Plasma Fusion Res. SERIES, vol.3 (2000) 238-242

Operation of TEXTOR-94 with a Tungsten Poloidal Main
Limiter System
posplEszczYK Albrecht*, TANABE Tetsuo5, PHILIPPS Volkera, SERGIENKO Genadija, OHGO Tadashir,
KONDO Katsumi2, WADA Motoi3, RUBEL MarekT, Von SEGGERN Jana, BIEL Wolfgang, HUBER Alexander,KIRSCHNER Andreas, BERTSCHINGER Giinter, RAPP Ji.irgen, SCHWEER Bernd and NODA Nobuki6
Institut filr Plasmaphysik, Forschungszentum Jiilich, TEC, Ass. Euratom-Jiilich, GermanylDepartment of Physics, Fukuoka University of Education, Munakata, Fukuoka, Japan
2Graduate School of Energy Science, Kyoto IJniversity, Uji, Kyoto, Japan
3Department of Electronics, Doshisha IJniversity, Kyotanabe, Japan
alnstitutefor High Temperatures of the MS, Assoc. IWAN, Russia
5Centerfor Integrated Research in Science & Engineering, Nagoya Univ., Nagoya, Japan
6National Institutefor Fusion Science, Toki, Gifu, Japan

TAyven Laboratory, Royal Institute of Technology, SE-10044 Stockholm, Sweden(Received: 18 January 2000 / Accepted: 12 May 2000)

Abstract
In TEXTOR-94 experiments have been performed with the upper and lower poloidal limiter tiles
made of vapor sprayed (VSP) tungsten (about 0.5 mm) deposited on graphite with a Rhenium interlayer.

A series of discharge conditions have been performed (density scan, scan of the auxiliary heating power,radius scan). There has been no restriction for operation at any density with auxiliary heating. For Ohmic
conditions the same density with testlimiters - and even higher - could be reached. At high discharge
power losses by radiation no severe accumulation of tungsten in the plasma centre could be detected. Theblocks could in general stand surface temperatures below 1700 K. However, most of them survived also
temperatures above 3000 K without exfoliation. However, some blocks showed severe damage bymelting or exfoliation probably due to not sufficient contact of the tungsten layer with the graphite.

Keywords:
tokamak, poloidal limiter, tungsten, heat loads

1. Introduction

twin tungsten test limiters (one half made of graphite

Tungsten is one of the high -Z materials, which isfavored as wall or divertor material in the future. The
reason for this is that both its erosion is much lower andits redeposition higher than for carbon based materials.
This can be a considerable advantage for long pulse or

and the other half of tungsten)[], which can be inserted

into the main plasma or withdrawn on a shot-to-shotbasis. However, the active area of these limiters iscomparatively small and their influence on the main
plasma may not be typical. Therefore, in order tosteady state fusion devices where erosion andincrease the interactive tungsten surface area up to arelevant number, the ten blocks of the upper and lower
poloidal TEXTOR-94 limiter have been replaced from

redeposition may significantly limit the lifetime ofplasma facing materials. In TEXTOR-94 severalexperiments have already been carried out using full or@2000 by The Japan Society of Plasma
Science and Nuclear Fusion Research*Corresponding author's e-mnil: a.pos @fz-juelich.de238

Pospieszczyk A. et al., Operation of TEXTOR-94 with a Tungsten Poloidal Main Limiter System

pure graphite to vacuum vapor sprayed (VSP) tungsten(about 0.5 mm) deposited on graphite with a Rhenium

the limiter surface was observed spectroscopically bymeans of a CCD-spectrometer and video cameras incombination with interference filters [2]. The surface
temperatures were measured via a CCD-camera with acut-off filter at 850 nm. which could be cross calibrated
by a pyrometer aiming at the same spot on one of theinterlayer. These kind of limiters also allow the study ofthe behaviour of these coatings under strong thermalloads (i.e. more than l0 MWm2), which might be
typical for excessive power loads on divertor tiles.

blocks.

2. Experiment
TEXTOR was normally operated under theCore plasma parameters are monitored with thestandard TEXTOR diagnostics. In addition an XUVspectrometer has been installed for the observation of
the spectral emission ofthe quasicontinuum radiation offollowing discharge conditions: Ip = 350 kA, B, = 1.752.25 T, NBl-heating power of 1.3 MW and ICRH power
up to I MW. The l0 poloidal limiter blocks have each alength of 130 mm, a height of 80 mm (bottom) 65 mm

WXXVIII around 5 nm from the plasma center.

3. Results and Discussion(top) and a curvature of 50 mm on both sides in toroidal
direction. Within about 100 ms they can be moved intoor retracted from the plasma, the radius of which isnormally defined by the toroidal belt limiter ALT-II atr7;^ = 46 cm. In general the "active" surface of theThe two main subjects, which have been studied,
are the following: How much is the power / temperature

which such limiters can bear during plasma operation and - is there any harming influence on the generalpoloidal limiter amounted to about 200 cm2.
Boronization (in the first part of the campaign) andperformance. Other subjects of importance are e.g. thesiliconization (in the second half) was routinely applied
for surface conditioning of the inner wall and nonremovable limiter components.

particle release rates for tungsten, oxygen, carbon and

The individual blocks were equipped with

Figure I shows the arrangement of the upper
limiter blocks and their appearance before and after

uptake of these WJayers concerning hydrogen and theirhydrocarbons. Parts of the latter subjects will bepresented in [3].

thermocouples and heaters. The particle emission fromabout 150 discharges. In order to prevent these blocks

from too high heat loads at the beginning of thecampaign, they have been very carefully exposed to theIIC?e€-soEt.9U'ITime / s
Fig.2limiter at 45 cm:2-3 s; NBI (1.3 MW) 1-4 s; at
2.5 s: fAlro = 2 x 1018/(cm s), f t'"o = 2.8 x 101s /(cmFig. 1 Tungsten poloidal limiter blocks: virgin (a) andafter about 150 discharges {b).

sl, Puo= 0.6 MW, lt't*,/T't'"o = 0.0002, Ft'".,'/Tt'*o= 0.01, fLrMou/T'''"o = 0.003.

239

Pospieszczyk A . et al., Operation of TEXTOR-94 with a Tungsten Poloidal Main Limiter Systemagainst intensive carbon deposition during "normal"
plasma operation and behaved in many respects asnormal carbon limiters except at surfaces, where thecarbon layer had been sputtered away during intensive
impinging impurity fluxes. However, these - sometimes
intentional - "cleaning" procedures proved to be very
sensitive to the power loads on the individual blocks;
either the cleaning effect was small or there was adanger that the W-layer suffered by exfoliation. This
relative independence of relative W-fluxes could be seen
during scans of the auxiliary power (both for ICRH andNBI). As main reason for the release of tungsten
sputtering by impurities could again be found. This isshown in fig. 4 when an unintentional drop of the upper
limiter occurred by interference of ICRH into the limiter
control system. However, it demonstrates that not somuch the energy of the impinging particles is the causeof tungsten sputtering but the kind of impurities. From

plasma for a restriced time during the flat top phase ofthe discharge. During other programmes they wereretracted nearly to liner position to prevent unintentional

overload and destruction.
The ratio of convective power ( I - y) lT = radiatedpowerl to total input power proved to be quite different
for boronized and siliconized conditions - for the latter it
was about a factor of 3 smaller - which led to incomparable discharge scenarios.
Figure 2 shows the behaviour of a plasma just after
a fresh boronization, which resulted in discharges with 7< 0.3 i.e. high convective loads. From the temperaturerise in the bulk of AT = 35 K one can derive an energy
load of about 40 kJ in I s (i.e. a heat flux of l2MWlmz
for the "active" limiter surface), from which a maximum
surface temperature of about 1700 K can be calculated[4]. The resulting D-flux at T" = 80 eV is ILIMp = | .2 x
lOre/(cm2s), which is in good agreement with themeasured one under the assumption of an "active"
limiter surface of 100 cm2. During the insertion of thelimiter fAlro drops by about 107o, reducing the total
plasma energy by 90 kJ, which is 2 times more thancomparison of the D, CI and OI signals one canexpected; however, the energies of the scraped particles

conclude that oxygen is the constituent, which causes amajor contribution to the tungsten sputtering.
The general performance of the plasma discharges
with poloidal tungsten limiters proved to be better thanare less in the outer parts of the boundary layer (see Fig.

expected. One of the major reasons for this behaviour3).

was obviously the all-carbon surrounding of theThere are also unexpected findings in comparison

TEXTOR-94 vessel and built-in components. One of theimpressive results was that even Ohmic dischargesdisplayed at least the same or even higher density limits
as similar ones with tungsten test limiters. This is shownin fig. 5 with the poloidal W-limiter even 4 cm further

with the test limiter experiments. The tungsten flux ratioltt"*,/ft*o is about an order smaller and the flux ratio

fttt"/fttto for hydrocarbons one order larger. It may

well be possible that these two ratios depend on eachother: the poloidal limiters could not be protected

fvdD" (pol Limiter);4ox

II(t,cr5o (ool. \Limiter)'{Agz.cto\8(t'\EoD" (ALT Limiter)oo1(t) g!

oo

NBI-Power (1.4MW).g)cr(AL'l Limiter)

I04445

ICRH-Power (14647480r2345

Poloidal limiter position / cmTime / s

Fig. 3 Fluxes and absorbed heat during a poloidal
limiter scan for Ohmic,, low density discharges.

Fig. 4 Poloidal limiter fluxes during a short insertion
from 45 cm -> 42 cm between 1.2 s -> 1.6 s.
240

Pospieszczyk A. et al., Operation of TEXTOR-94 with a Tungsten Poloidal Main Limiter System1o€10-2oon

cogEU'ola''0"

xf

*

tra

tot.gcolsOI

E=tro. _-5 |'" 1.5

2.o

2.5

3.0
Line-averaged density 11013 cm{]

0246

4ddo

Fig. 6 W-concentration and -fluxes during a density scanwith ICRH-power of 1 MW {rg" = 46"r,.

Time / sFig. 5 Ohmic disch. with W-limiter at 42 cm from 1 .7 s.

!(

1700@

more in than the main limiter ALT-II. However, theradiated power was in this case very near to the totalc)
L=1500Einput power - a common result of a siliconisation
campaign which had been carried out before theseexperiments [5]; moreover, the increasing impurityq)
CLEo1300q)

fluxes of carbon and oxygen did not lead to a growing
tungsten release. On the contrary, at the maximum levelof n"(0) = 4.5 x l0r3 cm-3 at 3.1 s both the tungsten
fluxes and the tungsten concentration in the center(represented by the WXXVIII-quasi-continuum=E6p

1 100(L900oo

radiation around ,1, = 5 nm) are practically negligible - avery similar behaviour as in ASDEX-U near the density
limit [6]. Indeed, during that experimental campaign
mentioned the density limit for discharges was equal for
carbon and W-limiters. For higher convective powers

0246
Time / sFig. 7 Surface temperatures of the 5 bottom blocks
during NB|-heating (1.3 MW) with W-limiter at 42

cm from 1.7 s.

Ohmic discharges of only 75Vo of this density could bereached until the strong tungsten accumulation in thecenter lead to a radiation collapse. Again under nocircumstances led NBI- and/or ICRH-power heatedrestricted discharge duration this effect could not havebeen studied further in detail.

discharges to remarkable W-concentrations.
Figure 6 displays this behaviour for a density scan

At the end of the campaigns experiments had been
carried out to find the load limits of the tunssten limiter

during auxiliary ICRH-heating. There is obviously astrong screening effect with increasing density, whichblocks.

Figure 7 shows that, although the total energy
uptake is only half of that discharge displayed infig.2
because of the higher radiation losses, a nearly equal
temperature is reached. This measured temperature

prevents the tungsten to diffuse into the plasma center.

On the other hand one should add that during theseexperiments both the tungsten fluxes and concentration
continuously rose during the discharge time when theagrees well on average with calculated one [4].
However, one can already notice that not all blocks havepoloidal limiter was inserted; there seem to bemechanisms, which gradually change either thereached the same surface temperature. On the other hand

sputtering rate, -surface, or responsible particles during
longer power loads onto the limiters. Because of thethe absorbed energy, represented by the temperature risemeasured by the thermocouples, shows a reciprocal
241

Pospieszczyk A. et al., Operation of TEXTOR-94 with a Tungsten Poloidal Main Limiter Systembehaviour for the individual blocks. A lower surface
temperature is obviously a hint for a good contact
between W-layer and graphite substrate. When thepower load was further increased, those blocks withhigher absorbed energy were more resistant toexfoliation than the others, where a bad conduction led
to a cracking of the layer with subsequent melting of theeven higher - could be reached.

-At high levels of radiated power no severe
accumulation of tungsten in the plasma center
could be detected.

-the blocks could in general withstand surface
temperatures below 1700 K. However, most ofthem survived also temperatures above 3000 K
without exfoliation. For most of them larger cracks
appeared, but microcracks could not be seen.

tungsten.

However, it was found that most of the blocks
could stand energy loads up to about 100 kJ for I s,
which has led to surface temperatures well above 3000
K. This resulted in the appearance of several major

However, 4 blocks (from a total of 16 used)showed severe damage by melting or exfoliation.

cracks. Microcracks due to grain growth byReferences
V. Philipps et al.,J. Nucl. Mater.258-263 (1998)

recrystalisation were not observed. This is in contrast totll

ASDEX-Upgrade [6], but seems to confirm findings byheat load experiments with identical layers [7].

I2l A.Pospieszczyk, Diagnostics of Edge Plasma by

4. Gonclusions
The experiments performed with a poloidalOptical Methods, in R. K. Janev, H. W. Drawin,
Atomic & Plasma Materials Interaction Processes
in Controlled Thermonuclear Fusion, Elsevier,858.

tungsten limiter system on TEXTOR-94 did not only
confirm the previous results with tungsten test limiters,

Amsterdam, p.213.
t3l M. Wada et a/., this conference.
[4] Carslaw & Jaeger; Conduction of Heat in Solids;
Clarrendon Press, Oxford (1959).
[5] J. Winter et al.,Phys. Rev. Lett. 7l (1993) 1549.
16l K.Krieger et al., J. Nucl. Mater. 266-269 (1999)

but revealed even more promising characteristics.
However, one should have in mind that these may be
favoured by the wall conditioning procedures and theall-carbon surrounding. In particular- there has been no restriction for operation at any
density with auxiliary heating. For Ohmic
conditions the same densitv with test limiters - and207.

t7l K. Tokunaga et al., J. Nucl. Mater. 258-263 (1998)
998.

^tAa