Lipschultz, B.

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PFC/JA-94-012Plasma-Surface Interactionsin the Alcator C-Mod TokamakB. Lipschultz, J. Goetz, B. LaBombard,G.M. McCrakcen, J.L. Terry, D. Jablonski, C. KurzPlasma Fusion CenterMassachusetts Institute of TechnologyCambridge, MA 02139June 1994Submitted to Journal of Nuclear MaterialsThis work was supported by the U. S. Department of Energy Contract No. DE-AC02-78ET51013. Reproduction, translation, publication, use and disposal, in whole or in partby or for the United States government is permitted.1 1 th PSI Conference, Mito Japan, 1994Plasma-Surface Interactions in the Alcator C-Mod TokamakB. Lipschultz, J. Goetz, B. LaBombard, G. McCracken, J. Terry, D. Jablonski, C. Kurz,A. Niemczewski, J. SnipesPlasma Fusion Center, Massachusetts Institute of Technology, Cambridge Ma. 02139 U.S.A.AbstractAlcator C-Mod is a high-field tokamak with which experiments wereinitiated in April 1993. The divertor and the first-wall have plasma-facing surfacesof molybdenum. The divertor has a configuration which is closed in comparison toother, currently operating tokamaks. The combination of divertor geometry andhigh field (density) make Alcator C-Mod an ideal experiment to investigatedissipative (radiative/CX) divertor scenarios. Single-null divertor operation hasbecome the standard mode of operation. Relatively clean plasmas (Zeff 1.3) areobtained with short periods of baking and electron cyclotron discharge cleaningonly.Impurity source rates and screening have been investigated with an extensiveset of wavelength-filtered diode arrays and spectrographs. Carbon and oxygen andmolybdenum dominate the impurity levels. The molybdenum source from thedivertor proper is negligible.1I. IntroductionAlcator C-Mod is the first high-field diverted tokamak. As demonstrated inprevious Alcator experiments, the high magnetic field allows attainment of highdensities ('T e curreently < 3 x 1020 m-3) in the main plasma. In Alcator C-Mod, thecombination of divertor and high central density can produce ne,div > ne(0) of thecentral plasma.Two other characteristics combine with the high density to create a uniqueenvironment for divertor studies. First, the divertor geometry is relatively closedcompared to the flat-plate divertors currently available. Second, again because ofthe high magnetic field, the parallel heat flux in the SOL is > 100 MW/M 2, evenwith purely ohmic plasmas. Additional heating with ICRF with up to 8MW ofpower and several antennas is planned. Input powers of up to 1 MW [1] with asingle antenna have been already achieved.The goals of the experimental program are to explore different operationalregimes accessible by the unique features described above. In particular, the highdivertor density maximizes the loss of power from the edge plasma; radiation andneutral processes (charge exchange and elastic ion-neutral collisions) are bothproportional to n-e. The more general tokamak issues are discussed elsewhere [1].The focus of this paper is on the divertor and SOL.Alcator C-Mod has major and minor radii of .67 and .21 m respectively.During initial operation the achieved (design) capabilities are Bt = 5.3T (9T), plasmacurrent up to 1 MA (3 MA), and elongation of up to K = 1.65 (1.8). Well controlledx-point and strike point position were achieved early on with novel hybrid analog2computer control of the plasma shape and position [1].There are two very different divertor geometries in Alcator C-Mod (figure 1).The open, or flat-plate divertor is located at the top of the vessel. This divertor issimilar to most current tokamaks. The lower divertor is much More closed thanthat located at the top of the vessel. Single-null operation has so far been solelywith the closed divertor. The strike points are located on surfaces that face awayfrom the main plasma. This geometry minimizes the chance of neutrals reachingthe main plasma before being ionized in the divertor region and swept back to theplates. For the data discussed herein, the BxVB direction was oriented towards theclosed divertor. The first-wall surface is comprised of 7000 molybdenum tiles.There are some RF antenna surfaces that are TiC coated.II. DiagnosticsTransport in the SOL is studied using a large complement of Langmuirprobes. 48 fixed-position Langmuir probes are located on the bottom divertorsurfaces [2] at 16 different poloidal locations. During the 1993 run period, 43 probeshad a profile which was 'flush' with the divertor plate surfaces, and 5 had a'domed' profile protruding beyond the surface of the divertor. The 'flush' probesare used solely for determination of ion-saturation current ('SAT) profiles on thedivertor surfaces. The divertor plasma density and temperature are measured withthe 'domed' probes. There is one movable probe located in the SOL which can bepneumatically driven to the separatrix and back out of the SOL in a period of -100ms [2]. This probe provides density and temperature profile measurements in theSOL.3The power balance in the SOL and divertor is studied with 8 bolometerdetectors in two arrays viewing the divertor region [3]. These detectors provideinformation both about the spatial distribution and magnitude of the radiation.Further information about the main contributors to the radiation is provided bymultiple arrays of filtered diodes. Over 200 diodes with changeable filters view theplasma from a large number of angles [4]. The brightness information provided bythese multiple views and angles allows for an inversion of the data to obtainemissivities [5]. Together with additional spectral information from a 0.5 mspectrometer with an OMA detector array, these systems provide the details of thedivertor radiation composition.III. ConditioningThe methods required to prepare the C-Mod vacuum vessel for tokamakoperation are modest compared to tokamaks with carbon tile first-walls. Followinga vacuum vent, the vessel is baked at 120 *C for - 1 week. Electron CyclotronDischarge Cleaning (ECDC) is then applied for several days [6]. This technique,developed and tested elsewhere [7,8], utilizes a low-power (3 kW) 2.45 GHzmicrowave source to produce a low-temperature and low-density hydrogencleaning plasma. The toroidal field resonance (.088 Tesla) is swept back and forthacross the vacuum vessel. Performance of the system is such that 5 monolayers /dayof carbon can be removed from the walls (based on the measured pumping rate ofcarbon-based molecules). This technique is also used before each run day tocondition the walls which are at - room temperature. The walls are kept at thistemperature for the remainder of the day and no additional conditioning is done4between shots.IV. OperationMost operation of Alcator C-Mod has been with a single-null located at theclosed divertor. The central plasma Zeff (see figure 2) decreases with increasing neand thus is normally low(1.3) for standard Alcator plasmas. The apparent offset inZeff is due to different integration paths of the diagnostics involved(bremsstrahlung and interfereometer). Spectroscopic measurements have used todetermine central impurity densities from which we estimate a correction to Zeff of.3 - .4 above l- - 1 x 10 20 m-3. Carbon, present as an impurity on the molybdenumsurfaces, and to a lesser extent oxygen, are the largest impurity concentrations in thecore plasma. The levels of C, 0 and Mo in the plasma are typically .2%, .1% and.02% of ne(0) respectively. Molybdenum levels are higher at low fe where Zeffrises. The resultant radiation from the main plasma is proportional to the inputpower with PRAD,main =.3 - .4 xPOH [3.Two important source locations for these impurities are the inner walllimiter and upper surfaces of the divertor. Direct measurements of the H flux atthese surfaces [5] show that there is significant plasma surface interaction eventhough the the separatrix is normally more than 2 density e-folding lengths away (>2 cm). The resultant C & Mo source rates (based on spectroscopic measurements ofC-III and Mo-I) at the inner wall have been measured.There is also a significant source of impurities in the divertor region. Carbonsource rates from this region, measured using the filtered diode arrays [4], can be asmuch as 2-3 times that from the inner wall [9]. This is due to the much higher5particle fluxes in the divertor region. In contrast to the carbon source rates, themolybdenum source rate in the divertor is much less than that from the inner wall.Te in the divertor region is such that ion energies are at or below the sputteringthreshold. However, in the presence of injected argon, the molybdenum sourcerate from divertor surfaces is significantly enhanced [9]. We feel that this effect isdue to the lower Mo sputtering threshold for incident ions with higher mass.Though the impurity source rates at the inner wall are lower than over mostof the divertor surfaces, the relatively poor screening at the wall results in a greaterfraction of the impurity content in the main plasma (C, Mo and 0) being due to theinteraction with the inner wall. The poor screening, defined as the ratio of thecentral impurity concentration to the primary influx from the wall or divertor, isdue to several factors: Both the density and the scrapeoff length are large in thedivertor region. In addition, the divertor geometry is such that the trajectory of theneutrals, when they leave the divertor surfaces, is generally directed away from themain plasma.Much of the edge divertor program at Alcator C-Mod is focused ondeveloping methods of reducing the power concentration on the divertor platesthrough dissipative methods; radiation and ion-neutral collisions (elastic andcharge-exchange). The level of parallel power flux, 1 (< 250 MW/m 2 ) in theAlcator C-Mod SOL is high even with ohmic plasmas. These levels are significantcompared to that predicted for ITER (500-1000 MW / M2 ) providing a relevant test-bed for dissipative divertor concepts.Two forms of dissipative divertor studied in Alcator C-Mod are the radiativeand detached divertor operational regimes. Radiative divertor operation occurs6over most of the Alcator C-Mod operational space. Radiation in the divertor issuch that most of the power flowing into the SOL is radiated before reaching thedivertor plates; PRADdiv/PSOL ~ 0.8 determined from bolometry [3]. Pressure isapproximately constant along a flux surface[2]. The radiation is located in the SOLregions from near the x-point down along the separatrix legs to the divertorsurfaces. Carbon is the dominant impurity in the divertor.Divertor power losses due to the charge exchange process estimated on thebasis of a Monte-Carlo calculation of neutral-ion interactions are in the range 1 - 30MW / m 3 over a path length of a few centimeters depending on discharge conditionsand position along the divertor surface. This result is typically of the same order asour estimate of the local emissivity due to carbon radiation. A more accurateassessment will be made in the future.Because of the combination of high density and divertor radiation, largegradients in ne and Te can be sustained along a flux surface [2]. For ie below ~ 1 x1020 m-3 the parallel transport in the SOL is sheath-limited. ne and Te each areapproximately constant along a flux surface and ne (SOL and divertor) is - linearlydependent on fe. As nfe and divertor radiation increases, the parallel transportchanges over to being conduction-limited [2]. In this transport regime pressure,rather than ne and Te individually, is constant along a flux surface. The densityand temperature at the outer divertor plate become strong functions (powers of 3-4)of ne. The scaling of plate ne and Te with Ke in the two different transport regimesis exemplified by the data in figure 4. Application of analytic 2-point edge models [2]to this data, along with knowledge of geometry (e.g. connection lengths) and power(input and radiated) reproduces this nonlinear dependence.7As n5 e is increased, with fixed plasma current and input power, a thresholddensity is reached above which the divertor plasma becomes detached from theplates. When this occurs, the ion flow to the plates and ne,plate in the region of theseparatrix markedly decreases. The divertor radiation, which is initially spreadthroughout the divertor region rearranges to peak at and above the x-point (insidethe separatrix). The impurity and hydrogenic densities in the central plasmaincrease after detachment. This is consistent with a scrapeoff layer that is moretransparent to neutrals (recycled and sputtered). As a result the radiation in the coreplasma increases. A necessary condition for detachment is that Te in those sameregions (near the strike point) must be ~ 5 eV. The total radiated power (divertorand main plasma) typically accounts for > 90% of the input power. Details ofdetachment characteristics are given in reference [10].V. SummaryInitial operation of the Alcator C-Mod tokamak has been very successful.Significant progress has been made in achieving the multiple goals of the edgephysics program; understanding transport (hydrogenic and impurity) anddeveloping the dissipative divertor operation.Impurity levels in Alcator C-Mod are relatively low. The majority ofimpurities populating the central plasma originate from the inner wall rather thanthe high-recycling, heat-bearing, divertor surfaces. Molybdenum source rates at thedivertor surfaces are negligible, due to the low Te in the divertor.Radiative divertor operation is robust, the majority of the power flowing intothe SOL being removed from the divertor plasma before reaching the plates.8Detached divertor radiation has also been achieved. Even more power is removedfrom the plasma before reaching the plates.AcknowledgementsThe authors greatly appreciate the support of the rest of the Alcator group. Inparticular, the operations group, Ian Hutchinson, Steve Horne, Steve Wolfe andRobert Granetz, have brought about the operation of well-postioned divertedplasmas without which this work would not have been possible. This worksupported by U.S. D.oE. contract # DE-AC02-78ET51013.References1. I H Hutchinson, R Boivin, F Bombarda, Physics of Plasmas, 1 (1994) 1511.2 B Labombard, D Jablonski, B Lipschultz, et al, this conference.3. J Goetz, B. Lipschultz et al, this conference.4. J Terry et al Rev Sci Instrum. to be published, 1994.5. C Kurz, J A Snipes, J L Terry et al, Rev Sci Instrum. to be published, 1994.6. E L Marmar, C Christensen, J Goetz et al, Bull Amer Phys Soc 38 (10) 1993, paper3S18, pg 1956.7. Y. Matsuki, H. Ogawa, Y. Miura et al, J. Nucl. Mater. 145-147 (1987) 704.8. Y. Sakamoto, Y. Ishibe, S. Ishii et al, J. Nucl. Mater. 111 & 112 (1982) 485.9. C. Kurz, B. Lipschultz et al, this conference.10. B Lipschultz J Goetz, B Labombard and G M McCracken, this Conference9Figure CaptionsFigure 1: Layout of the Alcator C-Mod vacuum vessel and divertor hardware.Figure 2: Zeff of the central plasma (from Bremsstrahlung) vs. ne.Figure 3: Outer divertor plate density (a) and temperature (b) in the rgion of theseparatrix vs. ne.10VacuumVesselLimite~rClosedDivertorFigure11432N100.5 1.5ne (10 2 0 m-3 )Figure 2127El~o oF ElE C E .C -1.0 2.0 2.51000 n 700kA <I 900kAe p C?n 5OOkA < I < 700kA[n] t)o 0e p 0 00 013 (9DEIC]E) an 01 0 ................ ............ ................................................................ .............................. El El009p0-1 r.-,0 0EP 0 10 0.......... ........ . ..................... .................................................... ...................  m ......2 4 6 8 10ff (1019 M-3eFigure 3 (a). 13700 T ;700kA I 900kA50 E e pEl T ;500kA I 700kApElep30 El -El-ElEl 00w El0 - 010 .00 ...........0008 DlOO0G6E E o 0EJaEl 0 CD4 ' E-2 4 6 8 10:f (10 19 m-3 )eFigure 3(b)14

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