wall interaction, PWI, PSI, TPE-2M

1. Introduction

divertor plate surface is measured by a Langmuir probearray. The observed unique divertor plasma profile andthe increased toroidal loop voltage are discussed inrelation to the enhanced masnetic fluctuation in thedivertor region.

The fusion plasma confinement concept based on
the reversed fi,:ld pinch (RFP) is considered to be one ofsimple and economic fusion reactor core concepts, inwhich it is claimed that the ignition may be attainedwith Ohmic heating alone as well as with a normal
conductor magnet [1,2]. Many works are now devoted

2. Device Structure and Wall Conditioning
The cross-sectional view of the vacuum vessel,
shell and coil assembly is shown in Fig. 1. The

to confinenrent improvement and/or physics

understanding. While, the control of plasma-wallinteraction (PWI) will be quite important for a highcurrent and long time discharge in the near future, forwhich pioneering works had been carried out with theengineering aspect of device is described in ref. [7]. The
aluminum stabilizing shell of circular inner cross-section(28 cm in I.D. and 2 cm in thickness) with anmagnetic divertor in [3,4]. To pursue it moreelaborately, 1'PE-2M (R/r = 87 cm/28 cm) has been

axisymmetric poloidal cut (approx. l8 degrees wide) is
installed in the rectangular SUS vacuum vessel (10 mm/2O mm thick) to ensure both a good shell-plasma surface

working for the divertor RFP discharge [5]. The global
stability had already been confirmed [6]. In the recent
series of experiment, the characteristic behaviors ofdiverted plasma and the global effect on the maindischarge are investigated. Here, the wall conditioning isproved essential to reduce the neutral gas effect on PWIstudy. The donsity profile of effused plasma on theC orre spondin 11 author' s e -mail : k. hayas e @ ais t. goproximity and a diverting space. The minimum distance
between the divertor plate surface and the shell innersurface is l2 cm. The position of X-point is controlled
by varying the plasma to divertor coil current ratio.
Typical calculated separatrix surfaces are drawn in the@2002 by The Japan Society of Plasma
Science and Nuclear Fusion Researchjp+JJ

Hayase K. et al., Characteristic Plasma Behaviors in the Divertor Reversed Field Pinch TPE-2MEPlasma cunenl

Bv coilBs

Bv_\OH

Toroidal loopvoltage

X"*.r**-r+.r-l1*lr\^ ..;,

Toroidal flux of olasma

a_uToroidal magnetic fieldon the shell inner aurface

14Time (ms)Fig. 2 Typical waveforms of discharge with divertor (fulllines) and without divertor (dotted lines) field.

Fig. 1 Cross-sectional view of vacuum vessel, shell andcoils of the divertor RFP, TPE-2M. Two separatrix
lines are shown for different X-point locations; 4
cm from the divertor plate surface and 3 cm fromthe shell inner radius.

limited usually by the volt-second capacity of OH
circuit, however, the discharge terminates earlier insome cases, presumably due to the uncompensated error

same figure. An array of l1 double probes (14 cm fromfields. To alleviate it, the external DC horizontal andvertical fields are elaborately added as well as thetop to bottom positions) is mounted on the divertor platesurface The divertor plasma and the edge plasma of
main core are also observed by a visible spectroscopy,
an inserted Langmuir probe and inserted/pre-installedpulsed vertical field.

Typical signals of plasma current, toroidal loop
voltage, etc are shown in Fig. 2. The plasma current
decays faster and the loop voltage is slightly higher inmagnetic probes.
In order to reduce the effect of neutral gas on thethe divertor discharge. The higher loop voltage implies adivertor plasma and its interaction with the divertor
plate surface, the inner wall surface of vacuum vessel

larger helicity loss. The helicity loss comes from thenormal and anomalous resistivities of plasma, and thelatter is considered to originate from such as anomalous
magnetic fluctuation. The presently observed loop
voltage is rather high even in the non-divertor operationcompared with usual spatially closed shell devices ofsimilar radius and plasma current (normally 20-50 V).
The specific character is that the magnetic fluctuationlevel is much higher in the open shell region than in theincluding the divertor plate surface and the outer surfaceof shell are activated by Ti-gettering for every 3-4
shots. The discharge D2 gas is injected by a fast acting

short-pulse puff with an intense glow discharge preionization, instead of a quasi-steady puffing, to reduce
the surrounding neutral gas density and to keep thedivertor surface fresh. Besides, the inner surface of shell(the plasma-facing wall) is boronized by mixeddischarge of trimethyl-boron and He gases. The
boronization is very effective to suppress the impurity
contamination of especially oxygen and aluminum in thesetup phase, and leads to the overall enhancement ofplasma conductivity [8].

shell-covered region as is described later. The magnetic

fluctuation in the open shell region is more intensive inthe divertor discharge, which may lead to an additionalhelicity loss. So that the difference of loop voltage may
qualitatively be explained by the existence of X-point in
the open shell structure. We, however, need a detailed
physical calculation to estimate it quantitatively. The
current terminates much earlier (< 4 ms) and sharply
when the X-point approaches to the shell inner radius.
In Fig. 3, some spectroscopic lines are shown. AII(396.2 nm) and CIII (464.7 nm) lines are from the edge
region of main plasma, and Do from the divertor region.

3, Experimental Results
3.1 General Properties of Discharge
The experiment is carried out typically with aplasma current of 50-70 kA, the duration of around 5ms, and the loop voltage of 100 V. The duration is434

Hayase K. et al., Characteristic Plasma Behaviors in the Divertor Reversed Field Pinch TPE-2M

This is the case without boronization. Withstage, however, is smaller in the non-divertor operation,
which indicates that the plasma effuses out more due tothe divertor field. The impurities, Al and C, from thewall surface seem to be suppressed by the divertor field.
In Fig. 4, profiles of ion saturation current of arrayedprobes on the divertor plate surface are plotted for theboronization, the AII intensity is below the backgroundlevel and the CIII intensity is also slightly reduced even

without the divertor field. Comparing the divertor andnon-divertor discharges, all the intensities are nearly thesame at the beginning of the discharge immediately after
the setup. As far as Do intensity outside the shell in thedivertor region is concerned, the peak in the initial stage
may come from a burst of plasma particles effused out
from the main plasma through the open shell portioneven without the divertor field, when the RFP magnetic
configuration sets up. This burst of particle is observed
also by an inserted Langmuir probe in the divertor
region. The burst is further steeper in the case ofdivertor discharge. The Do line intensity in the latera.u.

early stage immediately after the set-up (-2 ms) and thelater stage (4-5 ms). The divertor effect is evidently
seen in the later stage. The difference of signals withand without the divertor field is taken as the signal fromthe divertor effect.

3.2 Divertor Plasma Profile
In Fig. 5, the spatial profiles of saturated probe
current on the divertor plate surface in the later stage arecompared for two X-point positions; 4 cm from thedivertor plate surface and 4 cm from the shell inner
radius. Although the data are largely scattered, theobserved density profile is not doubly peaked, but rathersmoothed to a single hump, which seems unexpected in
the present divertor magnetic field configuration. When
the X-point lies at 4 cm from the divertor plate surface,
the profile is sharply peaked. As the X-point moves
toward the plasma surface (4 cm from the shell innerradius), the profile becomes broader. The observed
single hump may be explained by overlapping of two
widely spread humps, which seems to be caused byanomalous particle scattering due to a large amplitude
fluctuation of magnetic field in the divertor region. InFig. 6, typical plots ofB. at 5 cm outside from the shellinner radius in the open shell region with and without
the divertor field and on the shell inner surface(horizontally outboard) are shown. It is much higher incm

420a u.

AII

4: !: . l:.

0246
Time(ms)the open shell region than on the shell-covered surface.
It is further enhanced with the divertor field. To estimate

Fig. 3 Spectroscopic line intensities with divertor (fulllines) and without divertor {dotted lines) field.

az\ -oE4Eo!o0-d2^oEadrU)(Bottom)ll}with diwrtor

ot{J^6-O.l(4-5ms)

L

4 cm fTom shell inner radius.' 8.,o1'oot_ftrl0(Middle)

trElo
6E 26

Early stage(-2 ms)rtr

E 4 cm ftom diwrtor platecI!rO

Liler sage IE llo

4=6

trOwodNertor

a-7(Botom)

Oop)

Probe position (ctn)

D!
ll,a^,' E^., Itr

E=
67uT

La
tro7(Middle)

Crop)(Probe position (cm)Fig. 4 Profiles of saturated probe current on the divertor

Fig. 5 Spatial profiles of saturated probe current on thedivertor plate surface for two positions of X-point;

plate surface at early and later stages of discharge. The full and empty marks correspond to

4 cm from the divertor plate (D) and 4 cm fromthe shell inner radius (A).

divertor and non-divertor discha rges, respectively.
435

Havase K. et al.. Characteristic Plasma Behaviors in the Divertor Reversed Field Pinch TPE-2M

main plasma core and the magnetic fluctuation is more
intensified.
The parameters of divertor plasma are estimated.
The temperature is estimated from V-I characteristics
and saturated current of double probe signal as shown inFig. 7. It is as high as -50 eV, while the temperature at
the edge of main plasma of the shell covered region isaround -30 eV irrespective with divertor field (The coreTime (ms)temperature is assumed at a few hundred eV from the 1oZ" scaling). The fact suggests the effusion of hot plasma

Fig. 6 B, signals (a) at 5 cm outside from the shell inner

through the X-point region is significantly large. Theradius (slightly outside the X-point) with thedensity of divertor plasma is only a few times smaller thandivertor field, (b) at the same position with (a) but
without the divertor field and (c) on the shell innersurface (shell-covered region).

the edge density of main plasma. The relatively highoAl plasma edge in the shell region

oAldivertorplalesufacej.

density on the divertor surface may be accounted
qualitatively by the large amplitude fluctuation of local
magnetic field in the divertor region and/or by the shortconnection length of magnetic field line (.B, << Bo in RFP).

t'od4. Gonclusion
In summary, the divertor discharge of RFP has
been operated in TPE-2M, and some characteristic
behaviors are newly found. An anomalous particle

acglaoe(!

effusion due to a strong magnetic fluctuation throughthe X-point region is suggested. This feature ofmagnetic fluctuation also explains the slight increase ofo2040@80
Voltage (V)toroidal loop voltage by the additional helicity loss. The
appropriate positions of X-point are limited in theFig. 7 Probe characteristics for estimating the electron
temperature at the divertor plate surface and theplasma edge of shell-covered region.

present geometrical configuration due to a kind ofinstability.

the level of B. fluctuation, the Bn fluctuation level is also

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When the X-point moves further toward the plasma

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t4l

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