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ELM triggering by deuterium pellets

G. Kocsis 1) Acknowledgement: A. Alonso 2) , L.R. Baylor 3) , G. Huysmans 4) , S. Kálvin 1) , K. Lackner 5) , P.T. Lang 5) , J. Neuhauser 5) , M. Maraschek 5) , B. Pegourie 6) , G. Pokol 7) , T. Szepesi 1) ,

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ELM triggering by deuterium pellets

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  1. G. Kocsis1) Acknowledgement: A. Alonso2) , L.R. Baylor3) , G. Huysmans4), S. Kálvin1), K. Lackner5) , P.T. Lang5), J. Neuhauser5), M. Maraschek5), B. Pegourie6) , G. Pokol7) , T. Szepesi1), 1) KFKI RMKI, EURATOM Association, P.O.Box 49, H-1525 Budapest-114, Hungary 2) Laboratorio Nacional de Fusion, Euratom-CIEMAT, 28040 Madrid, Spain 3) Oak Ridge National Laboratory, Oak Ridge, TN, USA 4) Association Euratom-CEA Cadarache 5) MPI für Plasmaphysik, EURATOM Association, Boltzmannstrasse 2., D-85748 Garching, Germany 6) CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France 7) BME NTI, EURATOM Association, P.O.Box 91, H-1521 Budapest, Hungary ELM triggering by deuterium pellets

  2. Outline Introduction: ELM problem, pellet pacemaking Radial localisation of pellets at ELM trigger Pellet plasma interaction: Summary and outlook Pellet caused MHD perturbation in type-I, type-III and OH discharges Pellet caused plasma cooling in type-I ELMy H-mode Nonlinear MHD modelling of pellet ELM triggering Poloidal scan of pellet triggering capability

  3. Introduction

  4. ELMs can cause critically high power load Type-I ELMs can cause critically high transient power load on plasma facing components. Full performance ITER plasma W pedestal can be as much as 100 MJ leading to ~20 MJ ELM loss if ν*ped is the controlling parameter. Loarte et al., Nuclear Fusion, ITER Physics Basis,Chapter 4 «Edge Localised Modes» (ELMs) are MHD instabilities destabilised by the pressure gradient in the H-mode edge pedestal: Losses up to 10% plasma energy in several 100 micro-seconds Filaments evolving during ELM on MAST. Kirk, et al. PRL 2004.

  5. ELM pacing is mandatory in presence of medium-Z radiators ELMs are not only harmful, but also remove impurities from the plasma preventing impurity accumulation and radiation collapse. In presence of medium-Z radiators the ELM pacing is mandatory. Failure of two consecutive ELMs (pellets) are already problematic in scenarios with medium-Z radiators A. Kallenbach, Ringberg seminar,2006

  6. Solving the ELM problem: mitigation by frequency enhancement It was recognized that ELM energy loss scales inversely with ELM frequency ►►► envisaged solution for the ELM problem: Mitigate ELMs below the damage threshold by enhancing their frequency by pellet injection Observation: ΔWELM ~ PHEAT f ELM for many scenarios with natural ELMs Approach: Split large ELMs into many small ones A. Herrmann, PPCF 44 (2002) 883

  7. Pellet ELM pacemaking Injection of frequent small and shallow penetrating cryogenic pellets has been found a promising technique to mitigate this effect. The technique works but the underlying physical processes of the ELM triggering are not well understood. Therefore our aim is to study where and how a pellet triggers an ELM to be ableto make predictions for future machines and to optimise the pellet pacing tool.

  8. Poloidal scan of pellet triggering capability

  9. V H L Fueling pellets injected from all locations on JET trigger ELMs P.T. Lang et al., NF 47 (2007) 754 Pellet size: 4mm cube velocity: 150-300m/s Fueling pellets injected from all locations (V,H,L) on JET trigger prompt ELMs detected by Mirnov coils and on divertor Hα radiation

  10. Fueling pellets injected from all locations on DIII-D trigger ELMs Pellets injected from the 5 different injection locations on DIII-D are observed to trigger immediate ELMs. Pellet size: 2.7mm diam Pellet velocity:150-800m/s L. Baylor et al., POP 7 (2000) 1878

  11. Pellets injected from all locations on ASDEX Upgrade trigger ELMs Prompt pellet ELM trigger for HFS injection. ELM event released 20–50μs after ablation onset with a minor part of the pellet mass ablated. HFS: pellet size: 1.4-2.1mm velocity: 240-1000m/s LFS: pellet size: 1-2mm velocity: 100-200m/s P.T. Lang et al., NF 47 (2004) 665

  12. Radial localisation of pellets at ELM trigger

  13. Assumption: to trigger an ELM pellet has to reach a certain magnetic surface (lseed) independently of the pellet mass and velocity Every pellet injected into type-I ELMy H-mode plasma of ASDEX Upgrade triggered an ELM This ELM was observed only if the pellet entered into the confined plasma crossing the separatrix: dtELM ONSET = tELM ONSET – tpellet @ separatrix >0 dtELM ONSET depends on pellet velocities Pellet velocity scan: t0 , lseed can be determined Radial localisation of HFS pellets at ELM trigger instability growth rate pert1 pert2 Pellet path (l) lseed lsep lped PELLET 500 ms separatrix crossing time Ablation monitor Pick-up coil + ELM-delay ELM onset time Perturbation spreads: finally an instability starts to grow which develops into an ELM. B31-14 + ELM (type-I) dtELM ONSET =(lseed – lseparatrix) / VP+t0

  14. Radial localisation of HFS pellets at ELM trigger dtELM ONSET =(lseed – lseparatrix) / VP+t0 t0 = 50 ± 7 μs lseed = 2.7 ± 0.4 cm Consistently with peeling-ballooning model of the ELM predicting instability onset localized to the pedestal steep gradient region Only 5-15% of the expected pellet mass is ablated until the position of the seed perturbation Still question: smaller pellets still reaching the same location of pedestal gradient region can trigger an ELM or not (according perturbation reduces with pellet size) pedestal G. Kocsis et al., NF 47 (2007) 1166

  15. HFS Pellet Induced ELM Details from DIII-D ELM Triggered ELM End Pellet Da represents ablation of pellet in the plasma with assumed constant radial speed. DIII-D 129195 1.8mm pellet injected from inner wall is ~10% density perturbation 0.10 Pellet Enters Plasma 0.08 Pellet Da 0.06 0.04 0.02 0.00 1.9 neL(1014m-2) 1.8 Divertor Da 1.7 1.6 1.5 80 60 dBr/dt (a.u.) Inner wall 40 No MHD precursor to pellet induced ELM 20 0 -20 -40 200 150 dBr/dt (a.u.) Outer shelf 100 50 0 -50 2773.0 2772.2 2772.4 2772.6 2772.8 Time (ms) The ELM is triggered 0.05 ms after the pellet enters the plasma or 147 m/s* 0.05 ms = 7.4 mm penetration depth. L.R. Baylor et al, Fuelling WS, Crete, 2008

  16. Localisation of pellet when ELM is triggered in DIII-D Electron Pressure Pedestal 30 DIII-D 129195 2.772s Pre-pellet and ELM 25 Non-RMP HFS RMP HFS Non-RMP LFS RMP LFS 20 Pe (kPa) 15 10 5 0 0.6 0.7 0.8 0.9 1 1.1 r Pellet Location when ELM Triggered Data from DIII-D indicates that the pellet triggers an ELM before the pellet reaches half way up the pedestal (% Ped is % of Te pedestal height). Note: Pellets must penetrate deeper to trigger an ELM when RMP is applied. L.R. Baylor et al, Fuelling WS, Crete, 2008

  17. Radial location of pellets at trigger on JET Pellet size: 4mm cube velocity: 150-300m/s Intrinsic ELM Triggered ELM For all H track pellets an ELM onset delay of 200 ± 30 μs is derived, related to position of s = 30 ± 4.5mm along the pellet trajectory or r = 23 ± 3.4 mm radilly on the outer mid-plane, respectively. Until the ELM trigger only 1% (or less) of the initial pellet mass is ablated. P.T. Lang et al., NF 47 (2007) 754

  18. Is the ELM triggered at the location of the pellet? Is the perturbation localised polidally/toroidally?

  19. JET fast framing camera observation geometry Inter limiter box Limiter box 1 2 3 4 5 6 7 pellet box LFS pellet injection View: full poloidal cross section covering 90 degrees toidally and seeing the plasma facing components (limiters) on the outboard wall. The cross section of the LFS pellet injection falls into the middle of the view. Time resolution: 10-15μs Kocsis et al, EPS 2009

  20. Filament observation during ELM trigger for LFS pellet injection Inter limiter box Limiter box 1 2 3 4 5 6 7 pellet box 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 LFS Pellet triggered ELM: ♠ field line elongated filament is growing out from the pellet cloud ♠ bright spots on limiter elements: interaction of this filament with the plasma facing components ♠ appearance of this field aligned structure coincides with the MHD ELM onset. ♠ after detachment from the pellet cloud - bright spots in limiter elements move upward - slowed down after 50-100μs: rotates toroidally/poloidally Kocsis et al, EPS 2009

  21. Localisation of the seed perturbation for LFS pellets Natural type-I ELM (100-200kJ) Pellet triggered type-I ELM (100-200kJ) Plasma wall interaction footprint evolution is more regular Plasma wall interaction footprint evolution is irregular No delay relative to magnetic ELM onset Up to 80 μs delay relative to magnetic ELM onset The seed perturbation for the ELM triggering may be the pellet cloud The ELM is probably born at the location of the pellet ablation but this picture should be further refined/confirmed investigating HFS pellet ELM triggering

  22. Pellet – plasma interaction

  23. Pellet-plasma interaction → diamagnetic pellet cloud launches broadband Alfvén waves (vA~5 ·106m/s) that communicated on the whole magnetic surface on a few 10µs -100μs → ionised cloud elongated along field lines expansion vel.: ion sound speed < 105m/s localised in non-axisymetric filament < 10m high pressure → pellet cloud absorbs the incoming energy flux, cooling wave travels with electron thermal speed ~ 107m/s → homogenised on the whole magnetic surface in a few 10µs-100μs Geometry at ASDEX Upgrade: Type-I ELMy H-mode pedestal along the pellet trajectory ~ 6cm Pellet cloud radius perpendicular to the magnetic field ~ 1cm Pellet velocity: 240-1000m/s, pellet mass: 3-9(20) 1019 deuterium → neutral cloud formed on µs timescale → further complicated by grad B caused cloud drift to LFS Pellet ablation in the first 50-100µs: pellet ionised cloud incoming heat flux Incoming spherical neutral cloud

  24. Axisymmetric plasma cooling

  25. Axisymmetric plasma cooling I Pellet trajectory ELM onset vP=240m/s Pellet trajectory ELM onset vP=600m/s Pellet caused local cooling is homogeneously distributed on the magnetic surface in a few 10µs-100μs (fast electron cooling wave travels with electron thermal speed ~107m/s). The local cooling appears on fast ECE electron temperature measurement located toroidally 90o from the location of the pellet injection in a few 10µs. 200µs Pellet trajectory ELM onset vP=1000m/s Kocsis et al, EPS 2008

  26. Axisymmetric plasma cooling II Pellet trajectory • Pellet caused cooling: • ● appears almost immediately after the pellet • reached the according magnetic surface • ● causing remarkable temperature drop on a • short timescale • ● the cooling front moves together with the • pellet for all pellet velocities. ELM onset vP=240m/s 200µs ● pellet plasma cooling lasts until the pellet is completely ablated and the plasma starts to recover but on a ms timescale. ● relative temperature drop is in the range of few 10% seems to depend on the pellet velocity. ● temperature decrease caused by the triggered ELM is slower than direct pellet one, therefore they can be discriminated. Pellet trajectory ELM onset vP=600m/s Kocsis et al, EPS 2008

  27. Pellet caused magnetic perturbation

  28. Pellet caused magnetic perturbation in OH, HD type-III and type-I H-mode 500 ms distance from separatrix separatrix crossing Dt ELM-delay B31-14 (type-I) ELM pellet → but only for long-life pellets • Pellet-driven MHD masked by type-I ELM footprint: • in the early phase of the ablation • (100- 200 μs) the ELM is triggered • the pellet-driven perturbation is small, • masked by the ELM • after the ELM the pellet-driven mode is visible • if the pellet lifetime is long enough Ablation monitor Pick-up coil Therefore: the pellet driven magnetic perturbation was analyzed in different scenarios: OH, HD type-III and type-I H-mode Pick-up coil signals were analyzed: high frequency component: bandpower in 100-300 kHz called ‘envelope’: magnetic perturbation strength spectra and toroidal mode number spectra were studied Szepesi, EPhF 2009

  29. type-I type-III OH ELM pellet Magnetic perturbation strength • → no dependence on pellet mass • → slight/no dependence on pellet speed • → depends on pellet penetration •  BUT only in one scenario •  implies dependence on plasma parameters instead of penetration OH type-III Magnetic perturbation strength [a.u.] type-I Electron pressure [Pa]  ELM- and pellet-related MHD activity can be separated Szepesi, EPhF 2009

  30. Spectral properties of the magnetic perturbation magnetic spectrum coherent mode spectrum Pellet-induced perturbation: pel OH • → mode frequency: 100 - 300 kHz • → tor. mode number: n = -6 • (ion diamagnetic drift direction) • TAE: the same parameters • but: ELMs are different • n = 3, 4 (Neuhauser, NF 2008) TAE oscillation Maraschek, PRL79 pel type-III → in type-I the ELM makes the plasma prone to the n = -6 oscillation, and this is enhanced by the pellet (cooling of plasma edge, emergence of turbulence) → type-III case: more similar to OH → the pellet produces no new phenomena, only enhances what is already present pel type-I Onliving mode n=-6 Washboard n = 3, 4 Szepesi, EPhF 2009

  31. Nonlinear MHD simulation of pellet ELM triggering

  32. Non linear MHD code JOREK JOREK has been developed with the specific aim to simulate ELMs • domain with closed and open field lines • non-linear reduced MHD in toroidal geometry • Density, temperature, electric potential (perp. flow),parallel velocity, poloidal flux • Ideal wall conditions on walls • Mach one, free outflow at divertor target Aim of the pellet related simulations is to answer the following questions: ELM occurs when plasma crosses linear MHD limit after which plasma returns to stable state How can a pellet trigger an ELM in the stable inter-ELM phase? Is pellet a trigger or a cause for an ELM? flux-aligned grid (reduced resolution) Huysmans, EPS 2009

  33. Non linear MHD code JOREK: modelling of pellet ELM triggering Pellets are approximated as an initial perturbation to the density profile: • poloidally and toroidally localised • Amplitude typically 25 times central density • Total amount of added particles 3-6% • constant pressure (i.e. low temperature) Pellet triggered MHD in H-mode pedestal: • Initial non-linear MHD simulations of pellets injected in H-mode pedestal show: • Destabilisation of medium-n ballooning modes • High pressure in pellet plasmoid drives MHD instability forming a single helical perturbation at the pellet position • suggests pellets can cause ELMs (instead of being a trigger) Huysmans, EPS 2009 ~0.5μs x

  34. Summary Millimeter sized fuelling pellets trigger ELMs independently of the poloidal injection angle (LFS, HFS) Pellets are located in the mid-pedestal at ELM trigger, and only a small fraction of their mass is ablated until the trigger, but it is still not known that smaller pellets just reaching the same radial location are still trigger ELMs For LFS pellets the high pressure pellet cloud formed around the pellet seems to be the seed perturbation but for HFS injection the situation is not yet clear The high pressure pellet cloud, the axisymmetric plasma cooling and the magnetic perturbation of the pellet cloud have been recognized as candidate perturbation for trigger mechanism, and were investigated in details Initial nonlinear MHD simulation for pellet ELM triggering shows destabilization of medium-n ballooning modes. High pressure in pellet plasmoid drives MHD instability forming a single helical perturbation at the pellet position for LFS injected pellets, which agrees with the observations on JET

  35. Outlook HFS-LFS asymmetry of the ELM triggering will be further investigated ►► on ASDEX Upgrade with LFS injection (measurement of the internal delay for LFS) ►► on JET with HFS injection (fast visible imaging of filaments) The investigation of pellet caused magnetic perturbation and plasma cooling is an ongoing project on JET, results will be published soon. The simulation should be run to clarify the determining mechanism of the pellet Elm triggering (HFS/LFS) Further investigations would be necessary with reduced pellet size or with small impurity pellets.

  36. Backup transparencies

  37. ne cs L Te qe L Pe L q f What Causes an ELM Trigger from a Pellet? • Pellet cloud releases from pellet and expands along a flux tube. • Density from the cloud expands along flux tube at the sound speed cs. • Temperature ‘cold wave’ travels along the flux tube at the thermal speed. Heat is absorbed in the cloud resulting in a temperature deficit far from the cloud. • Pressure decays and expands along the flux tube with a lower pressure far from the cloud. • Strong local cross field pressure gradients result along the flux tube that form on ms time scales. LRB Crete Jun2008 38

  38. Experimental set-up I on time: 10µs off time: 90µs Ablation monitor signal Cam. exposures time [s] Trajectory recons- truction separatrix Vide angle view photodiode long exposure short multiple exposures exposures Injection geometry tpellet @ separatrix Pellet localization (space, time): - fast CCD cameras + spatial calibration → def: penetration = distance from separatrix

  39. Experimental set-up II Poloidal pick-up coil set, 7 coils about 600 Calibrated Electron Cyclotron Emission (ECE) profiles are measured in second harmonic X-mode with a fast 60-channel heterodyne radiometer Toroidal pick-up coil set about 1800, 5 coils Mirnov coil set 3600, 30 coils PELLET 500 ms Ablation monitor separatrix crossing Pick-up coil signal ELM-delay • Processing of the pickup coil signals • eliminate LF component by moving box average • calculate the envelope of the remaining HF component (25 ms box) • assume: envelope ~ MHD perturbation magnitude B31-14 ELM (type-I)

  40. Experimental set-up III Processing of pick-up coil signals: spectral properties continuous analytical wavelet transform: time shift and scaling invariant short-time Fourier transform (STFT): time shift and frequency shift invariant Mode numbers as least squares fit to cross-phase as function of relative probe position :

  41. Low pellet injection frequency : trigger events occur at different times in the ELM cycle that is we perform the analysis as a function of the time elapsed after the previous ELM Pellets can trigger ELMs at any time in the ELM cycle: plasma edge is not stable against HFS pellet induced seed perturbation on ASDEX Upgrade Radial localisation of pellets at ELM trigger Radial localisation of HFS pellets at ELM trigger dtELM_ONSET saturates with increasing dtelapsed dtelapsed > 8ms: dt ELM_ONSET ~ constant G. Kocsis et al., NF 47 (2007) 1166

  42. Estimation of the pellet cloud pressure and plasma pressure drop Pellet-plasma interaction Pellet affects the plasma at a magnetic surface as long as t=DiamCLOUD /vP =20-100µs → the incoming energy flux is absorbed and concentrated in the HFS localised helical non axisymmetric cloud causing a cloud pressure higher then that of the target plasma rough estimate: >10 Pe → axisymmetric decrease of the plasma pressure causing a pressure gradient increase in front of the pellet rough estimate: 10-30% for AUG

  43. Injection scenarios Measurement of the ELM onset delay as a function of the pellet velocity → VP=240,600,880,1000 m/s HFS looping system: pellet erosion is velocity dependent → rP=0.71, 0.67, 0.58, 0.51 mm NP=9 , 7, 5, 3 x 1019 Characterization of the injection scenarios → Neutral Gas Shielding model calculation Designated pellet path dNP /dt = - Const. rP4/3 n e1/3 Te1.64 dNP /dl VP = - Const. rP4/3 n e1/3 Te1.64 P.B. Parks et al., PoP 21 (1978) 1735 Integrating this diff. equation along the designated pellet path the ablation rate (ablated particles /s) is calculated

  44. Injection scenarios 240 m/s, r=0.71mm, N=9 ·1019 600 m/s, r=0.67mm, N=7.4 880 m/s, r=0.58mm, N=5.e19 1000m/s,r=0.51mm, N=3.3e19 In the pedestal region the ablation rate is nearly similar for all injection scenarios Pellets penetrate deep into the plasma crossing the pedestal region completely Particle deposition scales with 1/VP

  45. Type-I ELMy H-mode el. diam. Drift, n>0 ion diam. Drift n<0 delayed triggered Prompt triggered 15 pellet induced ELMs have been analysed for their basic poloidal/toroidal structure. The single, upward rotating structure represents a rather specific case. Typically, two or more such pronounced structures occur simultaneously. Despite the fixed pellet launch position, they appear initially at a more or less random toroidal position relative to the pellet, i.e. they are not directly growing out of the pellet plasmoid. washboard-type precursor mode slowing down, n=4 No precursor Neuhauser et al, NF, 48, 045005, 2008

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