Boron and carbon fluxes in the SOL of TEXTOR depending on boron content and temperature of the wall elements

Vacuum/volume

Pergamon Pll:

SOO42-207X(96WO096-6

47Jnumbers 6-8/pages 935 to 938/1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. AH rights reserved 0042-207X/96 $15.00+.00

Boron and carbon fluxes in the SOL of TEXTOI? depending on boron content and temperature of the wall elements J von Seggern, V Philipps, P Wienhold, A Pospieszczyk, EURATOM-KFA, D-52425 Jiilich, Germany

H G Esser and J Winter,

IPP-KFA Jiilich GmbH, Ass.

A limiter made from boron-doped graphite (3.5%, Carbon Lorraine) has been used in TEXTOR as a single main limiter. Additional heating by neutral beam injection (NBI) was applied resulting in heat loads up to 20 M W/m’ on the limiter, increasing its temperature far beyond 2000°C. At these high temperatures strong thermal sublimation of boron and carbon from the limiter was measured spectroscopically. After the set of experiments, deposits have been studied on the test limiter and in the SOL on collector probes by interference fringe analysis (IFAI, Auger electron spectroscopy (AES), electron microprobe (EPMAI and energy dispersive X-ray analysis (EOX). IFA was applied to estimate the total thickness of the deposits. A very thick boron rich layer (6 pm) had grown near the tangency point of the limiter. Comparing the evaporated and deposited amount of the limiter material a high local deposition of boron and carbon of at least about 69-70% is deduced. Copyright 0 1996 Elsevier Science Ltd. Key words: Boron-doped

graphite,

limiter, high heat load, erosion, local deposition,

SOL, impurity fluxes.

Introduction

Experimental

Graphite. often coated with low-Z materials (e.g. boron, beryllium, lithium or silicon) is presently used for limiters, divertor plates and other first wall components.‘m3 While the good mechanical and thermochemical properties of graphite are maintained the application of protective coatings may improve some of the disadvantages of pure graphite. The amount of hydrogen stored in the near surface region during one discharge by interaction with the hydrogenic fuel (wall pumping, thermal desorption) could successfully be controlled, the chemical erosion of carbon due to interaction with oxygen (CO formation) and hydrogen (hydrocarbons), respectively, was minimized. However, these films are less tolerant to erosion. They are easily used up and have to be renewed. Solid boron-doped graphite may provide a source for continuous liberation of boron due to plasma-induced erosion and lead to a successive conditioning of inner walls. Limiters made e.g. of different boron-doped carbon bulk materials have been tested as main limiters during a series of high heat flux discharges in TEXTOR.4.5 This paper reports on experiments with a boron-doped graphite limiter exposed in TEXTOR to a sequence of discharges of high power fluxes. Local erosion and redeposition upon the limiter as well as the impurity transport in the SOL will be reviewed and compared with observations acquired in TEXTOR without additional heating.

For these experiments TEXTOR was operated in deuterium at Z, = 340 kA, B,= 2.2 T, and at averaged line density n,(O) = 2.2 x 10’3cmmi. Neutral beam injection was used for auxiliary plasma heating. The electron temperature T,at the radius of the test limiter tip (v = 45 cm) was about 50 eV, the electron density at this radius being -5.4 x lO”cm~‘, and the corresponding deuterium Aux In = 2.5 x 1O’“cm ‘s I.’ The test limiter, used as the main poloidal limiter, was manufactured from 3.5% boron-doped graphite (Carbon Lorraine S2508). It was a 10 cm long, 6 cm wide and 5 cm high assembly with toroidal and poloidal top curvatures of 8.5 and 6 cm, respectively. The construction was equipped with two thermocouples mounted along its length axis, each 2 cm out of the tangency point, 7 mm behind the surface. The limiter was attached to TEXTOR by a limiter lock at the bottom of the torus and established 1cm closer to the plasma than the main ALT II toroidal belt limiter system (Y = 46 cm). It was externally heated prior to plasma discharges up to 6OOC. The power load onto the limiter was altered by varying the power as well as the duration of the NBI pulses (co and co + counter injection) and occasionally also by modifying the radial position of the test limiter. For some discharges the temperature distribution across the limiter surface was determined by infrared thermography using a CCD camera with an edge filter transparent above 850 nm1.7For 935

J von Seggern et a/: Boron and carbon fluxes

the other discharges the surface temperature near the location of highest heat loads has been reconstructed from signals of the thermocouples mounted 7 mm behind the surface. Using these signals, the integral energy flow to the limiter has been analyzed involving the entire metal support isolated limiter head as a calorimeter. Local impurity release from the limiter has been studied spectroscopically by a high resolution spectrometer equipped with an intensified CCD camera as a detector.7 Light emission in the wavelength between 408 and 436 nm in front of the test limiter was used for the observation of particle fluxes towards the plasma. After the total exposure time of 114 s deposition and erosion areas on the limiter surface as well as the limiter bulk have been investigated post mortem by IFA” sputter AES, EPMA and EDX. During some of the discharges impurities transported in the SOL have been collected by the Stockholm TEXTOR-probe system equipped with aluminium targets. The probe tip was set into the SOL at the midplane 2 cm behind the last close flux surface (LCFS) defined by the position of the test limiter (r = 45 cm) and the sample holder oriented perpendicularly to the field lines. The toroidal position of the system was about I IO” away from the limiter head in the ion drift direction with no direct connection of the magnetic field lines. With this arrangement the connection length to the next limiting element is _ 1.4 m upon the ion drift (ALT II blades) and up to 40 m on the electron drift side, respectively. The Al targets approached on the ion drift side the LCFS up to 3 cm. They were analyzed post mortem by different methods. Power loading and impurity release In the course of the investigations the test limiter has been operated as a main plasma limiting element in the previously boronized machine. Discharges in deuterium were supplementarily heated by two neutral beam injectors operated at beam energy values of I .5 MW for up to 1.2 s. Only a fraction of about 445% of the total convective energy of the plasma (total heating energytotal radiation) were deposited onto the test limiter.’ Nevertheless, on the highly loaded areas the peak power loading reached values of about 20 MWlm’. The evolution of the surface

a)

temperature had been deduced shot by shot from the thermocouple signals, which were calibrated using a factor calculated by application of the semi-infinite one dimensional heat flow approximation.’ During the experimental campaign the surface temperature rose with increasing power load to T > 2600 C. Impurity release from the limiter has been studied spectroscopically. Figure 1 shows emission spectra collected in front of the test limiter at two differing surface temperatures. Quantifying the intensities of the spectral lines using sensitivity factors, the deuterium and impurity fluxes are obtained. At 800 C (Figure l(a)) the ratio of released carbon and boron to the hydrogenic flux is about 0.0 15.At limiter temperatures above 2600 C (Figure l(b)) the release of C and B rises drastically and deuterium becomes hardly detectable at this scale. The flux ratio of carbon and boron to the hydrogenic flux (C + B)/D reaches a value of about 2 while carbon and boron are released with a ratio of B/C =: 1.3. The increase of boron and carbon fluxes at these high surface temperatures is due to their thermal sublimation (above 2000 C)‘.“’ which. on the other hand, strongly decreases the ratios D: jC and D-;/B. Thirteen discharges with sublimation of boron and carbon have been performed and the limiter investigated post mortem. Both ion and electron drift side display well developed patterns of redeposition and erosion areas (insert in Figure 2). As determined by sputter Auger depth profiles taken at various locations, a deposit composed of B and C has been found on the surface. The redeposition is due to the local segregation and subsequent sublimation of B and C at locations of high heat load. Released atoms are ionized near the surface (mean free paths of boron and carbon are here -4 mm) and then driven along the field lines back to the surface; they induce around the tangency point a deposit which covers an area of about 20 cm*. According to Auger depth profiling, the thickness of the deposit varies between 3 I’m next to the tangency point and 6 /cm in its maximum, i.e. IO mm apart. These values show that an extremely high net redeposition with rates up to about 50 nmjs (- 3 cm/year) occurs. Applying an experimentally determined density of -6.5 x IO” cm-’ valid for carbon rich deposits,” a total of up to

- 6 x 10’” atoms (boron + carbon) have been estimated to be deposited upon this area. Based on the spectroscopy data taken

NBI-co

b)

T- 800°C

C)ll

fl 410

416

i 426

434

410

1.

temperatures. 936

418

426

wavelength [nm]

wavelength [nm] Figure 1. Emission spectra of the 3.5% boron-doped

NBI-co T> 2600”C

graphite

limiter in the wavelength

range 410436

nm accumulated

during

two different surface

J van Seggern

et al; Boron and carbon fluxes

Figure 2. AES depth profile taken on the ion drift side I mm out of the limiter centrc. Insert shows the appearance of the test limiter after II4 s operation tn TEXTOR.

in front of the limiter, and assuming the flux of the hydrogenic species to be at the plasma edge -2.5 x lO’“cm~‘s~‘, theamount of atoms releasing the limiter surface could be roughly estimated. It results in a total of not more than 7 x IO”’ to 9 x 10’” atoms sublimed during the high heat load discharges. Since about 6 x IO”’ atoms remain as a deposit on the limiter, only less than a third of the sublimed species can contribute to the impurity transport in the SOL. This points to a very substantial local redeposition under these experimental conditions. Microprobe analysis executed after the TEXTOR operation perpendicularly to the surface over a slice cut from the middle part of the limiter at the electron drift side, as well as EDX measurements taken upon the surt:dce, had shown boron rich (B _ 50 wt%) up to 10 /lrn large precipitates, embedded in the boron containing bulk material (3.5 wt%). The precipitates appear to be the dominant source for the supply of boron and carbon during the thermal sublimation. A typical example of the AES depth distribution of the layer constituents within the deposition pattern generated on the limiter well inside the plasma edge region is also shown in Figure 2. A very uniform composition with BiC _ I, typical for these deposits. can be stated. Compared with the bulk material (3.5%) the enhanced boron amount in the deposit (about 50%) agrees quite well with values found by visible spectroscopy just above the limiter surface. The second redeposition area can be identified close to the limiter edges and on the side walls of the limiter. It is a factor of > 10 thinner and boron depleted (t/ = 500 nm, deposition rate of this area is similar c4.5 nm I(. B:‘C 2 0.2). The composition to deposits acquired on collector probes in the SOL.” Impurity

During

transport

in the SOL

The thickness of the deposit. determined by the IFA, ranged from -400 nm at I- = 49.8 cm to 130 nm at r = 57 cm (Figure 3. left hand scale). Using repeatedly the above mentioned density of -6.5 x 1O”cm I, the deposition rate across the SOL (Figure 3. right hand scale) as well as the total amount of collected atoms can be calculated. The amount of impurities decays across fhc SOL with e-folding length, a behaviour expected for particles eroded at the limiters and then diffused across the ticld lines into the SOL. In the experiment described here they decay with i 2 2.6 cm. This is a somewhat higher value than that experimentally found at corresponding OH heated discharges (; : I .O cm) and is due to increased cross-field diffusivity of impurities at high plasma edge temperatures. During the exposure of the probe to the plasma discharges. a total amount of about I x IO’” atom> M’ere collected on the probe over 4 cm’ on the ion drift hide. The areal density within the gap between the plasma edge and the tip of the collector was extrapolated from the measured data by applying the decay length. A very rough estimate of the moral number of atoms transported in the SOL through a cross-section ( -_3200 cm’) could be evaluated. The calculations demon4lra~e a toral ofabout 2 x IO” atoms being transported. Thl:, number is at least tuo orders of magnitude higher than the number of atoms which sublime from the test limiter (about 4x IO”‘) during (he discharges, as determined spectroscopicalt~. C‘onsequencly. the ALT II belt limiter with its surface of 3.3 m’ might present the main impurity source. and not the rather small test limiter (67 cm’ surface area). This is consistent \\ith the fact that the total heat load on the test limiter comprised oni> ;I fraction (~1‘about 4% of the total power in the plasma,’ .4 mean deposition rate of 2 7 nm s was found at I’ = 52 cm. the same value as found in the SOL while collecting impurities during an experiment with comparable but l.jnly OH heated discharges. Thus. the high heal load on the limiter dots not Ggnificantly influence the Limount of impurities deposlted on the Stockholm TEXTOR-probe placed hIthin the SOL. There is onI1 a slight increase of impurities with incl-casing plasma convective energy. The composition of the layer collected during the part of the high heat load experiment discussed in this paragraph wtb also characterized by AES depth profiling (Figure 4). 4 IO0 Nan thick lager composed of carbon. boron and oxvgcn was grown at I’ = 52 cm while exposed to the 20 disch;rrges. A\ presented

of TEXTOR

a part of the operation (4 ohmic + 16 NBI discharges < ISOOC), impurities up to p,<>,,,= 2 MWs, limiter temperature in the SOL were collected by the Stockholm TEXTOR-probe. and investigated by IFA and AES. Since the Al-targets on the em drift side became molten by the extreme heat influx the redeposition on the ion drift side will exclusively be discussed here.

01

49 .

,

50

I

,

51

distance

I

5’2

5s

to plasma

?I4

center

3

55

’

/

56

.

I 57O

[cm]

Figure 3. Radial dependence of the layer thickness collected during 20 high pouer load discharges by the Stockholm TEXTOR-probe and of the deposition rate. respectively. determined h> interference fringe unaly’;i\ (IFA).” 937

J von Seggern

et al: Boron and carbon plasma

2.05

E

.;

1

convective

fluxes energy

1.85

[MWs] 1 1.22

1

1 OH

60

z!

50

3 e

40

8 g

30

0 20 10 .

-

0

depth Figure 4. AES depth profile of deposit operation in the SOL at r = 52 cm.

[nm] collected

during

76 s plasma

before,” fluctuations inside the deposit can be used as a marker for the incremental growth during a single exposure. Thus they facilitate the determination of the deposition rate of each single discharge. The estimated shot by shot increase in the deposit thickness is shown as short lines above the depth profiles. Calculating the deposition rates, the deposit collected through the experiment shows a rather constant, though from shot to shot slightly scattered, growth rate. Apart from the near surface region, there is on average no significant difference between OH and NBI heated discharges. The deposition rate yielded from the incremental growth is between 2 and 3 rim/s,, a value which corresponds well with the average deposition rate determined by the IFA (2 nmjs). Despite the fairly constant deposition rate the chemical composition of the film alters notably. Carbon amount in the deposit depends on the plasma convective energy (outlined at the top of Figure 4). A steady increase up to a factor of about 1.5 has been established between OH and NBI heated discharges of the peaked heat load. This observation is supported by the slightly increasing Cl signal measured spectroscopically on the ALT II limiter. On the other hand, boron supply depletes under the same conditions; its concentration approaches a value of -0.2 which is a value typical for quasi-stationary conditions in the boronized TEXTOR.” Summary

A test limiter made of 3.5% boron-doped graphite was exposed in the TEXTOR plasma boundary to high heat load ranging up to 20 MW/m’. Both deposits built on the limiter and collected

938

in the SOL were investigated after the exposure to the plasma discharges (114 and 76 s, respectively). Two types of deposition mechanisms on the limiter surface were established. The first one is a process localized on the small area within the plasma edge, i.e. an area of highest surface temperature. A very thick, boron rich layer (> 6 pm, B/C z 1) had grown within the section near the tangency point of the limiter. The B/C-ratio of about 1 seems to originate from the thermal sublimation of B and C including precipitates (B:C z 1: 1) within the bulk material, and the following ionization of these components. Due to their short ionization length (-4 mm) they become ionized close to the surface, move along the field lines and more than 65% are deposited on the limiter. The geometry (curvature) of the limiter body supports this local mechanism. A total of -6 x 10’” atoms and a net deposition rate up to 50 nmjs have been established in this region. The deposition at the edges of the limiter (r z 47 cm) points to a second, global process which originates in the background plasma similar to the codeposition in the SOL. The limiter edges act as a collector probe. Impurity collection in the SOL has shown that the probe mainly recognizes the global events happening at the plasma edge. These are actually determined by erosion processes which take place on the toroidal ALT II belt limiter (r = 46 cm). This results in almost constant deposition rates, and in lower boron content in the deposits. The increase of the carbon concentration might be due to the increased release of carbon from the ALT belt limiter with increasing heating power. References ‘J Winter, H G Esser, L Kiinen, H Reimer, J von Seggern, J Schhiter, E Vietzke, F Waelbroeck, P Wienhold, T Banno, D Ringer and S Vepiek, J Nucl Mater, 162-164, 713 (1989). ‘P R Thomas and the JET Team, J Nucl Muter, 176 and 177,3 (1990). 3J Winter, H G Esser, G L Jackson, L Kiinen, A Messiaen, J Ongena, V Philipps, A Pospieszczyk, U Samm, B Schweer, B Unterberg and the TEXTOR Team, Phys Ret) Lett, 71, 1549 (1993). 4V Philipps, A Pospieszczyk, U Samm, J Winter, H G Esser, M Erdweg, L KGnen, J Linke, B Schweer, J von Seggern, B Unterberg, E Vietzke and E Wallura, J Nucl Muter, 196-198, 1106 (1992). ‘V Philipps, A Pospieszczyk, B Unterberg, H G Esser, M Erdweg, J Linke, B Schweer, U Samm, J von Seggern, E Vietzke, E Wallura and J Winter, J Nucl Muter, 212-215, 1189 (1994). 6A Pospieszczyk, In Atomic and Plasma Interaction Process in Controlled Thermonuclear Fusion (Eds. R K Janev and H D Drawin). Elsevier, Amsterdam (1993). ‘V Philipps, U Samm, M Z Tokar, B Unterberg, A Pospieszczyk and B Schweer, Nucl Fusion, 33, 953 (1993). “P Wienhold and U Littmark, EMRS Meeting, Strasbourg, France, June 1987, Les Edition Physique Vol XVIII, p 441 (1987). ‘M S Carslaw and G C Jlger, Conduction of Heat in Solids. Clarendon, Oxford (1959). “‘E Vietzke, V Philipps and K Flaskamp, J Nucl Marev, 196198, 1I 12 (1992). “P Wienhold, J von Seggern, H G Esser, J Winter, H BergsPker, M Rubel, I Gudowska and B Emmoth, J Nucl Muter, 176 and 177, I50 (1990).