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Status of ELM trigger investigations on JET and AUG R. Wenninger IPP Garching, EFDA JET

Status of ELM trigger investigations on JET and AUG R. Wenninger IPP Garching, EFDA JET. Outline. Pellet technology at AUG and JET Pellets – a candidate ELM mitigation method Observation of pellet triggered ELMs in H-mode regimes Direct pellet driven MHD

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Status of ELM trigger investigations on JET and AUG R. Wenninger IPP Garching, EFDA JET

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  1. Status of ELM trigger investigations on JET and AUG R. Wenninger IPP Garching, EFDA JET

  2. Outline • Pellet technology at AUG and JET • Pellets – a candidate ELM mitigation method • Observation of pellet triggered ELMs in H-mode regimes • Direct pellet driven MHD • Careful considerations towards trigger mechanism • Pellet impact on tokamak (ITER) operation • Summary

  3. Outline • Pellet technology at AUG and JET • Pellets – a candidate ELM mitigation method • Observation of pellet triggered ELMs in H-mode regimes • Direct pellet driven MHD • Careful considerations towards trigger mechanism • Pellet impact on tokamak (ITER) operation • Summary

  4. AUG Pellet Centrifuge AUG fuelling system Volume to freeze and compress ice Extrusion nozzel Extrusion arm (s=0.25mm) Cutter VHFS

  5. AUG Blower Gun AUG pacing system • High frequency: Up to 143Hz • 2nd injection line for tangential transfer through edge plasma close in or outside separatrix

  6. AUG Injection Capabilities

  7. JET – High Frequency Pellet Injector High Frequency Pellet Injector (HFPI): • Worldwide 1st system fully optimised for ELM pacing • Deep fuelling also possible PELIN pellet injector

  8. V H L JET – Overall Pellet system • Injection from 3 poloidal locations • Centrifuge – Future option for parallel fuelling and pacing Pellet Injection Locations

  9. JET – HFPI Injection capabilities Values from HFPI specification

  10. Outline • Pellet technology at AUG and JET • Pellets – a candidate ELM mitigation method • Observation of pellet triggered ELMs in H-mode regimes • Direct pellet driven MHD • Careful considerations towards trigger mechanism • Pellet impact on tokamak (ITER) operation • Summary

  11. Reliable ELM control technology mandatory for ITER ELM control – a necessary requirement • Acceptable ELM size for ITER: • Target plate erosion negligible at 0.5MJ/m2 for CFC and W (Zhitlukhin 2007) • Assume strong asymmetry of deposition on inner/outer targets: Pout/Pin = 1 : 2 • Tolerable energy deposition on target plates per ELM: WELM,target=0.5MJ/m2  1.3m2  (1+1/2)  1MJ (Polevoi EPS 2008) • WELM,target fELM =   Psep,  = 0.2 – 0.4(Herrmann 2002) •  fELM = 20 – 40Hz (spont. ~ 2 - 4Hz) F. Federici et al PPCF 45 (2003)

  12. ELM control technologies • Edge ergodisation by resonant magnetic perturbation (Y. Liang PRL 98 (2007))  Plasma edge rarefaction without cooling (needs pellet fuelling) • Magnetic ELM pacing by vertical kicks – accelerating plasma in vertical direction (Sartori et al.) • Impurity seeding  Type III (higher frequency) + higher radiation fraction • Pellet ELM Pacing (P. Lang NF 2004) • … DIIID None of these technologies is yet proven to work at ITER

  13. Pellet ELM Pacing – Proof of Principle at AUG AUG JET • fELM≥ fPEL  fELM more than doubled at AUG • WELMfELM=const at const. Pheat confirmed for pellet triggered ELMs (Lang NF 2002)

  14. Scaling aspects: size, magnitude, location of required perturbation? • Local perturbation imposed by pellet particle deposition is strong enough for triggering ELMs at AUG and JET (Until now every pellet injected into ELMy regime triggered an ELM) • But does this still hold at ITER size? • More physics understanding necessary! • Threshold might be defined by • local  (e.g =T, n, p, j,...) and • relative extension (e.g. x/R) x Readjustment might be possible but at the expense of again stronger fuelling (and pumping) and hence convective losses.

  15. Outline • Pellet technology at AUG and JET • Pellets – a candidate ELM mitigation method • Observation of pellet triggered ELMs in H-mode regimes • Direct pellet driven MHD • Careful considerations towards trigger mechanism • Pellet impact on tokamak (ITER) operation • Summary

  16. Pellet can trigger an ELM at any time between Type-I ELMs • ELMs triggered by with feff up to 350Hz (temporal resolution problem in higher frequency) • Triggered ELM:  50s delay between pellet causes perturbation and ELM AUG G. Kocsis

  17. Quiescent H-Mode: Pellets don’t trigger ELMs in any H-mode regime AUG • QH: Obtained by counter injection • Good confinement (H98y~1) • High pedestal and core ion temp. • ELMs replaced by ‘edge harmonic oscillation’ (EHO, ~10kHz) + ‘high frequency oscillation’ (300 – 400kHz) • Even large pellets do not trigger ELMs

  18. Pellet can terminate phase free of spontaneous ELMS • Pellets trigger first ELM ~0.5s earlier than first spont. ELM occurs in reference shot • Avoid spontaneous Giant ELM (e.g. 1st after ELM free phase) • Each pellet triggered an ELM of smaller size, with <20% the loss in energy, than the spontaneous Giant ELM JET 14MW NBI

  19. Pellets lead to fastest ELM-growth (I) AUG Comparison of growth time of MHD signal up to its max. value (ELM rise time): spontaneous Type I  pellet driven between Type I   pellet driven between Type III < spontaneous Type III

  20. Pellets lead to fastest ELM-growth (II) AUG Type III Rad. cooled Type I Type I • ELM rise time of pellet driven ELMs  const. ( spont. Type I: fastest growth) • ELM rise time increases from Type I, via rad. cooled Type I to Type III • Correlated e.g. with parallel resistivity at pedestal top

  21. Outline • Pellet technology at AUG and JET • Pellets – a candidate ELM mitigation method • Observation of pellet triggered ELMs in H-mode regimes • Direct pellet driven MHD • Careful considerations towards trigger mechanism • Pellet impact on tokamak (ITER) operation • Summary

  22. Option 2: Direct pellet driven MHD Perturbation of plasma parameters (n, T, j,...) ELM Pellet Simplistic causal model Option 1: Perturbation of plasma parameters (n, T, j,...) Direct pellet driven MHD ELM Pellet

  23. Magnitude of required ELM trigger perturbation: Tiny in pellet terms? Magnetic signal for pellet driven ELMs can be separated in • ELM related MHD • Directly pellet driven part • observed even in L-mode • stops abrupt with burn out • @ trigger time below resolution • Direct pellet driven MHD sufficient, if threshold would be > x100, if option 1 JET

  24. ≈ 50 µs OH: Direct pellet driven MHD only • Ohmic plasma (OH)  Only direct pellet driven component AUG • Faintest pellet provokes stronger MHD than at typical ELM onset

  25. OH: No sign. Variation of directly pellet driven MHD with pellet parameters • Repeated use of same stationary scenario • Averaging of amplitude of dB/dt (MHD) over entireshot for all pellets (about 10) AUG • MHD clearly correlated to radial position of pellet and thus on local plasma parameters (e.g. p, j, Te, …) • No significant dependence on pellet parameters (mass, velocity) and thus on ablation / deposition  saturation effect? • Pellet driven MHD depends mainly on plasma parameters

  26. Outline • Pellet technology at AUG and JET • Pellets – a candidate ELM mitigation method • Observation of pellet triggered ELMs in H-mode regimes • Direct pellet driven MHD • Careful considerations towards trigger mechanism • Pellet impact on tokamak (ITER) operation • Summary

  27. Spont. type I and triggered ELMs – No significant difference on level of density fluctuation • Reflectometry at fixed frequency mode  density fluctuation • Compare frequency power spectra (Integration: 2.5ms) for • 3 different densities • LFS and HFS • Spontaneous and triggered ELMS •  No significant difference between spontaneous type I and triggered ELMs AUG

  28. Spont. type I and triggered ELMs – No significant difference in target power load pattern AUG • Infrared Thermography: • Observation of type I ELMs reveals non-axissymmetric stripes on divertor targets  mapped to filaments (Eich PRL 2003) • For later ELM phase mode structure can be identified •  No significant difference in patterns between spontaneous type I and triggered ELM

  29. J a Pellets in the Peeling-Ballooning-Picture • PB-Theory quantifies ELM dynamics via growth Rate  of driving mode • Type I ELM cycle (Connor 1998) • Type I ELMs: • Typical av. inter ELM time > ms • Pellet triggered ELMs: • t(pellet at location – ELM) < 0.1ms •  If there is a similarity in the mechanism, pellet must shortcut the type I ELM cycle significantly Instable >C Resistive time scale Stable <C Transport time scale

  30. Outline • Pellet technology at AUG and JET • Pellets – a candidate ELM mitigation method • Observation of pellet triggered ELMs in H-mode regimes • Direct pellet driven MHD • Careful considerations towards trigger mechanism • Pellet impact on tokamak (ITER) operation • Summary

  31. Pellet impact on tokamak (ITER) operation – Penetration depth and required pellet mass • Cumulative distribution function of trigger locations ~100% at ped. top • Assumption: Pellet penetration to pedestal top is required • Pellet mass required to reach pedestal top for different scenarios: • Reference: width 20cm, Te at top 4keV • Wide pedestal: width 30cm, Te at top 4keV • High pedestal: width 20cm, Te at top 5keV • Lower speed  More mass required but stronger perturbation • Assumption: Ref. scenario Kocsis 2007 ITER K.Gál (Hybrid-LLL-Code)

  32. X ? ITER total pumping rate: 40 – 50  1021 D/s Results from Hybrid-LLL code Assuming 40Hz pellet frequency Pellet impact on tokamak (ITER) operation – 3 injection scenarios

  33. Pellet impact on tokamak (ITER) operation – Convective power losses due to pellets • Pellet particles are heated by the plasma up to  75% Tped,ref • Steady state cond.: Pellet = add. loss • Padd. loss= 3 Pellet kB <T> • Assuming <T>=3keV (75% Tped,ref) • Padd. loss  200MW (P) • 60MW (OS) • 6MW (AO) ITER total heating power: 40MW • Conclusion: • Self-consistent modeling of the localized particle deposition and enhanced transport required for more robust figures • At least OS is needed – better AO

  34. A new idea – Beryllium pellet injection • Advanced Optimistic: 1000m/s of 1020D (1.6mm3) extremely challenging • High speed should be easier for Be pellet (melting point 1278°C, crystal structure) • Simulation with a C pellet (K.Gál) indicate 1 * 1019 Be (a Ø 500μm sphere) would be sufficient to reach pedestal top • Fuel dilution: Taking a plasma particle content of 1023e, P=1s, finjection=40Hz : • ΓP ≈ 4 * 1020/s Be, but ≈ 4 * 1021/s expected from wall • Need to demonstrate: • - ELM triggering by a Be pellet • - Pellet transfer through a tube ITPA Activity

  35. Summary • JET-HFPI: Milestone in development of pellet injector technology • Pellet ELM Pacing: Candidate technique for ITER ELM control • So far every pellet injected into ELMy regime triggered an ELM – not clear, if this holds for ITER • Pellet can trigger an ELM at any time between Type-I ELMs • Pellet triggered ELMs grow as fast as the fastest spontaneous ELMs (Type I) • Pellet driven MHD depends mainly on plasma parameters • If there is a similarity in the mechanism, pellet must shortcut the type I ELM cycle significantly • 500 or 1000m/s injection mandatory for tolerable pump load and additional power losses in ITER

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