R. Wenninger 1 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 Status of ELM trigger investigations on JET and AUG R. Wenninger IPP Garching, EFDA JET

R. Wenninger 2 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 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

R. Wenninger 3 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 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

R. Wenninger 4 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 AUG Pellet Centrifuge AUG fuelling system Volume to freeze and compress ice Extrusion nozzel Extrusion arm (  s=0.25mm) Cutter VHFS

R. Wenninger 5 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 AUG Blower Gun High frequency: Up to 143Hz 2 nd injection line for tangential transfer through edge plasma close in or outside separatrix AUG pacing system

R. Wenninger 6 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 AUG Injection Capabilities CentrifugeBlower Gun SpeciesH 2, D 2, dopedD2D2 FrequencyUp to 83HzUp to 143Hz Pellet speed m/s (limited by looping to 1000m/s) 100 – 350m/s Pellet size/mass (D 2 ) ~ 2.8 – 7.2 mm 3 ( atoms) ~ 1.7 mm 3 (10 20 atoms) Reliability  95% Injection lineVHFSLFS (Invest. line)

R. Wenninger 7 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 JET – High Frequency Pellet Injector High Frequency Pellet Injector (HFPI): Worldwide 1 st system fully optimised for ELM pacing Deep fuelling also possible PELIN pellet injector

R. Wenninger 8 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 JET – Overall Pellet system Pellet Injection Locations Injection from 3 poloidal locations Centrifuge – Future option for parallel fuelling and pacing V L H

R. Wenninger 9 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 FuellingELM pacing SpeciesH 2, D 2 Frequency≤15 Hz≤60 Hz Pellet speed m/s m/s Pellet size/mass (D 2 ) ~ 64 mm 3 ( atoms ) ~ 1.5 mm 3 ( atoms ) Reliability  98% JET – HFPI Injection capabilities Values from HFPI specification

R. Wenninger 10 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 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

R. Wenninger 11 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 ELM control – a necessary requirement Acceptable ELM size for ITER: Target plate erosion negligible at 0.5MJ/m 2 for CFC and W (Zhitlukhin 2007) Assume strong asymmetry of deposition on inner/outer targets: P out /P in = 1 : 2 Tolerable energy deposition on target plates per ELM:  W ELM,target =0.5MJ/m 2  1.3m 2  (1+1/2)  1MJ (Polevoi EPS 2008) F. Federici et al PPCF 45 (2003) Reliable ELM control technology mandatory for ITER  W ELM,target  f ELM =   P sep,  = 0.2 – 0.4 (Herrmann 2002)  f ELM = 20 – 40Hz (spont. ~ 2 - 4Hz)

R. Wenninger 12 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 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

R. Wenninger 13 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 Pellet ELM Pacing – Proof of Principle at AUG AUG JET f ELM ≥ f PEL  f ELM more than doubled at AUG  W ELM  f ELM =const at const. P heat confirmed for pellet triggered ELMs (Lang NF 2002)

R. Wenninger 14 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 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) Readjustment might be possible but at the expense of again stronger fuelling (and pumping) and hence convective losses. x

R. Wenninger 15 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 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

R. Wenninger 16 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 Pellet can trigger an ELM at any time between Type-I ELMs AUG ELMs triggered by with f eff up to 350Hz (temporal resolution problem in higher frequency) Triggered ELM:  50  s delay between pellet causes perturbation and ELM G. Kocsis

R. Wenninger 17 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 Quiescent H-Mode: Pellets don’t trigger ELMs in any H-mode regime 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 AUG

R. Wenninger 18 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 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. 1 st 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

R. Wenninger 19 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 Pellets lead to fastest ELM-growth (I) 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 AUG

R. Wenninger 20 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 Pellets lead to fastest ELM-growth (II) Type I Rad. cooled Type I Type III 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 AUG

R. Wenninger 21 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 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

R. Wenninger 22 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 Simplistic causal model Pellet Perturbation of plasma parameters (n, T, j,...) Direct pellet driven MHD ELM Option 1: Option 2: Pellet Perturbation of plasma parameters (n, T, j,...) Direct pellet driven MHD ELM

R. Wenninger 23 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 Magnitude of required ELM trigger perturbation: Tiny in pellet terms? JET 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

R. Wenninger 24 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 OH: Direct pellet driven MHD only ≈ 50 µs AUG Ohmic plasma (OH)  Only direct pellet driven component Faintest pellet provokes stronger MHD than at typical ELM onset

R. Wenninger 25 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 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 entire shot for all pellets (about 10) MHD clearly correlated to radial position of pellet and thus on local plasma parameters (e.g.  p, j, T e, …) No significant dependence on pellet parameters (mass, velocity) and thus on ablation / deposition  saturation effect? Pellet driven MHD depends mainly on plasma parameters AUG

R. Wenninger 26 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 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

R. Wenninger 27 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 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

R. Wenninger 28 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 Spont. type I and triggered ELMs – No significant difference in target power load pattern 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 AUG

R. Wenninger 29 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 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  J Stable  <  C Instable  >  C Transport time scale Resistive time scale

R. Wenninger 30 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 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

R. Wenninger 31 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 Pellet impact on tokamak (ITER) operation – Penetration depth and required pellet mass Kocsis 2007 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, T e at top 4keV Wide pedestal:width 30cm, T e at top 4keV High pedestal:width 20cm, T e at top 5keV Lower speed  More mass required but stronger perturbation Assumption: Ref. scenario ITER K.Gál (Hybrid-LLL-Code)

R. Wenninger 32 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 Pellet impact on tokamak (ITER) operation – 3 injection scenarios Optionv (m/s)FeasibilityPerturbation Min. pellet mass Additional fuelling rate Pessimistic (P) 100DemonstratedMax 35  D140  D/s Optimistic Standard (OS) 500 JET – HFPI at lower mass Mid 10  D40  D/s Optimistic Advanced (OA) 1000 To be demonstrated – straight injection line! Min 1  D4  D/s Results from Hybrid-LLL code Assuming 40Hz pellet frequency X ? ITER total pumping rate: 40 – 50  D/s

R. Wenninger 33 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 Pellet impact on tokamak (ITER) operation – Convective power losses due to pellets Pellet particles are heated by the plasma up to  75% T ped,ref Steady state cond.:  Pellet =  add. loss  P add. loss = 3  Pellet k B Assuming =3keV (75% T ped,ref ) P add. loss  200MW (P) 60MW (OS) 6MW (AO) ITER total heating power: 40MW Conclusion: 1.Self-consistent modeling of the localized particle deposition and enhanced transport required for more robust figures 2.At least OS is needed – better AO

R. Wenninger 34 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 A new idea – Beryllium pellet injection Advanced Optimistic: 1000m/s of D (1.6mm 3 ) 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 * Be (a Ø 500μm sphere) would be sufficient to reach pedestal top Fuel dilution: Taking a plasma particle content of e,  P =1s, f injection =40Hz :  Γ P ≈ 4 * /s Be, but ≈ 4 * /s expected from wall Need to demonstrate: - ELM triggering by a Be pellet - Pellet transfer through a tube ITPA Activity

R. Wenninger 35 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 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

R. Wenninger 36 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 …

R. Wenninger 37 (35) Sat. Workshop to EPS on Fuelling 16 th June 2008 …