Bombard, B.

Cracken, G.M.

Hutchinson, I.H.

Jablonski, D.

Lipschultz, B.

Lisgo, S.

Stangeby, P.C.

Terry, J.

DSpace@MIT

PFC/JA-96-18Local Impurity Puffing as a Scrape-off LayerDiagnostic on the Alcator C-Mod TokamakD. Jablonski, B. LaBombard, G.M. McCracken, S. Lisgo1 ,B. Lipschultz, I.H. Hutchinson, J. Terry, P.C. StangebylJune 1996'Institute of Aerospace Studies, University of Toronto, Ont. M3H 5T6, Canada.Submitted to Jour. 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.Local Impurity Puffing as a Scrape-off Layer Diagnosticon the Alcator C-Mod TokamakD. Jablonski, B. LaBombard, G.M. McCracken, S. Lisgo,*B. Lipschultz, I.H. Hutchinson, J. Terry, and P.C. Stangeby.*Plasma Fusion CenterMassachusetts Institute of TechnologyCambridge, MA 02139, USA.AbstractA capillary injection system has been installed on Alcator C-Mod which allows thelocalized introduction of gaseous species at a variety of poloidal locations. Anexperimental programme has been undertaken to observe impurity puffs with a CCDcamera through appropriate optical filters. The comet-like shape of an ion line emissionplume formed in the region of an injection graphically displays the direction of backgroundDeuterium flow as well as that of the poloidal impurity ion drift. Parallel and cross-fieldone-dimensional fluid models are used to characterize the plume shapes and extract thebackground plasma temperature and parallel flow velocity and the impurity ion poloidaldrift. Model results are benchmarked against simulations of DIVIMP, a Monte Carlocode, and fast scanning probe measurements. In addition to elucidating local edge impuritytransport, the experiments present a novel diagnostic technique.1. IntroductionObserving the line emission patterns formed in the region of the scrape-off layer ofan impurity puff provides unique opportunities for investigation of impurity transport aswell as diagnosis of the background plasma. The experiments on Alcator C-Mod [1] haveinvolved imaging plumes at a range of poloidal locations in a variety of plasma discharges,with both lower and upper x-points, in normal and reversed field configurations. Thispaper discusses general observations, focusing on analysis of the first charge state plumesformed at the inner-wall midplane viewed from the outboard.Such plumes have been observed and modeled on other machines. Two methodsof modeling these emission patterns have been employed: the use of a complex fluid orMonte Carlo code, as was done by Matthews [2], McCracken [3], and Pitcher [4], and theuse of simple analytic coulomb collision and ionization relations, as was done by Pitcher* University of Toronto IAS, Toronto M3H 5T6, Canada.- 1 -[5]. An intermediate approach has been taken in this work, namely using fluid modelswhich are more complete than the simple analytic relations, but simple and straightforwardenough for ease in implementation and interpretation. A benefit of this method is thepotential to analyze a large number of images in relatively short order, allowing thetechnique to be used as a regular diagnostic. The efficacy of this fluid paradigm isevaluated by comparison with calculations of DIVIMP [6,7], a Monte Carlo code, and withmeasurements of the C-Mod fast scanning probe [8,9]. It should be noted that while theproposed methodology was different, plasma parameter extraction from impurity plumeshapes has been previously discussed by Stangeby and Elder [10].2. ExperimentsThe primary tools for the experiments are shown in figure 1. The Neutral gasINJection Array (NINJA) is a capillary puffing system which can deliver gaseous speciesat 20 poloidal and 5 toroidal locations [11]. Puff locations discussed below are labeled A,B, and C for reference. Line emission patterns produced in the vicinity of the puffs areimaged with CCD cameras through selectable optical filters with the two views marked onthe figure. The cameras are located outside the vessel, with coherent fiber bundles leadingto a standard C-mount lens at the view. The cameras are controlled with MultipleAutomatic Camera Exposure Control and Display (MACECAD) units [12]. Profilemeasurements of density, electron temperature, floating potential, and parallel flow in theSOL are provided by the fast scanning probe at the poloidal location shown in the figure.All of the discussed experiments were performed in 800 kAmp, 5.3 Tesla on axis, single-null diverted discharges.The shape of the ion plumes observed is typically comet-like, with a tail extendingalong the field line towards the nearest strike-point. An example of this is shown in figure2. For this lower x-point shot, Nitrogen was puffed simultaneously at the inner and outerdivertors (locations B and C) while viewed with the top camera through an N-Il filter. Thetail of each plume is directed towards the appropriate strike-point, CCW viewed from thetop at the outer divertor and CW at the inner. This behaviour is observed through both -IIand -III filters, regardless of BxVB drift direction, x-point location, or other dischargeparameters. Figure 3 shows contour plots of C-II plumes during methane puffs at theinner midplane (location A) observed with the side camera in upper and lower x-pointshots. In both cases, the parallel flow is directed towards the appropriate strike point.Work has focused on the inner-wall midplane because of the excellent viewaccorded by the side camera, and because the geometry and transport can be easilyunfolded. Neutral emission (observed for He-I during Helium injection) is seen to be-2-circular in shape, approximately Gaussian in profile with a FWHM of 3.5 cm about theinjection site. The ion profiles, as mentioned above, are asymmetric. This asymmetry isattributed to coulomb collisions with flowing background Deuterium. One might positother explanations, such as a local radiative cooling effect, but such hypotheses aredisproved by the observation that a plume shape does not alter with a change in gasinjection rate. While the emission profile shapes are obviously dominated by paralleltransport of the ions (competing with ionization to the next charge state), evidence ofperpendicular transport is seen as well. In shots with a downward BxVB ion driftdirection, the plumes are seen to have upward deviations from the field line, indicatingpoloidally upward impurity ion drift.3. ModelTwo-dimensional plumes are integrated across and along the field line to obtainparallel and perpendicular profiles respectively. These profiles are modeled with the fluidequations discussed below. While the impurity ions lack self-collisionality, the highcollisionality these ions have with the background Deuterium lends validity to theapplication of local fluid parameters [111. Analysis is restricted to the first charge state ofthe impurities.The parallel transport of the ions is characterized with a set of continuity,momentum, and energy equations (where x is the parallel spatial variable):d-(nv)= Sn - nne < ov >i (1)dxd(nv2 +nT nvD v-nvne <'v >i (2)dx m Ivd (1 3  5nT TD-T-(-y+--v)=n +dx 2 2 m TTnvVD -(-nv 2+ 3- )ne <Gv>i (3)TV 2 2 mfor which unscripted variables represent impurity ion parameters, ne the backgroundelectron density, TD and vD the background Deuterium temperature and parallel velocity,and the rate coefficients the ionization to the next charge state. The appropriate coefficientsgiven by Bell [13] are used. The particle source term, Sn, constitutes ionization from theneutral state. Its shape is taken as that of the observed neutral emission profiles (Gaussian,FWHM of 3.5 cm). The characteristic momentum (Tv) and energy (TT) equilibration timesgiven by Spitzer [14] are employed to account for coulomb collisions with the backgroundplasma. The background plasma values are taken as constant. Three inputs are needed (ne,TD, and vD) to solve the equations for the three outputs (profiles of impurity ion n, T, and-3-v). To match an experimentally observed profile with a modeled density profile, n, theelectron density, ne, is taken as given (provided by scanning probe measurements), and thebackground plasma temperature and Deuterium ion velocity are found which provide thebest fit.The perpendicular behaviour of the ions is characterized with a continuity equation(where z is the spatial variable):-D +v+ne < v>i n= S e- (4)dz2  dzwhere D and v are the poloidal diffusion coefficient and drift velocity respectively. TheGaussian form of the particle source has been inserted (with X = 2 cm). Assuming D andv are constant, this equation can be solved analytically in terms of error functions. Becausethe characteristic diffusion lengths are much smaller than the width of the source, thesolutions are insensitive to the value of D employed. To match a given perpendicularprofile, the density (provided by the scanning probe or other means), temperature (foundwith the parallel solution), and diffusion coefficient are taken as given, and the value ofpoloidal drift velocity which provides the best fit is found. In summary, for a given two-dimensional plume and specified background electron density, equations 1-4 are used intandem to find the background temperature, the background Deuterium parallel velocity,and the impurity ion poloidal drift.DIVIMP (DIVertor IMPurities), a three dimensional edge impurity transportMonte Carlo code, is used to benchmark the model. DIVIMP accounts for phenomena notcovered by the fluid equations, namely partial collisionality and three-dimensional aspectsof impurity transport. Further, the comparison provides a check against a code with aproven track record. The results of one such comparison are shown in figure 4. For thiscase, typical edge density and temperature radial profiles, a parallel Deuterium velocity of13000 m/sec, and a poloidal impurity ion drift of 1500 m/sec are modeled to produce aplume which is integrated to obtain the parallel and perpendicular profiles. Using theplasma density at the radial location where DIVIMP gives the C-Il density as peaked (ne of2.3x10 19 m-3), the best fluid model fits are shown in the figure. The resultant temperature,16.2 eV, compares with 17 eV at the radial location where C-Il is peaked. The fluid modelfound velocity values of 12400 and 1800 m/sec are within 5 and 15% of the DIVIMPmodeled values respectively.4. ResultsAn example of the fluid analysis for an experimental plume is shown in figure 5.This plume was observed in a lower x-point discharge with a core volume averaged-4-electron density (ife) of 2. 1x10 20m- 3. The electron density measured by the scanningprobe at the appropriate flux surface is employed (5.6x10 19m- 3). While the scanning probeand plume are at different poloidal locations (see figure 1), one expects plasma pressureoutside of the divertor region to be conserved along a field line, meaning that if the plumederived temperature matches that measured by the scanning probe, the accuracy of thedensity used by the model for the inner-wall puff location is verified. The temperaturefound with the parallel solution, 10.2 eV, does show agreement with that of the scanningprobe (9.5 eV). A parallel Deuterium Mach number of .4 (towards the lower divertor) anda poloidal impurity ion drift of 365 m/sec (upward) are found. If this drift is due to ExB, itwould correspond to a radially outward electric field of 2920 V/m.Error analysis shows the uncertainty of resultant temperature and poloidal drift tobe relatively small. Uncertainty in the temperature, primarily due to possible error in theemployed density, is 10% or less, and uncertainty in the drift velocity, primarily due touncertainty in the ionization rate coefficients used by the model, is on the order of 20%.For Mach number, the accuracy of the assumed density is critical. Roughly speaking, theMach number will be known only as well as is the electron density at the plume location.If the uncertainty of that density is 20%, the error bar on the Mach number will be about20% as well.Parameters derived from plume analysis for one C-Mod run are compared withscanning probe measurements in table 1. All of the shots had lower x-points. The valuesof e, along with the measurements of temperature, Mach number, and electric field arelisted. Fast scanning probe Mach numbers are derived from the ratio of upstream todownstream ion saturation current with the Hutchinson formulation, which setsmomentum and particle diffusivities equal in its derivation of the appropriate fluidequations [15]. Radial electric fields at the scanning probe are calculated by taking thederivative of the plasma potential found with standard Langmuir probe theory.While the temperature comparison in the table is a quantitative one, the Machnumber and electric field comparisons should be viewed qualitatively. Excellenttemperature agreement is shown in the third and fourth columns, validating the model useof the scanning probe measured electron density. The scanning probe measurements ofMach number indicate the order of magnitude of flow velocity one would expect in the C-Mod SOL outside the divertor region. Mach numbers obtained from both the plumes andthe probe range between .1 and .5. The negative values for the scanning probemeasurements indicate flow towards the outer strike point (as opposed to the flow towardsthe inner strike-point indicated at the inner-wall). Electric field values on the order of 103are seen by both measures. The observation that plume and scanning probe measured-5-electric field are often much different is not surprising, both because different values areexpected at the two locations, and because the uncertainty of the calculated electric field atthe scanning probe is about 50%.5. ConclusionsThese experiments have demonstrated that plumes can play a unique role indiagnosing the C-Mod edge layer. The direction of flow of the background Deuteriumions and the direction of the impurity ion drift are clearly seen in the raw experimental data.This on its own is of value because of the great flexibility and ease of use of the technique.Plumes can be observed wherever a capillary puff location with a view available of it islocated. A relatively simple fluid model can be used to determine the backgroundtemperature, parallel flow velocity, and impurity ion poloidal drift. The model paradigmhas been shown valid through quantitative comparison with DIVIMP and scanning probemeasured temperature, and qualitative comparison with scanning probe measures of Machnumber and electric field. With a more complete system, density and other parameterscould in principle be extracted with this technique. However, as the plumes are nowviewed and modeled on C-Mod, a reliable independent measure of density is required foranalysis.AcknowledgmentsThe authors would like to express their gratitude to Bob Childs and Tom Toland fortheir work in construction and implementation of NINJA, to Aaron Allen for constructingthe PC based system which digitizes and archives the plume images, and to the otherscientists, engineers, technicians, students, and support staff of the Alcator C-Mod teamwhose dedicated efforts have made this work possible. This work is supported by the U.S.Department of Energy under contract No. DE-AC02-78ET5103.References[1] I.H. Hutchinson, R. Boivin, F. Bombarda, et al, Phys. Plasmas 1 (1994) 1511.[2] G.F. Matthews, D.N. Buchenauer, D.N. Hill, et al, 18th European Conference onControlled Fusion and Plasma Physics (1990), 111-229.[3] G.M. McCracken, U. Samm, S.J. Fielding, et al, J. Nucl. Mater. 176-177 (1990) 191.[4] C.S. Pitcher, P.C. Stangeby, D.H.J. Goodall, et al, J. Nucl. Mater. 162-164 (1989)337.[5] C.S. Pitcher, PhD Thesis, University of Toronto IAS (1987).[6] P.C. Stangeby, C. Farrell, S. Hoskins, et al, Nuclear Fusion 28 (1988) 1945.-6-[7] P.C. Stangeby and J.D. Elder, J. Nucl. Mater. 196-198 (1992) 258.[8] B. LaBombard, J. Goetz, C. Kurz, et al, Phys. Plasmas 2 (1995) 2242.[9] B. LaBombard, J. Goetz, I.H. Hutchinson, et al, these proceedings.[10] P.C. Stangeby and J.D. Elder, Plasma Phys. Control. Fusion 33 (1991) 1435.[11] D. Jablonski, PhD Thesis, MIT Plasma Fusion Center (1996).[12] D.H.J. Goodall and M.N. Price, J. Nucl Mater. 196-198 (1992) 1047.[13] K.L. Bell, H.B. Gilbody, J.G. Hughes, et al, Atomic and Molecular Data for Fusion,Part I, UKAEA Report CLM-R216 (1982).[14] L. Spitzer, Jr., Physics of Fully Ionized Gases (Interscience, New York, 1956).[15] I.H. Hutchinson, Phys. Fluids 30 (1987) 3777.-7-Fig. 1 - Layout of plume experiments on the Alcator C-Mod tokamak: the capillary pufflocations utilized in the experiments discussed are marked with arrows and labeled forreference; eyes indicate the CCD camera views; the fast scanning probe poloidal locationand trajectory are shown.Fig. 2 - Top camera view through N-Ifilter of simultaneous Nitrogen puffs at inner andouter divertor (locations B and C). The tail of each plume extends along the field linetowards the appropriate strike-point.Fig. 3 - Contour plots of images of methane puffs at inner-wall midplane (location A)viewed by side camera through a C-II filter in lower and upper x-point discharges;background Deuterium ion flow is indicated towards the appropriate strike-point in bothshots.Fig. 4 - Best fluid model fits to integrated parallel and perpendicular profiles of C-IIplume produced by DIVIMP. DIVIMP utilizes typical C-Mod edge profiles of density andtemperature. Using the electron density at the radial location where DIVIMP has C-IIions peaked, the fluid model fits a temperature of 16.2 eV (compared with 17 eV at the fluxsurface of the employed density), a parallel Deuterium flow velocity of 12,400 m/sec(13,000 input to DIVIMP), and an impurity ion drift of 1800 m/sec (1500 input toDIVIMP).Fig. 5 - Best fluid model fits to integrated profiles of an inner-wall midplane (location A)N-II plume viewed by the side camera. Using the electron density measured by thescanning probe at the appropriate flux surface, the model fits to a temperature of 10.2 eV(compared with 9.5 eV measured by the scanning probe), a Mach number of .4, and apoloidal impurity ion drift of 365 m/sec.Table 1 - Plume and fast scanning probe measurement comparison for shots of one run.While temperature comparison should be viewed quantitatively, comparisons of Machnumber and radial electric field are meant to be qualitative (since measurements are madeat different poloidal locations).Core ne Temperature (eV) Mach Number Er-Field (V/m)x1020m-3  Species Plume FSP Plume FSP Plume FSP1.5 N-II 7.5 8.0 .20 -.44 1360 11002.1 N-II 10.0 9.5 .35 -.32 2190 19002.1 N-II 10.0 9.5 .40 -.36 2920 15002.3 He-II 17.0 15.5 .15 -.20 1080 37001.7 He-II 20.0 20.5 .25 -. 14 420 50001.2 N-II 8.0 8.0 .25 -.36 520 1500-8-TopXCameraSideCameraViewScanningProbeFigure 19Figure 210liii 111111 iii'''0)3CO-0-a- CU- 0s-i)C.-11111111111111 III 111111C C (00CjEco~0C:D--0A00N~II10,1,,,c(D 0 (0c&j(wo) I-eoip9iAFigure(wo) Ie3!1.JeA3110ClC0C-'00C)CU-0I-0n-0.10 0.00Distance along Field0.10Line (m)-0.05Poloidal0.00 0.05Distance (m)Figure 4121.00.8l-a)cFluid Model- --DIVIMP......0.60.40.20.0 .-0.201.00.81-0.20CoCa)Ca)a)0.60.4Fluid Model- -- DIVMP0.21-0.010-0.10 0.10-0.05 0.00 0.05 0.10Distance along Field Line (m)-0.02 0.00 0.02Poloidal Distance (m)Figure 5131.0.4-C',0.80.60.40.20.0ExperimentFluid Model-0.15 -0.101.00.150.810.61>1CCaa)4-0.40.20.0(perimentuid Model-- E7-F Fl-. f-0.04 0.04

Local impurity puffing as a scrape-off layer diagnostic on the Alcator C-Mod tokamak

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Local impurity puffing as a scrape-off layer diagnostic on the Alcator C-Mod tokamak

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