Devaux, S.

Eich, T.

Huysmans, G.

Lang, P.

Wenninger, R.

English

MPG.PuRe

Filament footprints of pellet induced ELMs observed on divertor targetR. Wenninger1, S. Devaux2, T. Eich2, G. Huysmans3, P. T. Lang2 and JET-EFDA contributors41 Universitätssternwarte der Ludwig-Maximilians-Universität, 81679 München, Germany2 Max-Planck-Institut für Plasmaphysik, EURATOM Association, 85748 Garching, Germany3 CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France4 See the Appendix of F. Romanelli et al., Fusion Energy 2008 (Proc. 22nd IAEA FusionEnergy Conf., Geneva, 2008)IntroductionIt is currently assumed that a reliable control technique for Edge Localised Modes (ELMs) ismandatory for the success of ITER. Gaining information about the process of inducing ELMsby pellets and reaching some level of understanding is desirable for the design of next stepfusion devices and the preparation of successful operation of those. Experiments at ASDEXUpgrade have shown that deuterium ice pellets when reaching the pedestal top induce ELMswithin 60µs up to frequencies twice the spontaneous ELM frequency [1]. It is widely assumedthat the instability associated with spontaneous type-I ELMs has a peeling-ballooning character[2]. Spontaneous ELM filaments are observed to cover the entire toroidal circumference withoutpeaking if averaged over a sufficient number of ELMs [3].Pellet injection is done at one fixed toroidal position both in JET and ASDEX Upgrade from theouter midplane. This raises the question, if the spatial structure of the perturbation associatedwith pellet induced ELMs features a corresponding toroidal asymmetry. We show experimentalevidence of a significant asymmetry of the scrape of layer (SOL) heat transport in presence ofpellet induced ELMs and its absence (in line with [4, 3]) for spontaneous ELMs. We furtherlink these results to recent simulations from JOREK indicating that pellet induced ELMs retainthis externally forced structure peaking in toroidal direction around one location [5].Experimental results and analysis by field line tracingDivertor infrared tomography (IR) in combination with magnetic field line tracing has pre-viously been employed to investigate the filamentary structure of spontaneous ELMs [3]. Weapply this method in conjunction with a ramp of the toroidal magnetic field in order to ma-nipulate the position of the divertor imprint of a (transiently) toroidally localised filament. Anoptimised JET H-mode discharge employs a ramp in toroidal magnetic field from 2.2T to 2.8Tat fixed IP and hence decreasing both q95 from 4.8 to 3.6 and the SOL connection length. Themain discharge parameters are: IP  2  0MA, PNBI 8MW and δ   0  27 (low triangularity).Pellets of a nominal particle number of 2  1021 have been injected perpendicularly from theouter low field side midplane at 4Hz with velocities between 170m/s and 200m/s.Figure 1 displays the evolution of the heat load profiles on the outer divertor target throughoutthe BT -ramp. The profiles for the spontaneous ELMs (figure 1 a) all have one main peak with asteep shoulder towards the private flux region and a flat shoulder towards the far SOL. The radiallocation of the peak does not change with BT and lies close to the pre-ELM heat flux maximum.Profiles for other spontaneous ELMs (e.g. the ELMs succeeding the pellet induced ELMs) areidentical and not shown here. In clear contrast, the profiles for pellet induced ELMs (figure 137th EPS Conference on Plasma Physics P4.1732.7 2.75 2.82.22.32.42.52.62.72.80102030BT [T]RDIV [m]a)q [MW/m2]2.7 2.75 2.82.22.32.42.52.62.72.80102030BT [T]RDIV [m]b)q [MW/m2]Figure 1: Evolution of the deposition profiles on the outer target: a) ELMs preceding pellet inducedELMs and b) pellet induced ELMs. The heat flux densities are averaged over t=0 to t=0.4ms with t=0defined by the peak power deposition. Horizontal lines are to support the allocation with BT .b) are accompanied by additional dominant peaks located at a variety of radial positions in theSOL region. We identify a trend of such pellet induced additional peaks moving outwards withBT .An inverted toroidal magnetic field ramp (3.3T-2.1T) has been carried out in order to verify theradial movement of the pellet induced peak heat flux due to the change of the toroidal magneticfield only. Indeed, for this discharge the pellet associated peaks move towards lower radii withdecreasing BT .In order to investigate the question of the origin of the BT -depended peaks observed exclu-sively in conjunction with pellet induced ELMs SOL field line tracing has been employed. Wetraced field lines starting from the pellet injection location at the outer midplane (MP) at thefixed vertical and toroidal coordinates ZMP and φMP. The radial coordinate RMP varies from thevalue of the pellet entrance in the far SOL to the separatrix. Magnetic field lines obtained via theequilibrium reconstruction have been followed from (RMP, ZMP, φMP) until they hit the outertarget defining the coordinates (RDIV , ZDIV , φDIV ). In such a mapping of equilibrium 3D SOLfield lines both RDIV and φDIV are functions of RMP. The target end points of this field lines fora continuous variation of RMP build a spiral on the target [6, 7]. Changing BT at constant plasmacurrent approximately corresponds to a rotation of this spiral. A rotation of the spiral in returnleads to a radial movement of the heat flux pattern observed by IR.Figure 2 shows the result of this previously explained mapping function applied to equilibriaof such times, at which pellet triggered ELMs are observed. Hence for each time, the pellettrajectory outside the separatrix is mapped onto the outer divertor target observed by IR (blacksquares). Figure 2 shows additionally the radial target position of the largest peaks by defining athreshold criterion versus BT valus. The threshold criterion is fulfilled when the local heat fluxdensity exceeds 85% of the maximum heat flux density within the profile during an investigated37th EPS Conference on Plasma Physics P4.1732.2 2.3 2.4 2.5 2.6 2.7 2.82.72.712.722.732.742.752.762.772.782.792.8RDIV [m]BT [T]Figure 2: Evolution of experimentally observed peak positions for pellet induced ELMs (blue dots)and spontaneous ELMs (red dots) and predicted peak positions on the basis of field line tracing (blacksquares).period (t=0 to t=0.4ms with t=0 defined by the peak power deposition). Again the different dis-tribution of locations of peak heat fluxes for spontaneous and pellet induced ELMs is clearlyvisible. One can see in figure 2 that the radial peak locations on the target, which are predictedon the basis of pre-ELM equilibrium field line tracing, are in very good agreement with theexperimentally observed peak locations for the pellet induced ELMs with a slight systematicdeviation towards higher radii. This supports the conclusion that the pellet induced ELM en-ergy transport in the SOL is not distributed fully toroidally symmetric but largely concentratedalong the SOL field lines intersecting the pellet trajectory. It should be finally noted that thepellet associated additional peak in the heat flux profiles does not show a significant delay whencompared to the spatially integrated heat fluxes giving the ELM power flux (  200µs).Code predictions for pellet induced ELMs at JETThe code JOREK is evolving non-linearly a set of reduced MHD-equations [5]. The plasma isrepresented in X-point geometry and the calculation domain extends beyond the separatrix andthe far SOL. JOREK simulations of spontaneous ELMs have reproduced various ELM featureslike filaments [5], which previously have been observed experimentally.For the simulation of pellet induced ELMs a JET-like plasma has been chosen with an edgepressure gradient such that the plasma without pellet perturbation is stable against ballooningmodes. To describe an injected pellet an adiabatic plasma density perturbation with an amplitudeof 25 times the central density, which extends over 8% of the minor radius and which has inthe toroidal direction a full width half maximum of about 16% of the circumference, has beensuperimposed to the unperturbed plasma.The pressure increases within a few µs to a value 6 times the pressure on axis in a region slightlylarger than the size of the original pellet perturbation. The extent of the magnetic perturbationis large enough to ergodize the flux surfaces, where the density has been initially enhanced. Asshown in figure 3  50µs after the initial density perturbation a field aligned helical perturbation37th EPS Conference on Plasma Physics P4.173Figure 3: JOREK simulation of a pellet induced ELM about 50µs after the initial density perturbation:Temperature on a flux surface just inside the separatrix is illustrated from blue (low T) to red (high T).The density contour of twice the central density is shown in yellow. The initial density perturbation wassuperimposed in the pedestal on the left-hand side. (Figure taken from [5] - courtesy IOP)has evolved. This perturbation peaks at one location in toroidal direction, which is defined by thepellet injection location. The regions of peak density are aligned with the helical perturbation.DiscussionWe have measured the ELM induced divertor heat flux for spontaneous and pellet inducedELMs. We have found notably that the divertor heat flux footprints due to pellet triggered ELMsdo show a dominant additional peak.First we conclude that the observed additional structure can not be a consequence of materialablated by the pellet during its flight through the SOL and convected to the divertor since suchparticles should be low energetic and hence have a much larger parallel transport time. Thus atsome location along the field line, which we have traced to the pellet specific imprints on thetarget, there has to be a strongly enhanced radial transport across the separatrix. The results fromJOREK and in particular extent and location of the peak pressure and field line ergodisationsupport the hypothesis of such a high radial transport in the vicinity of the pellet injectionlocation only some µs after the initial density perturbation. This is in line with visible cameraobservations of hot spots on limiters in the vicinity of the pellet injection location [8]. In thisstudy for pellet induced ELMs no delay to the magnetic ELM onset has been found, while forspontaneous ELMs a distribution of delays up to 100µs has been measured.In summary we have provided experimental evidence that the SOL energy transport associatedwith pellet induced ELMs is dominated in the main chamber in a single filamentary structure.From our observation we infer that spontaneous ELMs are not associated with such a strongasymmetry.References[1] Lang P T et al 2003, Nucl. Fusion 43 1110-1120[2] Connor J W et al 1998, Phys. Plasmas 5 2687[3] Eich T et al 2005, Plasma Phys. Control. Fusion 47 815-842[4] Kirk A et al 2005, Plasma Phys. Control. Fusion 47 995-1013[5] Huysmans G T A et al 2009, Plasma Phys. Control. Fusion 51 124012[6] Eich T et al 2003, Phys. Rev. Lett. 91 195003[7] Punjabi et al 1992, Phys. Rev. Lett. 69 3322[8] Kocsis G et al (2010), This conference37th EPS Conference on Plasma Physics P4.173

Filament footprints of pellet induced ELMs observed on divertor target

http://hdl.handle.net/21.11116/0000-0008-B040-8

Filament footprints of pellet induced ELMs observed on divertor target

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