Goetz, J.

Graf, M.A.

Kurz, C.

Lipschultz, B.

Nachtrieb, R.

Terry, J.L.

DSpace@MIT

PFC/JA-94-015Power Balance and Scaling of theRadiated Power in the Divertor andMain Plasma of Alcator C-ModJ.A. Goetz, B. Lipschultz, M.A. Graf,C. Kurz, R. Nachtrieb, J.A. Snipes, J.L. TerryPlasma Fusion CenterMassachusetts Institute of TechnologyCambridge, MA 02139June 1994Submitted to Journal of Nuclear Materials.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.Power balance and scaling of the radiated power in thedivertor and main plasma of Alcator C-ModJ.A. Goetz, B. Lipschultz, M.A. Graf, C. Kurz, R. Nachtrieb, J.A. Snipes, and J.L.TerryPlasma Fusion Center, Massachusetts Institute of Technology, Cambridge,MA 02139, USAMeasurements have been made of the radiation in the main plasmaand divertor of Alcator C-Mod using bolometer arrays. It is found that thepower radiated from the main plasma is Prad,main I POhmic - 0.3 - 0.4 overthe range 0.2 iie 2.5 x 1020 m- 3 . There is significant radiated powercoming from the divertor region; Prad,divertor I POhmic - 0.4. The radiatedpower in the main plasma and divertor scales linearly with density. Whendivertor detachment occurs, the power flow to the divertor plates in theregion near the strike points is suddenly reduced, the fraction of the inputpower radiated from the main plasma and divertor increases, and theradiation in the divertor region is spatially rearranged.i1. IntroductionEnergy can be removed from a tokamak plasma through radiation andcharge exchange and is dissipated onto the surrounding walls and divertorsurfaces. A considerable fraction of the input heating power of a tokamakplasma may be lost by impurity line radiation, mainly in the VUV to softx-ray spectral range, and by neutral particles through charge exchangeprocesses. This radiation, whether from the main plasma or the divertor,reduces the power incident on the divertor plates which is carried byconduction and convection of the plasma in the scrape-off layer. This isimportant for divertor operation in that it will ease the problem ofremoving the heat and cause less erosion of the divertor plates in futuretokamaks and reactors. An overview of measurements of radiation fromthe initial operational period of the Alcator C-Mod tokamak [1] will bepresented in this paper. Although capable of operating with a closed oropen divertor, only results from operation with a closed shaped divertorwill be discussed.2. Bolometer systemsThe bolometer systems installed in the tokamak are used to determinethe local radiation emissivity and the total radiated power of the plasma(photons plus neutrals). The bolometers consist of blackened gold foilabsorbers separated by an insulating foil from gold meander resistors [2].The foils absorb neutrals and radiation from visible through x-rayenergies. For main plasma measurements a toroidally-viewing array isused. This bolometer array has 24 chords that span the main plasma inPage 1the midplane with two centimeter radial resolution. There are twopoloidally-viewing bolometer arrays in the closed divertor region (seeFigure 1), each consisting of four collimated detectors; one has horizontalviews that encompass the inner strike point (chords B1 - B4), and one hashorizontal views that encompass the outer strike point (chords B5 - B8).The third divertor array consists of sixteen XUV photodiodes that view theinner half of the closed divertor region (chords UVI - UV16). Thesephotodiodes are responsive to photons with energies from 50 eV to 10 keVbut do not measure the energy from neutral particles.3. Main plasma measurementsChord brightness data obtained from the toroidally-viewing array ofbolometers are Abel-inverted to obtain the radiation emissivity profilewhich is then volume-integrated to obtain the total radiated power fromthe main plasma. Data are selected from quiescent periods of dischargeswith constant current and density. A database has been constructed whichincludes data from discharges with plasma current in the range 0.4 Ip !1.0 MA and line-averaged density in the range 0.2 Wile 2.5 x 1020 m-3 .The Ohmic input power is a function of the current and density and is inthe range 0.4 < POhmic < 1.4 MW.Typically the radiation emissivity profile is hollow with wings at anormalized radius of approximately 0.8. The major sources of error in thisdata are: uncertainties in the calibration constants of the bolometers of ±20%, uncertainties in the electronics gains of ± 10%, and uncertainties inthe radial location of the chords of ± 2%. Emissivity profiles of the intrinsicimpurities, mainly carbon and molybdenum, obtained by modelling withPage 2the MIST impurity transport code [3] correlate well with the radiationemissivity profiles obtained from the bolometer data.In general it is found that the fraction of the input power radiatedfrom the main plasma is 0.2 Prad,main / POhmic 0.4 (see Figure 2)although there is a large variation in this fraction for low densities (He0.8 x 1020 m-3). This fraction does not change significantly as the densityincreases. However, the power radiated from the main plasma increaseslinearly with density when N. 0.8 x 1020 m- 3 (see Figure 2). This linearincrease in the radiated power with density implies that the impurityconcentration remains approximately constant over this range in density (Prad n e nimp Limp(T) ). The Zeff of the plasma remains constant when n0.8 x 1020 m-3 , which also implies that the impurity concentration isconstant ( Zeff a nimp / ne ).Spectroscopic measurements of the fractional abundances ofimpurities indicate that carbon, which originates from vessel and tilesurfaces, is the dominant impurity with the oxygen concentration a factorof about three less than the carbon. The fractional abundance of carbon is0.5% and that of molybdenum, which is the first wall material, isapproximately 0.02%. These abundances indicate that carbon andmolybdenum contribute about 0.1 and 0.2 respectively to Zeff. Thesevalues are in good agreement with the measured Zeff.Modelling using the MIST impurity transport code together with dataobtained from a multi-layer-mirror based molybdenum diagnostic [4] andan absolutely calibrated grazing incidence VUV spectrograph [5] indicatesthat the molybdenum radiation accounts for about 30% of the radiatedPage 3power from the main plasma and that the contributions due to carbon andoxygen are 5% and 10% respectively. This code assumes a circulargeometry which may not properly account for the elongated plasmas anddivertor of Alcator C-Mod. The uncertainties in the atomic physics relatingimpurity densities to radiated power, the uncertainties in determining theradiated power from the main plasma, and the neutral particlecontribution may explain this discrepancy in radiated power fractions.4. Divertor plasma measurementsThe shaped divertor of Alcator C-Mod is not instrumented withenough bolometers to unambiguously obtain local emissivities withoutrelying on assumptions about the shape and extent of the emissivityprofile. An alternate method of calculating the radiated power from thedivertor is to assume that the bolometer chords of an array span the entireemitting region of the divertor. For each array, individual chordbrightnesses are multiplied by their poloidal widths, integrated in thetoroidal direction, and then summed to give the total radiated power in thedivertor.There is significant radiated power coming from the shaped divertorregion when the ion VB drift direction is toward the x-point (see Figure 3).This power increases linearly with density when iie > 0.8 x 1020 m- 3 .When the ion VB drift direction is away from the x-point, limited run-timeindicates that there is no significant radiation in the divertor. The fractionof the input power radiated in the shaped divertor region is nearlyindependent of density and is in the range 0.4 Prad,divertor / POhmic 50.6. Of the power available to the scrape-off layer, PSOL - POhmic ~Page 4Prad,main, the majority ( 60 - 80% ) of it is radiated in the divertor region(see Figure 3), indicating that the divertor is operating in the radiativedivertor regime.Vertically-viewing filtered diode arrays [6] have been used to viewthe plasma emission from the CIII multiplet around 4650 A and from Da'Together with the horizontally-viewing bolometers these measurementsdetermine the location of the impurity radiation in the divertor. Thecarbon emission is strongest from the region near the outer divertor tilesand the D. emission is most intense in the region near the inner divertortiles. Spectroscopic measurements indicate that the molybdenum sourcerate and radiation from the divertor region are negligible under normaloperating conditions.There are also filtered diode arrays that view the plasma in thehorizontal direction. Inversions of the approximately 200 brightnesschords can be performed using the D. or CIII data to give local emissivities[7]. The calculation of total hydrogen radiation is based on the Da signalsand Johnson - Hinnov formalism [8]. Determination of the total carbonradiation is less straightforward. Estimates of the local carbon emissivityare based on measured CIII emissivity, calculated charge-exchangeprofiles, and MIST modelling. Charge exchange volumetric losses havebeen calculated using a simple Monte Carlo code [9]. These calculationsindicate that the charge-exchange neutrals are comparable to carbonradiation as contributors to the energy loss in the divertor. The deuteriumradiation contribution is negligible. There is rough agreement betweenthese local emissivities and the emissivities calculated from the divertorbolometer chord brightness data.Page 5When divertor detachment occurs, the power flow to the divertorplates in the region near the strike points is suddenly extinguished [9].Signatures of this detachment are: the fraction of the input power radiatedfrom the main plasma and divertor increases and the radiation in thedivertor region is rearranged spatially. The spatial rearrangement of theradiation in the divertor is illustrated by the bolometer chordal viewspresented in Figure 4. Divertor detachment occurs just after 0.8 sec for thedischarge depicted in Figure 4. In general, it is found that for thebolometer arrays, chord B1, which is viewing well above the x-point, hasan increase in brightness and all other chords have a decrease. All of thechords of the XUV array show an abrupt decrease at divertor detachment.For the particular discharge shown in Figure 4, chord B2 increases atdetachment and then later decreases as the radiation moves further insidethe separatrix. Overall these bolometers, together with the visiblediagnostic arrays, indicate that the carbon radiation has coalesced into tworegions: one near the outer divertor plate and one above the divertor butinside the last closed flux surface. The Da radiation has also moved insidethe last closed flux surface but is closer to the x-point than the carbonradiation.5. Power balanceThe power balance equation is POhmic = Prad,main + Prad,divertor +Q divertor, with the latter not directly measured at the present time. Thereare thermocouples embedded in the divertor plates and the bulktemperature rise of the molybdenum tiles on the plates after a high OhmicPage 6power discharge can be used to set an upper limit to the heat flux to thedivertor. For a few such discharges (POhmic - 1.4 MW) it is estimated thatthe heat flux to the divertor plates is ; 40% of the input power, while theradiated power from the main plasma is ~ 30% of the input power, and theradiated power from the divertor is - 45%.Divertor detachment gives rise to a unique opportunity to check thepower balance of the plasma. The reduction of the power flowing to thedivertor plates, Qdivertor ~ 0, means that the power balance equationbecomes POhmic ~ Pradmain + Praddivertor. Figure 5 illustrates thisphenomenon. When the plasma detaches from the divertor plates 85% ofthe input power is accounted for by radiation from the main and divertorplasmas in contrast to only 60% for an attached plasma.6. ConclusionsDuring Alcator C-Mod's initial operational period it was determinedthat typically 30 - 40% of the input power is radiated from the mainplasma and 40% of the input power is radiated from the divertor region.These fractions are almost independent of density while the radiatedpower from these regions is linearly dependent on density. Highlyradiative divertor conditions (~ 80% of the power in the SOL is radiated)have been achieved. Spectroscopic identification of impurities shows thatwhile molybdenum is not very abundant in the main plasma it is thedominant radiation loss channel. Molybdenum source rates in the divertorregion are typically too low to measure and the radiation in the divertorcan be accounted for by charge exchange neutrals and deuterium, carbon,and oxygen radiation. Power balance accounting has been performed andPage 7there is relatively good agreement between the input power and themeasured power leaving the plasma. When divertor detachment occurs,the radiation in the main plasma and divertor increases. Also, the spatiallocation of the radiation in the divertor is rearranged with some emissionfrom D. and CIII coming from inside the last closed flux surface.AcknowledgementsA special thanks to Brian LaBombard for creating the Alcator C-ModEDGE Database. This work is supported by U.S. DoE Contract # DE-AC02-78ET51013References[1] I.H. Hutchinson and the Alcator Group, Physics of Plasmas, 1, 1511(1994).[2] E.R. Muller, G. Weber, F. Mast, G. Schramm, E. Buchelt, and C.Andelfinger, Design of a Four-channel Bolometer Module for ASDEXUpgrade and Tore Supra, Max-Planck-Institut fur Plasmaphysik IPP Report1/224 (1985).[3] R.A. Hulse, Nuclear Technology/Fusion, 3, 259 (1983).[4] M.J. May, A.P. Zwicker, H.W. Moos, M. Finkenthal, and J.L. Terry, Reviewof Scientific Instruments, 63, 5176 (1992).[5] M.A. Graf, J.E. Rice, J.L. Terry, E.S. Marmar, J.A. Goetz, G.M. McCracken, F.Bombarda, and M.J. May, submitted to Review of Scientific Instruments,(1994).[6] J.L. Terry, J.A. Snipes, and C. Kurz, submitted to Review of ScientificInstruments, (1994).Page 8[7] C. Kurz, J.A. Snipes, J.L. Terry, B. LaBombard, B. Lipschultz, and G.M.McCracken, submitted to Review of Scientific Instruments, (1994).[8] L.C. Johnson and E. Hinnov, J Quant Spect Radiat Transf, 13, 333 (1973).[9] B. Lipschultz, G.M. McCracken, J.A. Goetz, B. LaBombard, J.L. Terry, J.Snipes, A. Niemczewski, and R. Nachtrieb, these Proceedings (PSI-11).Figure Captions1. Schematic of the Alcator C-Mod closed shaped divertor showing theviewing chords of the bolometer systems. Chords labeled BI - B8 arecollimated bolometers and chords labeled UVI - UV16 are collimatedphotodiode detectors.2. Scaling of the radiated power from the main plasma (squares) and thefraction of the Ohmic power radiated in the main plasma (circles) withline-averaged density for standard discharges.3. Scaling of the radiated power from the divertor region (squares) andthe fraction of the power in the scrape-off layer radiated in the divertor(circles) with line-averaged density for standard discharges.4. Spatial rearrangement of the radiation in the divertor duringdetachment of the divertor. The detachment occurs at 0.805 sec. The slowtime response of chords B1, B2, and B5 arises from the digital filteringused to process the bolometer data. See Figure 1 for the location of thechords shown in this Figure.5. Scaling of the fraction of the Ohmic power radiated from the mainplasma plus the divertor with line-averaged density for standarddischarges (squares) and for discharges with a detached divertor (circles).Page 9-- -B2-B5-- B6B8UTV16UV1Figure 1100.6 1EEl0.5 -0.80.4 0E0000 0] 0.6 0 . 1 0 . 2- -o- 0.20~ 00 p I . . p I0.1-0 - - - - ' - - - -- ' - - ' - - - - 00 0.5 1 1.5 2ffe (1020 m-3)Figure 211000 0000 (ZD 00001........................................... ............................... 0 0   0  .......................!0 0 0n QO 00(bo 0A00 9j0........................................... ......  ...........................................---------------------- 0 .........00 OOW 00010 0...........................................   ................. ....................00..........00................ ........ . ............................................ cl 0J10.810.80.40. 60.40.200.60.40.201.50 0.5 1 2ne ( 1020 M-3Figure 312Chord B5Chord B2C h ord B 5= 1Chord UV1 6I,00.130.80time(s)Figure 470 0.75 0.85 0.901 0..................................    0 . .....................0 0 ------------------------------------0 1Ea...................................   .  .................. .....................................  ........ .......................................... ...------------------------XEl................................... ...............  I .................. ........ ... ..................................... ............... . .............................. .......1.21Cd4-$0P-10.80.60.40.200.5 1 1.5 2.501020 M-3 )Iffe (Figure 514

Power balance and scaling of the radiated power in the divertor and main plasma of Alcator C-Mod

http://dspace.mit.edu/bitstream/1721.1/95206/1/94ja015_full.pdf

Power balance and scaling of the radiated power in the divertor and main plasma of Alcator C-Mod

Abstract

Similar works

Full text

Available Versions

DSpace@MIT