Results of experimental measurements of arcing in ISX-B are described. Samples of stainless steel were exposed at the plasma edge and arcing currents to the sample were measured. Currents from several amperes up to 100 amperes were observed. It was possible to trigger arcs during well-behaved discharges by applying a high voltage to the sample. Elemental analysis of surface features of samples has shown that sulfur segregates to surfaces of arc craters in type 304 stainless steel. Detection of sulfur on surface samples may be related to arcing during cleanup processes in tokamaks

Clausing, R.E.

Emerson, L.C.

Heatherly, L.

Clausing, R. E.

Emerson, L. C.

UNT Digital Library

Arcing Studies in ISX-B*R. E. Clausing, L. C. Emerson and L. HeatherlyMetalsvand Ceramics DivisionOak Ri dge National LaboratoryOak Ridge, Tennessee 37830AbstractResults of experimental measurements of arcing in ISX-B are described.Samples of stainless steel were exposed at the plasma edge and arcing currentsto the sample were measured. Currents from several amperes up to 100amperes were observed. I t was possible to trigger arcs during well-behaveddischarges by applying a high voltage to the sample. Elemental analysis ofsurface features of samples has shown that sulfur segregates to surfacesof arc craters in type 304 stainless steel. Detection of sulfur onsurface samples may be related to arcing during cleanup processes intokamaks.-DISCLAIMER .This book was prepared as m account of work sponsored by an agency of me United Elates 'Oovwnment.Neither the United States Government nor any agency thettaf. nor viy at their employees, tttakes anywarranty, tunas or implied, tr auumn any l ? ^ ' liability o ' r«oonsit»ii!y f c Ute accuracy.CofD'erenvn. o> uselulrMu o l any ir*ntma1ion. ooparatus. uroduct, or [iroces Oiscloied. orrenreienis that >U use m i t id not ^ifringe urivately owned rigttts. Relefenu: Hetein to any specificcoitnvrcial product, nrocess. or service by trade name, t raderv t . .TUnufacfurer. or otherwitf. doesnot necessarily constitute or imply its endorsement, recommendation, or favorino by the UnitedStates Gove-nment or any agency thereof. The «ev« ana epmions 0 ' autnors eit>ress«d herein do notnecessarily stale or reflect IWM of the United States Government or any agency thereof.•Research sponsored by the Office of Fusion Energy, U.S. Departmentof Energy under contract W-7405-eng-26 with the Union Carbide Corporation.Bv acceptance of this article, thepublisher or recipient acknowledge*tha U.S. Government'! right toretain a nonexclusive, royalty-freelicense in and to any copyrightcovering the article.DISTRIBUTION OF THIS 00CUMEW IS U K H H f BIntroductionStudies of arcing phenomena in ISX have been reported [1] and wereextended to a more detailed laboratory examination [1,2]. In the laboratorystudy i t was shown (1) that erosion rates during arcing are 10"5 to 10~6grams per coulomb, (2) that arcing rates decrease exponentially with time,(3) that insulating particles and adsorbed gas or vapor sources formpotential arcing sites, and (4) that arcing can be minimized through surfacecleanup and high voltage conditioning.The ISX studies have been continued to further characterize arcing inthe tokamak and to develop techniques to reduce arcing in future devices.The approach being taken includes (1) correlation of arcing phenomena withplasma conditions, (2) an attempt to measure impurity introduction into theplasma by arcs and (3) studies of wall conditioning methods to minimizearcing. Techniques have been developed using the surface analysis station [3]to carry out measurements of arcing on specially prepared samples. Arc currentsand voltages can be measured and, under certain conditions, arcs can be triggeredat preselected times during a tokamak shot by the application of a timed biasvoltage pulse. Auger electron spectroscopy has been used to observe changesin the surface composition of samples as a result of arcing. In particular,the unusually high surface concentrations of sulfur observed following certainmachine operations [4] have been investigated and the results are reported here.Experimental ApparatusThe experimental apparatus is very similar to that used in the previousstudy [1 ] . The sample is introduced into the plasma edge region through thesurface analysis station port and is positioned at any desired radius between—f.'T-• • ? •the edge of the plasma and the wall. The sample is electrically isolatedfrom the ISX vacuum vessel, with a path for the arc current provided via acoaxial lead through the vacuum wall to an instrument module, a diode and asmall resistance to the vacuum vessel. A current probe is used to measurethe current pulse. The voltage on the sample is obtained by means of avoltage divider.Arcs were triggered by applying a negative bias voltage to the sample.This bias was applied by means of an SCR in the circuit switched on by agating pulse from the ISX control computer at selectable times into thetokamak discharge. This provides a single arc at a known time.ResultsSelf-induced arcs in plasma devices occur randomly during times whenthere is significant magnetohydrodynamic activity (MHD) within the plasma.Figure 1 shows the arc current and volbage traces at the top and a numberof plasma characteristics at the bottom. This is a typical ISX-B shot of140 kA and lasting -v!60 ms. As shown by signals from both the PIN diode arraywhich monitors soft x-ray emission from the plasma and from the MHD activity,periods of plasma instability and arcing occur at the beginning of the shotbefore the current is stabilized and again at the end of the shot during thecurrent ramp down. When arcing occurs on the sample it is generally duringthese periods. Two exceptions W3re arcs observed at the instant ofeither pellet or neutral beam injection even with no major plasma disruptions.Four arcs with currents to the sample of more than 10 amps were observedduring this shot and are identified at points a, b, c, and d in Fig. la.The current probe saturated on the 10 amp scale to help identify the arcsignals so it is not possible to obtain the magnitude of the arc currents"as-"from this particular shot. An arc was deliberately induced during a periodof low MHD activity shown at c in Fig. la. This was triggered by a 200 Vbias applied to the sample 130 ms into the shot. The ability to triggeran arc depended upon the applied voltage, the condition of the sample andthe radial position of the sample. It was relatively easy to induce arcson freshly introduced stainless steel samples but more difficult, and oftenimpossible even with a 300 volt sample bias, to induce arcs after the samplehad been exposed to discharge cleaning and routine tokamak operation.Although the I5X arcing rate data are insufficient in quantity to demonstratethat arcing rates decrease exponentially with time, they do show a decrease withtime and thus are in qualitative agreement with our earlier laboratory study [1].The normal ranges of arc currents are 10 to 20 amps when the sample is3-5 cm behind the limiter. Self-induced arcs with currents approaching100 amps have been seen on samples exposed at a position near the edge ofthe outer limiter.A mechanically polished stainless steel sample was exposed in ISX-Bat a position to draw arcs and then transferred, under vacuum, to the surfaceanalysis station for Auger analysis and mapping. The adsorbed sample currentimage is shown in Fig. 2a for correlation with the Auger sulfur map shown inFig. 2b. Areas of high sulfur concentration were found to be associated withindividual craters on the surface. A high resolution SEM image of the samearea at approximately the same magnification is shown in Fig. 2c. There is1 one-to-one correlation between the areas of high sulfur concentration onthe map and the larger craters on the surface. Figure 2d shows a highmagnification image of the double crater in the middle of the upper righthand quadrant of Fig. 2c. Most of the craters with diameters larger than10 microns had sulfur segregated to their surfaces. Exposure of samples4to hydrogen discharges removes sulfur from the crater area and distributesi t around ISX. There is evidence that the sulfur was present in the machinein a gaseous form following the discharges since the build-up on surface monitorsoccurs even under high vacuum static conditions [5 ] .Sudden increases of sulfur levels to 10 to 13 at. % on surface monitorsoccur after machine vents. This is followed by a gradual reductionto less than .2% and is similar to the time dependent behavior of the arcingrate in our laboratory study. This suggests that surface sulfur levels canbe used as an "arcing indicator."There is no proof that arcing is associated with high sulfur levelson surface samples, but i t does appear to offer a logical explanation forthe sulfur observed on samples at the wall position. Melting or severeheating of stainless steel results in sulfur segregation to the surface [6 ] ,thus heating or melting of the l imiter could also be responsible for thepresence of sulfur on surface samples.Conclusions and Future PlansResults from arcing studies in ISX-B have shown excellent correlationwith those from earlier laboratory studies.Arcing rates on surfaces are reduced by continued exposure to the plasmaenvironment,confirming that conditioning processes exist by which arcingmay be minimized.A method has been developed to deliberately induce arcs, under controlledconditions, at predetermined times during a tokamak discharge. We plan to usethis technique in combination with other time-resolved diagnostic techniques(such as optical spectroscopic techniques and laser fluorescent techniques)to isolate and study the impurity introduction mechanisms associated witharcing.I t has been shown that, for tokamaks with stainless steel walls, highsurface concentrations of sulfur are found on the larger arc craters. Thissulfur can be distributed around the tokamak. Thus, high sulfur levels onwall samples may indicate high arcing rates in fusion devices.References[1] P. Mioduszewski, R. E. Clausing and L. Heatherly, J . Nucl. Mater.85/86 (1979) 963.[2] P. Mioduszewski, R. E. Clausing and L. Heatherly, accepted by J. Nucl. Mater.[3] L. C. Emerson, R. E. Clausing and L. Heatherly, J. Nucl. Mater. 76/77(1978) 472.[4] L. C. Emerson, R. E. Clausing and L. Heatherly, Bull. Am. Phys. S o c ,Ser. II, 22 (1977) 1262.[5] L. C. Emerson, R. E. Clausing and L. Heatherly, J. Vac. Sci. Tech. 16(1979) 766.[6] W. H. Swartz and D. M. Holloway, Appl. Spectrosc. 31 (1977) 210.Figure CaptionsFig. 1. Arcs obtained during a typical tokamak discharge. Arcs nearthe beginning and end of the discharge are self-induced and are associatedwith MHD activity while the single arc observed during the period of lowMHD activity was induced by a trigger pulse applied 130 ms into the shot.Fig. 2. Photographs of the sample image and the corresponding sulfurmap from the Auger system and SEM images of the same area (see text).-, 4 *; > ^ : V .CURRENT 10 A/DivVOLTAGE 20 V/Div(200 V TRIGGER)PIN DiodeMHDVert. PositionPlasma CurrentPiADSORBED SAMPLECURRENT IMAGESULFUR AES MAPSEM IMAGE SEM IMAGE

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