J. Plasma Fusion Res. SERIES, Vot.3 (2000) 293-296

Particle Recycling at Garbon Deposited Tungsten Surfaces
Facing TEXTOR-94 Plasmas
WADA Motoi*, POSPIESZCZYK Albrechtr, OHGO Tadashi2, TANABE Tetsuo3, HUBER Alexanderr,
SERGIENKO Genardi4, PHILIPPS VoIkerI, OHYA Kaoru5 and NoDA Nobuaki6
Department of Electronics, Doshisha University, Kyotanabe, 610-0321 Japan
tlnstitute of Plasma Physics, Jiilich Research Center, TEC,Jiilich, D52425, Germany
2Department of Physics, Fukuoka
IJniversity of Education, Fukuoka, 8II-4192, Japan
3CIRSE, Nagoya tJniversity,alnstitute

Nagoya, 464-8603, Japanfor High Temperatures of the RAS, Assoc. IVTAN, Russia

sFaculty of Engineering, Tokushima University,6National Institute

Tokushima, 770-8506, Japanfor Fusion Science, Toki-shi, 509-5292 Japan(Received: l8 January 2000 / Accepted: l0 May 2000)

Abstract
The W poloidal limiter was placed beyond the LCFS determined by the carbon toroidal limiter
ALT-II to form a C layer on the W surface. The formed C layer could be removed by plasma radiation
about 1.5 x lOle cm-2 deuteron fluence when the limiter was kept in the shadow of ALT-II for about 60
discharges. When the surface was kept in the shadow for about 1600 discharges, the C layer became
stable against plasma exposures and the intensity ratio of the line spectnrm emission of WI to DT wasreduced by one order of magnitude, while the e-folding lengths of spectrum intensities toward the plasma
center for Dy, WI, CII and OII were unchanged. The surface showed an increase in intensity of CD band
spectrum as the limiter was exposed to plasma heated by a 5 s NBI pulse.

Keywords:
tungsten, particle recycling, TEXTOR-94

reemission from the surface due to a mutualcontamination between C and W, a test limiter half
made of C and another half made of W had been put

1. Introduction
There is the possibility that both low-Z andhigh-Zmaterials are used as first wall materials for a future
fusion experiment device. When both low and high-Zmaterials are present in the inner surface of theconfinement device, they are removed from the wall,
penetrate into the edge plasma, and are deposited onother parts of the wall made of different materials. The
deposition layer of alow-Z element onhigh-Z material

into the edge plasma of TEXTOR-94 beyond the LCFS[]. The change in the impurity emission from the part
directly exposed to an edge plasma radiation had beenobserved after several tens of shots. The post-mortem
analysis had shown the formation of W implanted C,
and C implanted W layers on the limiter surface [2].
Meanwhile, the amount of impurity C-flux in theTEXTOR edge plasma can be up to 2 to 3Ea of deuteron
flux because of C released from the toroidal belt limiteror vice versa should show particle recycling different
from the pure material, and affect the condition of theedge plasma.

To see the change in the characteristics of particle*Corresponding author's e-mail: mwada@ mail.doshisha.acjp293@2000 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchWada M. et al., Particle Recycling at Carbon Deposited Tungsten Surfaces Facing TEXTOR-94 Plasmas

covered the wavelength range from 415 to 437 nm. A
CCD camera observing the W poloidal limiter surface
through a 1.5 nm bandwidth interference filter at 400.9
nm was used to monitor a two dimensional spatial

ALT-II. The poloidal limiter made of vacuum plasma
sprayed (VPS) W deposited on graphite with a Re
interlayer was installed to study its impact upon theTEXTOR plasma [3]. After the experiment the limiter
had been withdrawn from the plasma, and kept outsideof the LCFS. At the time of vacuum opening the limiter

distribution of WI neutral line spectrum emission. Therewas another CCD camera installed to observe the optical

was taken out of TEXTOR, and the surface wasconfirmed coated with C layer in the postmortem

image of the limiter.

analysis [2]. The thickness of the C layer on the poloidal
limiter was larger than that of a twin limiter, and theparticle recycling can be completely different between

3. Results and Discussion
3.1 Short term accumulation of C on W
surface
The top 5 blocks of the W-poloidal limiter were
replaced to new ones during the vacuum opening ofrwo limiters. To investigate the change in impurity
emission due to the formation of this C layer, a series ofexperiments has been conducted by installing newTEXTOR. Before the exposure of the W-poloidal limiterto the plasma, 53 discharges had been made for otherresearch missions by keeping the limiter position at 49cm minor radius.
Prior to the experiment a He glow discharge wassustained for 40 minutes to reduce oxygen. After theglow discharge treatment, four ohmic discharges weremade keeping the limiter position at 49 cm. Then thepoloidal limiter was positioned at 48 cm minor radius
for shot No. 81679 and at 47 cm minor radius for shot
No. 81680. No line radiation of WI was observed on theblocks of W poloidal limiter, and coat it with C bykeeping in the shadow region of the ALT-II belt limiterfor specified numbers of shots.

2. Experimental Arrangement
TEXTOR-94 was operated with the standard
discharge conditions of I,-35O kA and Br-2.24 T. Core

plasma parameters were obtained from the routine
diagnostics, while the edge electron temperature anddensity were obtained from He atomic beam diagnostic
method [4]. The ALT-II main limiter fixed the plasma
minor radius to be 46 cm. The poloidal limiter wasmoved further into the plasma beyond 46 cm minor
radius to be exposed to TEXTOR plasmas.
The experimental set up for the local particle
emission measurement from the poloidal limiter surface
is schematically illustrated in Fig. l. The surface of thelimiter surface for these shots, but the CD band
spectrum was observed indicating the presence of C on
the surface. Toward the end of the shot No. 81681 when

the limiter was located at 46 cm minor radius. a
noticeable signal of WI was recorded on the limiter
surface.

The peak of the line spectrum intensity of WI

poloidal limiter was observed from the toroidal

became comparable to those of CrI and OII, as thelimiter was moved up to 45.5 cm minor radius in shot
81682. During shots 81682 and 81683, the intensity of
WI had slowly increased. In Fig. 2, the spectrum for thefinal stage of the shot 81683 with the poloidal limiter
positioned at 45 cm minor radius, is shown to compare
it with the spectrum for the beginning stage of the shot
81682. As shown in the figure, the intensities of the WIlines became larger, and the intensity of the CD band
spectrum became smaller. The CD band spectrum had
been reduced to less than a half of the orisinal valueafter shot 81683.
The deuteron fluence onto the limiter integrated
over the toroidal direction by shots 81681, 81682, and81683 was estimated to be 4.3 x 10re cm-r. If thetangential direction through an optical spectrometer thatSp€ctrorneter
WI image

Wtest limiterSp€ctrorneter

Vitleo fumge
2D W rmAge

geometrical length of the limiter in the toroidal
direction, which is 3 cm for the poloidal limiter is

ALT-II rnain limiterassumed to be the total length in the line-of-site for theparticle emission, the deuteron fluence can be roughlyFig. 1 Experimental arrangement.

294

Wada M' et al.' Particle Recycling at Carbon Deposited Tungsten Surfaces Facing

TEXTOR-94 plasmas

300='250t0; 200o-9 rsoeo

3 too

coot50.E04&Sevcengttr
1nm1430

300

300
250ct

€obg=.250

1 eoo

200

e9t so150

Erooc r00

cEo5so50

00125w avohnglh (nm)

rt30

120

125430,t35

wawlength (nm)Fig. 3 Comparison of the spectra between shots 83332

Fig. 2 Comparison of the spectra between shots 81682
and 81683.

and 833u14.

estimated to be 1.5 x l0re cm-2. The C layer on the W
poloidal limiter could been removed with this magnitudeof deuteron fluence when the C layer on the W surface
was formed by about 60 shots.
J3.2 Long-term accumulation of C on W surface;

After shot 81739, the W poloidal limiter was kept
at 49 cm minor radius. It was moved up to 45.5 cm
minor radius in shot 83332. This means that the poloidal
limiter was kept in the shadow region of the ALT-IIlimiter for about 1600 shots. As shown in Fig. 3, thepeak of WI could never be identified, but a clear CDband spectrum was observed. After l0 shots with thepoloidal limiter positioned deeper than the ALT-II, theoFo 10goac

WI peak was recognized on the spectrum at shot 83344.
The line integrated deuteron fluence for these l0 shots
was about 6.5 x 1Ole cm-r. However, the intensity of WI
did not become as strong as that observed after theremoval of C layer formed on the newly installed W4644424038

Minor radius (cm )Fig. 4 The intensity distributions of line spectra near thesurface of the C covered W poloidal limiter.

limiter block like the case of shot 81683.
The intensity ratio of WI to CII was about 0.06 forshot 83344, while it was 0.208 for shot 81705 before thethan that of shot 81683. Meanwhile the intensity ratio of

long-term surface accumulation of C. The plasma
exposure was continued until the line integrated D

WI to Dy of shot 83344 was smaller than that of 81683
by an order of magnitude. (See also Fig. 4.)

fluence exceeded 4.4 x 7020 cm-l, but no apparent signof enhancement in WI intensity was observed. The flux
ratio of C to D of shot 83344 estimated from the line
spectrum intensities was about l.2Vo and was smaller

3.3 Particle release from C deposited W
surface
Figure 4 shows the intensities of spectral lines of295

Wada M. et aI., particle Recycling at Carbon Deposited Tungsten Surfaces Facing TEXTOR-94

Plasmasthe CD band spectrum intensity was strong at the top of

D1, CII, OII and WI plotted as functions of radial
position for shot 83344. To find the effect of the longterm deposition of carbon on the surface of the W
limiter, data in Fig. 4 are compared with the similargraph obtained for shot 81683. As shown in the figure.

the limiter and also at the position about 3 cm awayfrom the top. The intensity had increased at both
positions in accordance with the passage of time while
the Dy flux stayed almost constant during the discharge.

the signals fit reasonably well to exponential functions,

The increase of CD signal is attributable to theand e-folding lengths could be determined from thetemperature rise of the bulk limiter from 635 K to 725

K, which can cause thermal desorption and/orgraph.

The obtained e-folding lengths were 3.3 cm forCll,2.8 cm for OIl, 4.7 cm for D1, and 0.5 cm for WI,

enhancement in chemical sputtering on C layer.

respectively. The local electron density and temperature
measured by the atomic beam method at the position ofthe limiter werc 1.2 x 10r8 m-3 and 60 eV, respectively.
When we compare the e-folding length of WI with thedata obtained from the solid W test limiter [5], it isslightly larger, but essentially the same within the4. Summary
The C layer formed on the W-poloidal limiter
surface by 1600 TEXTOR discharges in the shadow ofALT-II could not be removed by D fluence about 1.5 xlgzo .--2. Due to the formation of the C layer, the ratioof WI intensity to DY was reduced by one order ofmagnitude. Meanwhile, the e-folding length of WI as
well as those of OII, CII and D1 did not change by thelayer formation. The C coated limiter surface showed
CD emission as the limiter was exposed to plasma

experimental accuracy.
In shot 81683 the local electron density was 1.2 x
l0l8 m-3 and the electron temperature was slightly higher
than that of shot 83341 and was 70 eV. The e-folding
lengths under these conditions were 3.3 cm for Cll,2.9

heated with NBI for 5 s.

cm for OII, 4.6 cm for Dy, and 0.5 cm for WI,References

respectively. Therefore, the e-folding lengths of CII,
OII, Dy and WI were not affected by the formation oftll

Controlled Fusion and Plasma Phys. (1999).
[2] T. Tanabe, V. Philipps, A. Pospieszczyk et al.,Tobe published in J. Nucl. Mater.

the C layer.

3.4 CD release during long NBI pulse
The CD release from the C coated W surface for adischarge with the 1.3 MW, 5 s NBI heating wasinvestigated for shot 83363 by locating the limiterl2l A. Pospieszczyk et al., in these proceedings.
t4l B. Schweer, G. Mank, A. Pospieszczyk et al., J.
Nucl. Mater. 196-198, 174 (1992).
t5l M. Wada, V. Philipps, A. Pospieszczyk et al., J.
Nucl. Mater. 241-243,1164 (1997).

position at 44 cm minor radius. The electron density andtemperature at the position of the limiter top was 1.2 xlgle ttr-3 and 45 eV, respectively. As shown in Fig. 5,IC?lolloo30
E
ltoo20oo310oc€o50

48

40

11Minor radius (cm )12A.Huber, V. Philipps et al., Proc.26th EPS Conf.

40Fig. 5 Change in the spatial intensity distribution of CDband spectrum. Top of the limiter is located atabout 44 cm minor radius.

296