Devaux, S.

Eich, T.

Huysmans, G.

Jachmich, S.

Köchl, F.

Lang, P.

Wenninger, R.

English

MPG.PuRe

Scrape off layer heat transport and divertor power deposition of pelletinduced ELMsRP Wenninger1, TH Eich2, GTA Huysmans3, PT Lang2, S Devaux2, S Jachmich4, F Köchl5and JET EFDA Contributors JET-EFDA, Culham Science Centre, Abingdon, OX14 3DB, UK1Universitätssternwarte der Ludwig-Maximilians-Universität, 81679 München, Germany2 Max-Planck-Institut für Plasmaphysik, EURATOM Association, Garching, Germany3 ITER Organization, Bat 519, CEA Cadarache, 13108 St. Paul-lez-Durance, France4Laboratory for Plasma Physics, Ecole Royal Militaire/Koninklijke Militaire School, B-1000Brussels, Belgium5Association EURATOM-ÖAW, Atominstitut, TU Wien, 1020 Wien, Austria See the Appendix of F. Romanelli et al., Proceedings of the 23rd IAEA Fusion EnergyConference 2010, Daejeon, KoreaIntroductionIt is currently assumed that a reliable control technique for ELMs is mandatory for the successof ITER to meet restrictions in power loads to the target [1, 2, 3]. In view of next step fusiondevices it is of paramount importance to reach some level of understanding in the process ofinducing ELMs by pellets. Due to the technically given toroidal asymmetric nature of pelletinjection the question arises, if the spatial structure of the perturbation associated with pelletinduced ELMs features a corresponding toroidal asymmetry in the SOL and in the divertor. Thepaper recapitulates the finding of a toroidal asymmetric divertor deposition pattern during pelletinduced ELMs employing a new JET divertor infrared system and compares these to nonlinearsimulations.Experimental observationsDivertor infrared tomography (IR) in combination with magnetic field line tracing has pre-viously been employed to investigate the structure of spontaneous ELMs [3]. We apply thismethod in conjunction with a ramp of the toroidal magnetic field (2.2T to 2.8T, q95 4.8 to 3.6)in order to manipulate the position of the divertor imprint of a (transient) toroidally localisedfilament. Some key parameters of the discharge analyzed in this paper (79573) are: IP 2  0MA,PNBI 8MW and δ  0  27 (low triangularity). Figure 1a displays the evolution of a number offurther parameters. Pellets of a nominal particle content of 2  1021 (  30  trigger threshold[4]) have been injected perpendicularly from the magnetic low field side mid plane at 4Hz withvelocities between 170m/s and 200m/s.As already shown in [5] significant differences according to the divertor deposition struc-ture have been observed during spontaneous and pellet induced ELMs. Figure 1b shows theradial target position of the largest peaks of the divertor power flux density profiles defined bya threshold criterion versusBT. The threshold criterion is fulfilled when the local power fluxdensity exceeds 85% of the maximum power flux density within the profile during an investi-gated period (t=0 to t=0.4ms with t=0 defined by the peak power deposition). One can see thatthe locations of most peaks associated with pellet induced ELMs show a clear correlation withBT, while spontaneous ELMs have peaks scattering arround the same location for all valuesofBT.38th EPS Conference on Plasma Physics (2011) P5.071−3−2.5−2BT [T]00.511.52Dα [a.u.]14 15 16 17 18 19 20 21010203040Pdiv,out [MW]t [s] 2.2 2.3 2.4 2.5 2.6 2.7 2.82.72.722.742.762.782.8RDIV [m]|B  |T  [T]Figure 1: a) Time traces of toroidal magnetic field, Dα -radiation, power to the outer target, edge lineintegrated density and pellet monitor signal. b) Observed peak positions (criterion is described in the text)for pellet induced ELMs (blue dots) and spontaneous ELMs (red dots) as function of  BT  in discharge79573 (Fueling size pellets / LFS): Squares indicate predicted and corrected peak positions on the basisof field line tracing with Φmp  Φpel .A comparative analysis of various divertor power load aspects has been carried out [5, 6]. Herespontaneous and pellet induced ELMs chosen on the basis of selection criteria described in [6]have been analyzed. A maximum power flux density on the outer target during the entire ELMof 23 	 5MW  m2 for spontaneous and 30 	 7MW  m2 for pellet induced ELMs has been found.Code predictions from JOREKThe non-linear MHD code JOREK [7] evolves a set of reduced MHD equations in 3D.Thecomputational domain includes the plasma center, the separatrix, the x-point and the open fieldlines in the SOL. For the simulation of pellet induced ELMs a JET-like H-mode plasma [8] hasbeen chosen with an pedestal pressure gradient such that the plasma is marginally stable withrespect to low to medium-n ballooning modes. In the simulations the pellet is described by alarge local particle source located in the middle of the pedestal at the magnetic low field sidewith a half-width of 2 cm and acting adiabatically on the plasma.Figure 2a) shows a contour of the density as well as the temperature on the separatrix and theheat flux convected to the target  50µs after the start of the pellet source. A helical pertur-bation with the dominant toroidal mode number n=1 can be observed. At this time the pressurein the pellet cloud at the mid plane is about twice the pressure at the top of the pedestal andremains relatively constant. Figure 2b) shows a close-up of the density profile at the toroidalangle where the pellet cloud is passing the x-point. Here as well the contours of the electrostaticpotential (ExB flow lines) from the JOREK calculation are illustrated. From the high densityregion of the helical pellet cloud there is an ExB flow across the separatrix close to the x-pointdirected towards the divertor. This ExB flow is found by the model to be clearly stronger to-wards the outer divertor leg, which is the one with the higher simulated power flux densities onthe divertor.The ExB flow leads to prompt losses of particles close to the x-point. Since the particles cross-ing the separatrix have been heated by parallel transport inside the separatrix, we expect that38th EPS Conference on Plasma Physics (2011) P5.071Figure 2: a) The density contour (n  1  1  nmag  axis) at t  50µs, the temperature at the separatrix(red-blue scale) and the convected heat flux to the target. The pellet injection location is on the LFSmid plane on the side of the plasma, which is turned away from the viewer. b) Close-up of the densitycontour together with the contours of the electrostatic potential (ExB flow lines, black contours). Theheat convected to the target is illustrated in both figures.there is also an associated convected heat transport. After the separatrix crossing the paralleltransport might have a more dominant role.Figure 3a) illustrates the power flux density on the outer divertor calculated by JOREK fora time  40µs after the onset of the particle source as a function of radial divertor positionRdiv and toroidal angle Φ. At a radial position of 3.25m there is high or even maximum depo-sition for all toroidal angles. This position corresponds to the separatrix mapped on the outerdivertor target. In addition to this symmetric deposition component another component chang-ing maximum power flux density and profile shape with the toroidal angle can be observed.This component is spreading toroidally towards a symmetric shape with approximately the lo-cal sound speed in the x-point region.In order to isolate the asymmetric component we identify the toroidal angle, for which thepower flux density integrated along the radial direction is minimum (fig. 3c). We select the ra-dial power flux density profile for this toroidal angle as the profile, which describes in the bestway the symmetric component. We subtract this profile of the power flux density profiles foreach toroidal angle and obtain the profiles of the asymmetric component, which are shown infigure 3b). Figure 3d) illustrates the profiles for the toroidal position with the maximum asym-metric contribution.The maximum power flux density of the total deposition evaluated for each toroidal positionhas a maximum variation of 66% of the mean value, which is a much larger variation thenthe experiment describes. Thus a considerable toroidal asymmetry in the maximum power fluxdensity is predicted by JOREK. Figure 3d) indicates the location of the maximum power fluxdensity for all toroidal locations for the total deposition and the asymmetric component. In realspace coordinates the line for the total deposition would appear as a structure similar to a spiral.By means of JOREK calculations and SOL field line tracing a number of indications regard-ing the nature of the toroidally asymmetric component of the transport associated with pelletinduced ELMs has been gathered. As further discussed in [5] there are two candidate positionsfor the separatrix crossing: a) X-point region and b) pellet injection location.38th EPS Conference on Plasma Physics (2011) P5.071φdiv [rad]Rdiv [m]a)0 2 4 63.153.23.253.33.353.43.453.50 2 4 63.153.23.253.33.353.43.453.5φdiv [rad]Rdiv [m]b)3.15 3.2 3.25 3.3 3.35 3.4 3.45051015x 10−6Rdiv [a.u.]q [a.u.]2.68 2.7 2.72 2.74 2.76 2.78 2.8 2.82 2.8400.511.522.53x 107Rdiv [a.u.]q [a.u.]c)d)Figure 3: Heat flux density on the outer divertor target from JOREK simulation: a) Total deposition b)Asymmetric component. Pellet injection location is at Φ  0. c) Profiles of total deposition (solid) andasymmetric component (dashed) at toroidal position with minimum (red) and maximum (blue) asym-metric contribution. d) Deposition profile of individual ELMs averaged over t=0 to to t=0.4ms with t=0defined by the peak power deposition: Pellet induced ELM (black) and spontaneous predecessor (ma-genta)SummaryWe recall that the ultimate goal of inducing ELMs by pellets is the reduction of the deterio-rating effect of the ELMs on the plasma facing components [9]. In the light of this the strikingfeature of pellet induced ELMs to cause an asymmetric divertor power load deserves particularattention. In the simulation the toroidal variation of the maximum power flux density is 66%for the total deposition. In qualitative comparison the experiment with asymmetric componentin the IR-view the peak heat flux of pellet induced ELMs is found to be considerably largerthan for spontaneous ELMs. It is not clear, if the asymmetric component is of similar extent forELMs induced by pellets injected from the high field side, or ELMs induced by smaller pelletsand how it scales with major radius. Consequently it can not be excluded that pellet injectionat fixed q95 in next step fusion devices such as ITER causes considerable toroidaly localisedenhanced divertor degradation.This work was supported by EURATOM and carried out within the framework of the EuropeanFusion Development Agreement. The views and opinions expressed herein do not necessarilyreflect those of the European Commission.References[1] Federici G et al 2003, J.Nucl.Mater. 313-316 11-22[2] Loarte A et al 2003, J.Nucl.Mater. 313-316 962-966[3] Eich T et al 2005, J.Nucl.Mater. 337-339 669-676[4] Lang P et al 2011, Nuclear Fusion 51 1-16[5] Wenninger R P at al 2010, 37th EPS Conf. on Controlled Fusion and Plasma Physics[6] Wenninger R P at al 2011, Submitted to Plasma Phys. Control. Fusion[7] Huysmans G T A et al 2009, Plasma Phys. Control. Fusion 51 124012[8] Huysmans G T A et al 2010, 37th EPS Conf. on Controlled Fusion and Plasma Physics[9] Lang P et al 2004, Nuclear Fusion 44 665-67738th EPS Conference on Plasma Physics (2011) P5.071

Scrape off layer heat transport and divertor power deposition of pellet induced ELMs

http://hdl.handle.net/21.11116/0000-0008-8D2D-8

Scrape off layer heat transport and divertor power deposition of pellet induced ELMs

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