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ELM Pacing with Vertical Kicks for Access to Stationary H-mode with H98~1 on JET-E

This study investigates the effect of ELM pacing using vertical kicks on accessing stationary H-mode with an H98~1 on JET-E, focusing on plasma performance and the triggering mechanism. The results show that vertical kicks can reliably trigger ELMs at input power marginally above Pth, leading to stationary H-mode with H98~1.

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ELM Pacing with Vertical Kicks for Access to Stationary H-mode with H98~1 on JET-E

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  1. The effect of ELM pacing by vertical kicks on the access to stationary H-mode with H98~1 on JET E. de la Luna 24th IAEA Fusion Energy Conference, 8-13 October, San Diego, USA

  2. Contributors E. de la Luna1,2, R. Sartori3, G. Saibene3, P. Lomas4, F. Koechl5, F. Maviglia6, S. Saarelma4, V. Parail4, R. Albanese6, R. Ambrosino6, M. N. A. Beurskens4, E. Delabie7, D. Dodt8, J. Flanagan4, C. Giroud4, A. Loarte9, C. Maggi8, M. Mattei6, F. Rimini4 and JET-EFDA contributors* 1) EFDA-CSU, Culham Science Centre, Abingdon, OX14 3DB, UK 2) LaboratorioNacional de Fusion, Asociacion EURATOM-CIEMAT, Madrid, Spain 3) Fusion for Energy, Joint Undertaking, 08019 Barcelona, Spain 4) EURATOM/CCFE Fusion Association, Culham Science Centre, OX14 3DB, U.K. 5)Association EURATOM-ÖAW/ATI, Atominstitut, TU Wien, 1020 Vienna, Austria 6) Assoziacione EURATOM-ENEA sullaFusione, Consorzio CREATE, Napoli, Italy 7) Dutch Institute for Fundamental Energy Research, Association EURATOM-FOM,, Nieuwegein, The Netherlands 8) MPI fürPlasmaphysik, EURATOM Association, D-85748 Garching, Germany 9)TER Organization, Route de Vinon sur Verdon, 13115 St. Paul Lez Durance, France * See the Appendix of F. Romanelli et al, 24th IAEA Fusion Energy Conference, San Diego, 2012, paper OV/1-3

  3. Motivation • ITER will operate at powers close to the H-mode threshold power scaling Pnet ~1.3xPth(Martin08) to access H-mode with H~1 • JET experience on operation at low power margin over Pth: • H-mode plasma has a transient behaviour (cyclic transition to Type III ELMs or L-mode) H-mode with H~1 in stationary conditions is achieved above a critical input power (Ploss ~aPth, with a~1.3-2, CFC-wall) [Sips- ITR/P1-11] H~1 PIN increases

  4. Outline of the talk • Motivation: can ELM pacing methods help to sustain H-mode with H98~1 at heating powers marginally above Pth? • How do kicks work in JET? • Modelling of kick cycle. Mechanism responsible for the ELM triggering • Use of vertical kicks (ELM pacing) at powers marginally above Pth in JET with CFC wall • Plasma performance at powers marginally above Pth in JET with Be/W wall • Summary + outlook for ITER

  5. Vertical kicks in JET Before kick After kick DV Kick: Plasma moves downwards, plasma volume decreases • Minimum kick size is required to trigger ELMs • In JET (and AUG) ELMs are triggered when the plasma is moving down (towards lower X-point)

  6. Kick cycle modelling Pre-kick equilibrium solved with CREATE-NL (includes eddy current, magnetics only) Transport code (JINTRAC) where the plasma profiles are evolved (0.5 ms steps) Plasma boundary dyext/dt Work in progress to perform closed-coupled simulations

  7. Kick: ELM triggering mechanism Simulation shows that current driven instabilities are enhanced by increase in Jz (and lower shear). j increases at y~1 (dytot/dt>0) Current perturbation propagating inwards j reduces at the edge when plasma moves down

  8. Use of vertical kicks (ELM pacing) at powers marginally above Pth in JET with CFC wall

  9. ELM pacing (kicks) at Ploss~Pth Ploss ~ 1.2 x Pth dav ~0.4, q95=3 • Vertical kicks can reliably trigger ELMs at input powers marginally above Pth • Prad,bulk and W/t are reduced with the appearance of the triggered ELMs (10-20 Hz) Ploss = Pabs- W/t Pnet = Ploss-Prad,bulk The increase in fELM with kicks is sufficient to control particle and impurity density  stationary H-mode with H98~1

  10. ELM pacing (kicks) at Ploss~Pth Ploss ~ 1.2 x Pth dav ~0.4, q95=3 Pre-ELM profiles The increase in fELM with kicks is sufficient to control particle and impurity density  stationary H-mode with H98~1

  11. Pnet vs. Ploss in the access to H~1 Power scan (no gas fuelling) q95=3, dav=0.4 Minimum Ploss/Pth to access stationary H-mode conditions with H~1 is reduced with kicks H~1 (Type I ELMs, stationary) Ploss/Pth≥1.8 Low ne Type III ELMs Transient H-mode LH

  12. Pnet vs. Ploss in the access to H~1 Power scan (no gas fuelling) q95=3, dav=0.4 Minimum Ploss/Pth to access stationary H-mode conditions with H~1 is reduced with kicks H~1 (Type I ELMs,stationary) Ploss/Pth≥1.8 Low ne Type III ELMs H~1 (kicks ) Ploss/Pth≥1.2 LH

  13. Edge current and MHD stability Stability analysis (MISHKA-1): With kicks: From marginally stable (no ELMs) to low n peeling unstable when additional edge current, as predicted by simulations, is included amax With additional jf (simulations) amax

  14. Edge current and vertical kicks Ploss/Pth=1.1 • Minimum kick size required for successful ELM triggering at Ploss~Pth • Increasing velocity of plasma movement improves the probability of triggering an ELM (larger induced current) Kick size scan Increase in gas puffing  reduction in edge current (larger collisionality) • For fixed kick size, the ELM triggering probability increases with increasing edge Te (lower resistivity, larger bootstrap current). Encouraging in view of ITER (high Te,ped) Successful ELM triggering

  15. Plasma performance at powers marginally above Pth in JET with Be/W wall

  16. fELM and core W accumulation in ILW PIN/Pth ~1.4 • ELM frequency control essential for JET operation with Be/W wall to prevent core W accumulation (same as AUG)[Joffrin- EX/1-1] • Gas fuelling typically used to keep ELM frequency high and reach stationary conditions (H98<0.9) • At low gas fuelling (and low power) • cyclic transitions to L-mode at constant PIN are typically observed (similar to CFC-wall).

  17. ELM frequency control in JET-ILW Kicks exp. in JET with Be/W have just started Kicks: ELM control without affecting any other plasma parameter (no added impurities or density) • Kicks (increase in fELM) can help in reducing W core accumulation in gas fuelled H-mode plasmas in JET with Be/W wall • Te,ped lower with the Be/W wall  kick size larger than in previous exp. • Be/W (gas fuelled) : Te,ped < 0.6 keV • CFC (unfuelled): Te,ped ~ 1 keV [Beurskens-EX-P7-20]

  18. Conclusions • Vertical kicks can reduce the power requirements for achieving H-modes with H~1 (with type I ELMs) in JET (in CFC-wall so far) • Inter-ELM dynamics and radiation to be included in the evaluation of the minimum power required to access stationary H-modes with H98~1(PnetvsPloss) • Large variation in PH~1 in JET remains to be explained (Ploss~(1.3-2)x Pth) • Experimental and modelling results suggest that the mechanism by which vertical kicks trigger ELMs is related with the increase in edge current and its effect on edge stability. • Encouraging in view of ITER: The induced current in ITER (lower edge resistivity) would be relatively large compared to that at JET for the same perturbation (vz) • Use of vertical kicks may provide a potential route towards minimizing the W impurity build-up during the early phase of the H-mode in ITER (no penalty on confinement). • Vertical kicks using the existing coils are an option in ITER at Ip<10 MA (current ramp up/down) [Loarte- ITR/1-2] H~1

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