Boivin, R.L.

Bombard, B.

Bombarda, F.

Christensen, C.

Cracken, G.M.

Fairfax, S.

Fiore, C.

Goetz, J.

Golovato, S.

Graf, M.A.

Granetz, R.S.

Irby, J.

Kurz, C.

Lipschultz, B.

Marmar, E.S.

Rice, J.E.

Snipes, J.A.

Terry, J.L.

Welch, B.

DSpace@MIT

PFC/JA-94-35Performance of the Alcator C-Mod ClosedDivertor in Ohmically Heated PlasmasJ.L. Terry, F. Bombarda', J. Goetz, M.A. Graf, C. Kurz,B. LaBombard, B. Lipschultz, E.S. Marmar, G.M. McCracken,J.E. Rice, J.A. Snipes, B. Welch2 , R.L. Boivin,C. Christensen, S. Fairfax, C. Fiore, S. Golovato,R.S. Granetz, J. Irby, M.J. Greenwald, S. Horne, T. Hsu,A. Hubbard, I.H. Hutchinson, D. Jablonski, T. Luke,M.J. May3 , A. Niemczewski, P. O'Shea, M. Porkolab,J. Schachter, P. Stek, Y. Takase, R. Watterson, S. WolfeSeptember 1994' ENEA Frascati, Via Enrico Fermi, 27, 00044 Frascati (Rome), ITALY2 Institute for Plasma Research, U of Maryland, College Park, MD 20742 USA3 Dept. of Physics, The Johns Hopkins Univ., Baltimore, MD 21218 USAThis paper will be published in Plasma Physics and Controlled NuclearFusion Research 1994 (Proc. 15th Int. Conf. Seville, 1994), Vol. 1, IAEA,Vienna (1995).This 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.AbstractAlcator C-Mod operation with a single null closed divertor producesclean, Zeff <1.3 plasmas and two dissipative divertor regimes. For;1e > 0.7x 1020 m- 3 the divertor plasma is highly recycling and radiativewith high densities and low temperatures near the strike points. If thetemperature near the target plates drops to <5 eV, this plasma detachesfrom the plates over most of their surfaces. The plasma pressure therecan drop by a large factor (-50-80). Known amounts of trace impurities(Ar, Ne, He) have been puffed into the high-recycling divertor, and thefraction reaching the main plasma have been measured. The "impurityshielding efficiency" of this closed divertor configuration is high in eachcase, with N m /N "f, < 2% for Ar and Ne, and <20% for He.1. IntroductionAlcator C-Mod [1], a high field, compact tokamak with a molyb-denum first wall, produces highly shaped, diverted discharges. For theresults described here, the main plasma parameters were 0.6< I, <1.1MA, 0.2x102 0 m- 3 < 7ie(gas-fueled) <2.8x10 2 0 m- 3 , and 1.4< K <1.7with Bt=5.4 T. The device is designed to operate with a single or doublenull divertor. While in the single null configuration a number of differentmagnetic divertor geometries ("open", "closed", "closed with slot") arepossible[1]. Results reported here are for the "closed" divertor, shown inFig. 1, where the scrape-off layer (SOL) plasma flows around structureswhich baffle it from the core. A unique feature of this geometry is the"vertical" orientation of the divertor plate surfaces at the strike points.This geometry causes recycled particles in the common flux region to bedirected toward the separatrix and away from the main plasma. Electrontemperature and density profiles over the inner and outer divertor platesare measured with arrays of probes. As shown in Fig. 1, there are 6probes along the inner plate and 10 probes along the outer. Also shownis a fast-scanning probe which measures scrape-off plasma parametersabove the divertor. SOL parallel heat flows to the probe are as muchas ~200 MW/m 2 to date, and field line lengths from the probe to thedivertor plate are ~5 m. At the plates, grazing angles of incidence canreduce the heat flow by a factor of ~50. However, actual heat flows tothe plates are much lower than this as a result of heat dissipation in ornear the divertor.1-0.2-0.3Inner ProbeArray |-0.4Z (M)-0.5-0.60.4 .5Impurity Gas Puff0.6 0.Rm j,, .(m )Outer Probe ArrayFigure 1. A cross-section of the Alcator C-Mod "closed" divertor.The poloidal flux surfaces are constructed from magnetics measure-ments. Embedded in the divertor plates, but with their domed tipsextending above the surface, are 16 probes. Also shown are the fast-scanning probe (above the divertor) and the location of the gas pufftube used in the impurity injection experiments.2. Dissipative Divertor ConditionsDivertor conditions in which much of the heat flux is dissipatedwithin the divertor before reaching the strike points are currently of in-terest because of the need to reduce the peak heat loads which are pre-dicted for the ITER divertor plates. Two types of "dissipative" divertoroperation are observed in C-Mod. With the ion grad-B drift directed to-20.8[$canning Probe~ | Divertor "nose"I- 0- 82ward the divertor, a high-recycling, radiating divertor plasma is observedfor H, > 0.7x 1020 m-3. Above this density, the sheath electron densityat the plate and the D, emission from the divertor begin to increase non-linearly, ni sheath (CX(Te/ p), indicative of high recycling. Spatial profilesat the plate show that the density is strongly peaked near the separatrixand can be as high as n heih=6x1020 m~3 . Within the divertor cham-ber (i.e. below the outer "nose") the temperature is observed to increaseaway from the separatrix[2]. This is opposite to the profile in the SOLabove the divertor, where the temperature falls outside the separatrix.Electron pressure is approximately constant on the SOL flux surfaces,and total radiated and neutral-particle power (measured bolometrically)from the divertor is typically -50% of the input power in these cases.With no significant change in magnetic geometry, if the main plasmadensity exceeds a critical value which is proportional to the input power,the high-recycling, radiating divertor plasma "detaches" [3,4,5] from bothdivertor plates, and a second type of dissipative divertor operation isobtained. The detachment occurs when the temperature at the platesdrops to -5 eV over a significant fraction of the surface. This transitionto "global" detachment occurs rapidly (<10 ms), but is preceded (ona -100 ms time scale) by a gradual evolution in which pressure at theplate falls for flux surfaces near the separatrix and is no longer constanton those surfaces. This gradual process is a local, partial detachment.A typical evolution of the sheath pressure profile at the plate through"global" detachment is shown in Fig. 2. The pressure near the strikepoint drops gradually as the peaks in the pressure and density profilesmove away from the separatrix into the common flux region. At "global"detachment, the pressure and the parallel heat flux at the plates dropsharply (by factors of ~10-80) over a large fraction of the surface, in-cluding the strike point. The temperature falls, but remains above 1 eV.In addition, the spatial distribution of the radiation within and near thedivertor changes suddenly, with the bulk of the radiation moving up to-ward the X-point. The rate of density rise in the main plasma increasesconcurrently, indicative of increased fueling. (see Fig. 2 inset.) Thus thedensity is not limited by the detachment, which occurs in ohmic plas-mas at -25% of the Greenwald density limit[6]. The pressure on fluxsurfaces significantly further out in the SOL (usually at those strikingabove the divertor "nose") remains constant or increases slightly duringthe time leading up to and including the "global" detachment. "Global"3detachment is not full poloidal detachment, but only detachment at thevertical surfaces of the target plates. The dynamics of the transition fromthe gradual, partial detachment to the sudden "global" detachment arenot well understood. The main plasma density threshold for "global"detachment is a linear function of the ohmic input power. This, however,is probably a result of atomic processes which become dominant whenthe temperature near the plates falls to a threshold of -5 eV.Shot I 940623021A .60sec0 .69secSeparatnix 0 .77 secA .83 secE0..50.6 0.7 02fime (s)probe 80.0-5 0 5 10 15P.dp.,(Mm)Figure 2. Evolution of the pressure profile at the outer divertor platefrom a high recycling condition through to "global" detachment att=.81 s. The pressure is calculated as 2 ne,sheathTe, assuming Te=Ti,with density and temperature measured by the probe arrays. Theprobe positions on the plate are mapped to midplane coordinates,p, with the separatrix at p=O. When plotted in this way, large (-3cm) changes in the peak position at the plate appear as small changes(~2 mm). In the inset is Tie(t) for the time period of the pressureevolution.43. ImpuritiesFor H, above 1.5x 1020 m- 3 , the Zef f of the main plasma is <1.3.The major low Z impurity is carbon, in concentrations of <1%. Theinflux of molybdenum, the first wall and divertor plate material, decreasesrapidly for ie >1.5x 1020 m- 3 . Above that density, Mo is present inthe core in concentrations of <0.01%. When the plasma configurationchanges from limited (on the inner wall) to diverted, the influx of Mo tothe main plasma typically falls by a factor of > 2. The Mo influx fromthe target plates is usually negligible; it comes predominantly from theinner wall[7].Because shielding of divertor impurities from the main plasma is an-other key aspect of ITER divertor design, experiments to measure the"impurity shielding efficiency" of the divertor in the closed configura-tion were performed by puffing a known, trace amount of recycling Ar,Ne, or He into the private flux region of the divertor (see Fig. 1).Theamounts of Ar and Ne appearing in the main plasma were measured spec-troscopically, while an upper limit for the amount of He was set by theabsence of a measured change in core Zeff (AZeff <0.05) The ratios ofthe number of ions in the main plasma to the number of atoms puffedare found to be -2% for Ar, -1% for Ne, and <20% for He, with Hein the range 1-2x1020 m-3 . Additional experiments with Ar pufling[8]showed that the shielding efficiency decreases after "global" detachment,with Nmain/Np"ff increasing by about a factor of 2.AcknowledgementsWe gratefully acknowledge the excellent support of the Alcator C-Mod engineering and technical staff. This work is supported by US DoEContract # DE-AC02-78ET51013.References[1] Hutchinson, I.H., et al., Physics of Plasmas, 1 (1994) 1511.[2] Hutchinson, I.H., et al., 1994 Controlled Fusion and Plasma Physics(Proc. 20th Eur. Conf. Montpellier), to be published.[3] Petrie, T.P., et al., J. Nucl. Mater. 196-198 (1992) 848.[4] Janeschitz, G., et al., Proceedings of the 19th European Conf. on5Controlled Fusion and Plasma Physics, Innsbruck, 1992, (EuropeanPhysical Society, Petit-Nancy, Switz., 1992), Vol 16C, Part II, 727.[5] Lipschultz, B., et al., Proc. 11th International Conf. in PlasmaSurface Interactions, Mito, J. Nucl. Mater., to be published.[6] Greenwald, M., et al., Nucl. Fusion 28 (1988) 2199.[7] Kurz, C., et al., Proc. 11th International Conf. in Plasma SurfaceInteractions, Mito, J. Nucl. Mater., to be published.[8] McCracken, G., et al., Proc. 11th International Conf. in PlasmaSurface Interactions, Mito, J. Nucl. Mater., to be published.6

Performance of the Alcator C-Mod closed divertor in ohmically heated plasmas

http://dspace.mit.edu/bitstream/1721.1/95224/1/94ja035_full.pdf

Performance of the Alcator C-Mod closed divertor in ohmically heated plasmas

Abstract

Similar works

Full text

Available Versions

DSpace@MIT