14 th Pedestal and Edge Physics ITPA Topical Group Meeting

1. 14th Pedestal and Edge Physics ITPA Topical Group Meeting P.T. Lang et al (V1.5.) ● ELM control: AUG & JET status ●Can pacing like in AUG be maintained in an ITER like parameter regime at JET? ●From JET to ITER: how to scale up the required local perturbation? ● ELM features and directly pellet driven MHD perturbation: is there ample of headroom? ●Three different scenarios ●A silly idea? ●Summary & Outlook Status of the pellet ELM triggering investigations at JET and ASDEX Upgrade and possible conclusions for ITER AUG

2. Full control for fPel > 1.5 x f0, raising fELM reduces WELM ELM control status: pacing demonstrated at AUG Triggered and spontaneous type-I ELMs are essentially identical if they have the same frequency AUG

3. ELM control status: pacing demonstrated at AUG Pellet pacing can break the intrinsic relation between edge parameters (n,T,ν*,…) and fELM  fELM and hence PELM become control parameter AUG

4. ELM control status: triggering demonstrated at JET JET Pellet triggers ELM also already by local perturbation  pacing will work at JET as well

5. ELM control status: AUG – JET – ITER step ladder Pellet pacing works at AUG up to about fPel/f0 = 2 and will also work at JET, but will it still hold on at - ITER parameters (fPel/f0 = 10, ν*Ped,… )? - ITER size (“size dilution of perturbation”)? The first task will be tackled in the “Pellet ELM pace making” and “Pellet ELM pacing scenario integration” experiments this year in JET, the second is subject to several dedicated experiments and analyses but should be investigated by theory and modeling (XGCO?) as well!

6. Pelin Injector (Tore Supra design) Microwave cavities Valves V Collector Tracks H Main support frame L Selectors “ITER-scenario” at JET:the new HFPI & 3 launch sites JET Fuelling: 64 mm3 pellets at up to 15 Hz Pacing: 1.5 mm3 (~ 1020 D) at up to 60 Hz

7. When doing pacing at JET, due to the macroscopic pellet size it will still come with noticeable fuelling… ~ 5 x 1021 D/s could cause Δne/ne~ 0.1 Pacing at JET:still some convective losses expected AUG AUG JET …and probably with some confinement loss as well

8. Pacing at ITER:convective losses to be expected? AUG: mm pellet size and high rates required for pacing do cause strong fuelling and hence confinement reduction (20 - 25 %) JET: same pellet parameters are expected to have only a very modest impact on fuelling and confinement (2 – 10 %) but already at ITER like fP/f0! ITER: same pellet size but lower required rates would have no significant fuelling and confinement impact ( < 1 %) But the question is: can such pellets still trigger ELMs?

9. Scaling aspects: size, magnitude, location of required perturbation? Local perturbation imposed by pellet particle deposition is strong enough for triggering at AUG and JET, but does this still hold at ITER size? Perturbation amplitude is determined by local parameters, reduction with plasma size below a critical level? Readjustment might be possible but at the expense of again stronger fuelling (and pumping) and hence convective losses.

10. Scaling aspects: size, magnitude, location of required perturbation? There are some indications that perturbations created by the pellets used at AUG and JET are far stronger than required for just “get off triggered” an ELM - Trigger at any time within the natural ELM cycle “Explosive” growths following pellet trigger “Saturated” MHD perturbation monitor Our hypothesis: ELMy H-mode edge always non-linear unstable with mm size pellets creating abundant perturbation magnitude to kick local conditions into the explosive growth regime

11. Trigger at any time within the natural ELM cycle “Perturbative” ELM triggering: “scan” elapsed time to previous spontaneous ELM AUG AUG

12. Triggered ELMs remain fast even in regimes the spontaneous ones do not Triggered ELM: “explosive” growth kept over different ELM regimes Type-III Type-I AUG

13. Triggered ELM stay “explosive”: radiative edge cooled type-I Hot edge type-I: triggered/spont. ELMs virtually identical Cooled edge: triggered keeps characteristics of its regime, but keeps fast hot-edge type-I MHD onset AUG

14. Taking the MHD monitor as perturbation monitor: does it make sense? “Saturated” perturbation?: pellet and ELM driven MHD response ≈ 50 µs AUG Indeed it is found the pellet creates “direct” MHD activity (declining rapidly after the pellet has burn out) stronger that those at the onset of an ELM: the method seems to be at least plausible

15. Pellets in OH:what drives the MHD ? Variation of pellet mass and/or velocity – changes magnitude of MHD amplitude… AUG

16. Pellets in OH:what drives the MHD ? … but neither pellet ablation nor deposition correlate with the MHD amplitude. → Local pressure perturbation seems to play (no more) role AUG

17. Pellets in OH: impact on MHD already “saturated” The only correlation is found between penetration depth and MHD amplitude. → no more effect of stronger pellet perturbation, reached “saturation” level AUG

18. Pellets in OH:correlated MHD activity AUG Coherent MHD activity (drift Alfvén turbulence driven TAE, p≈ 0.8) in ohmic phase, enhanced by a factor > 100 reaching ELM onset level during pellet

19. Magnitude of required ELM trigger perturbation: tiny in pellet terms? JET The large, slow pellet “survives” the ELM it has triggered and the directly driven MHD becomes visible

20. Magnitude of required ELM trigger perturbation: tiny in pellet terms? Directly driven MHD? -vanishes with pellet burn out. -when the ELM is triggered drops below the resolution. Remind: -pellet changes the magnitude of some local parameters -compare to e.g. Δx/x << 1 from fluctuations triggering spontaneous ELM JET

21. Required ELM trigger perturbation: possible conclusions for ITER Assumption: the magnitude of pellet imposed perturbation required to provide a suitable ELM seed structure is quite massive Consequence: ELM pacing at ITER would be feasible indeed almost without any adverse effects Detailed investigations at JET can confirm or challenge this assumption further (the final answer has to wait anyway for ITER), but for the moment we can regard it as one (plausible?) option. For operational safety reasons, we should not only rely on.

22. Considering 3 different scenarios: required pellet penetration Gas jet: close to and slightly beyond LCFS not sufficient Pellet: mid-pedestal to top-pedestal, arrives at 100% trigger fraction around pedestal top → Assume penetration to pedestal top is required for any scenario

23. Considering 3 different scenarios: required pellet mass Using Hybrid-LLL-Code (K.Gál), take ITER profiles and pellet velocity; "Tailor" pellet mass to achieve pedestal top penetration ITER

24. Considering 3 different scenarios: max. safety to max. performance Pellet velocity (and path) to optimize 100 m/s: this represents the slowest technical feasible injection speed, imposing the maximum possible perturbation and hence the safest option for ELM triggering. This option is considered as pessimistic suitable. “P” 500 m/s: assuming smaller perturbations can trigger ELMs, this option stands for the best one available with the currently planned pellet system. Therefore, consider it as the optimistic standard one. “OS” 1000 m/s: assuming smaller perturbations can trigger ELMs, this option stands for the best one available with an upgraded pellet system allowing for straight injection (no significant bend in the guiding tube) at the horizontal mid plane from the LFS. Therefore, this 1000 m/s case can be considered as the optimistic advanced one. “OA”

25. Considering 3 different scenarios: pellet particle fluxes Taking the (straight LFS injection: adapted!) values from the code runs: mP→ Pessimistic: 35 * 1020 D Optimistic Standard: 10 * 1020 D Optimistic Advanced: 1 * 1020 D Taking a 40 Hz pellet rate (A. Loarte: WELM < 1 MJ) ΓP → Pessimistic: 140 * 1021 D/s Optimistic Standard: 40 * 1021 D/s Optimistic Advanced: 4 * 1021 D/s This was the required input for a pumping system assessment, but what is of coarse also of great interest is …

26. Considering 3 different scenarios: loss power due to convective losses … is the impact of this particle loss on the confinement. In steady state conditions ΓP = Γloss the pellet fuelling (and pacing!) operational area boundary can be explained by taking into account the convective heat flux carried by the extra pellet particle flux Ti = Te→ Ploss = 3 ΓP kB <T> JET AUG

27. Considering 3 different scenarios: loss power due to convective losses Assuming <T> = 3 keV one gets (of course not self consistently) Ploss≈ Pessimistic: 200 MW Optimistic Standard: 60 MW Optimistic Advanced: 6 MW showing immediately how severe the impact on confinement could be. A more profound statement requires a decent self-consistent modeling of the localized particle deposition and enhanced transport (forced losses!). But it is already evident from such a (stupid) estimate that we need at least optimism, even better advanced optimism, or a silly idea…

28. Considering scenario 4: Be pellet injection • The “Optimistic Advanced” scenario has two shortfalls: • new pellet track required • high launch speed might be troublesome • Such a high speed should be easier achievable by a solid state pellet, and (higher evaporation energy) a much smaller pellet size is sufficient as well. • Simulation with a C pellet (K.Gál has code available) indicate 1 * 1019 Be (a Ø 500 μm sphere) would be sufficient. • Now of course the fuel dilution is the main concern…

29. Considering scenario 4: Be pellet injection Fuel dilution as the main concern: ΓP ≈ 4 * 1020/s Be, but ≈ 4 * 1021/s expected from wall Taking a plasma particle content of 1 * 1023 e, 1 s confinement time (edge & ELM), 40 Hz rate, this causes a ΔZeff≈ 0.01 The approach would further need the demonstration of - ELM triggering by a Be (C) pellet - Pellet transfer through a tube Thinking of supersonic gas jet or LBO instead? Does not trigger prompt ELMs…

30. Supersonic gas jet, LBO: no local/prompt trigger Insufficient penetration and/or too dispersed perturbation: only trigger via change of global plasma parameters (no sense) AUG JET

31. Summary & Outlook ● ELM pacing demonstrated at AUG, local triggering at JET ● Any pellet in ELMy H-mode triggers instantly with fast growth ●Pellets create a perturbation visible on MHD monitors stronger than according one present at ELM onset ●In pellet parameter regime applied the MHD perturbation is saturated, no correlation with local pressure change ●Indication for “massive oversized” pellet perturbation with respect to trigger requirements, optimization headroom left → Demonstrate ELM pacing in ITER parameter regime →Allow scaling up by physics insight →Investigate technical headroom left (“aggressive” pellets)

32. Pellet triggered ELMs:review of type-I results Every pellet triggers an ELM (at any time within the natural ELM cycle!), but we could not find striking differences between triggered and spontaneous ELMs, neither with respect to temporal (fast onset) nor spatial (statistical onset position) structure, onset within ≈ 50 µs after imposed perturbation inside ETB, onset visible at any poloidal and toroidal location within ≈ 20 µs. [G. Kocsis et al., Nucl. Fusion 47 (2007) 1166] Spontaneous Pellet

33. Pellet triggered ELMs: edge radiative type-I A.Kallenbach et al., J. Nuc. Materials Vol. 337-339, 732-736 During control phase here fP = 44 Hz, fELM≈ 80 Hz mixture of spontaneous and triggered type-I ELMs

34. ELMs features: comparison hot/type-I to cold/type-III onset Triggering recovers the prompt / fast onset of type-I again! Triggering from "quiet" phase hard to find, at high fELM impossible, but few clear phases show rapid onset like for type-I ELMs.

35. Beyond baseline scenarios: pellets in quiescent H-mode plasmas Pellets to increase density in QH mode [W. Suttrop et al., NF 45, 721] Pellet in QH: burst like MHD of small size, no significant out flux of energy (small fuelling induced effects) Spontaneous type-I ELM: clear burst like events in MHD and out flux of energy

36. Trigger potential in H-regime:results Every pellet launched into a type-III, ELM-free and type-I plasma triggers a prompt ELM at any time in the natural ELM cycle -> edge is never stable against strong seed perturbations The growth time of the triggered ELM is always as fast as that of a hot edge type-I ELM, the fastest onset time observed anyway -> triggered and spontaneous type-I ELMs evolve with the fastest observed onset times anyway, presumable in the "explosive non-linear" regime characterizing the natural evolution in a "deeply unstable" regime or the forced evolution with a strong seed perturbation kicking the edge locally in such a regime. QH (and RMP) plasmas with H-mode confinement seem to be in an edge stable (or saturated instable) regime, even a hard external perturbation cannot cause further mode activity (beyond the transient directly driven one). Pellets can probe edge stability (but careful, they can also change it!) Pacing requires fast small pellets, just reaching the pedestal top.

37. Features of MHD:trigger of n=3, m=2 NTM H. Zohm et al., Nucl. Fusion 41, 197 Pellet fuelling creates global trigger conditions (ρ*, ν*) and then triggers NTM => Almost perfect method to map out the onset boundary

38. Features of MHD:trigger of NTM Pellet with low frequency onset and "lethargic" phase. Pellet created seed island with low rotation in lab frame Unlikely pellet or plasmoid hits q=1,5 surface Mode grows several 10 ms on transport time scale NTM triggered, but local and prompt triggering cannot be claimed

39. Features of MHD:pellet trigger of n=m=1 snake AUG

40. Features of MHD:trigger of n=m=1 snake JET J.A. Wesson., PPCF37, A337 Again growth on time scaleof several 10 ms, so prompt local seed perturbation cannot be claimed

41. Summary of pellet trigger features:ELM specific and general Local pellet impact creates saturated seed perturbation, then growing at maximum speed even under conditions (type-III, radiation) spontaneous ELMs show significantly slower growth, provided edge can be destabilized - not e.g. in QH and RMP phases. Triggering of MHD instabilities seems to be common feature, but this cannot be claimed for the initial local perturbation.