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Role of Edge Current in ELM Behaviour: Modelling of Recent Current Ramp Experiment in JET

Role of Edge Current in ELM Behaviour: Modelling of Recent Current Ramp Experiment in JET. V.V. Parail 1 , G. Bateman 6 , M. Becoulet 4 , G. Corrigan 1 , A.Y. Dnestrovskij 2 , G. Huysmans 4 , A. Kritz 6 , A. Loarte 5 , J. Lonnroth 3 , T. Onjun 6 , G. Saibene 5 , H. Wilson 1

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Role of Edge Current in ELM Behaviour: Modelling of Recent Current Ramp Experiment in JET

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  1. Role of Edge Current in ELM Behaviour: Modelling of Recent Current Ramp Experiment in JET V.V. Parail1, G. Bateman6, M. Becoulet4, G. Corrigan1, A.Y. Dnestrovskij2, G. Huysmans4, A. Kritz6, A. Loarte5, J. Lonnroth3, T. Onjun6, G. Saibene5 , H. Wilson1 1UKAEA Fusion Association, Culham, UK; 2 Kurchatov Institute, Moscow, Russia, 3Association Euratom-Tekes, Finland; 4Association Euratom-CEA, Cadarache, France; 5 EFDA-CSU, Garching, Germany, 6 Lehigh Univ., PA Peeling vs. ballooning stability; Recent current ramp-up experiments; Predictive modelling of JET ELMy H-mode plasmas; Conclusions and future plans 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  2. Current ramp-up experiment on JET Btor=3.4T Ip=2MA =0.4 =1.7 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  3. Peeling vs. ballooning stability (1) • We adopt the following approach in predictive modelling : • we assume certain width of the ETB (either constant or dependent on plasma parameters); • we simulate all main plasma profiles (ne, Te,i, jII, + neutrals) up to and beyond the separatrix by using a coupling of two codes: JETTO+EDGE2D; • we can make different assumptions about ELM trigger (ideal ballooning with and without access to a second stability, peeling mode); • we use ad hoc model for ELM amplitude and duration 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  4. Peeling vs. ballooning stability (2) 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  5. Peeling vs. ballooning stability (3) Jedge/Javge 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  6. Peeling vs. ballooning stability (4) • Simple analytical criteria for ballooning and peeling mode stability are used in JETTO as an ELM trigger: • Ballooning mode: with adjustable parameter to account for an access to a second stability region; • Peeling mode: 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  7. Peeling vs. ballooning stability (5) • We can use either one or both criteria in our predictive modelling; • the caveat is that ELM-event itself is independent on the ELM trigger - it appears as a short sharp increase in heat and particle diffusivities (but not in electrical resistivity) within the ETB; • it means that ELM quickly destroys the edge bootstrap current but it takes much longer to dissipate the Ohmic current, which substitutes bootstrap current; 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  8. Peeling vs. ballooning stability (6) • We study the relative role of ballooning and peeling mode instability as ELM trigger in our predictive modelling; • In experiment the easiest way to decouple the edge current from the pressure gradient (bootstrap current) is to ramp the current during the ELMy H-mode; • JET recently conducted a dedicated experiment, in which current ramp-up technique was used in low density Optimised Shear plasmas; 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  9. Selection of shots (1) Btor=3.4T Ip=2MA =0.4 =1.7 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  10. Selection of shots (2) 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  11. Predictive Modelling of ELMs (1) First we demonstrate that we can reproduce experimental data with ballooning stability trigger; Ballooning stabilitytrigger with JET #53298 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  12. Predictive Modelling of ELMs (2) First we demonstrate that we can reproduce experimental data with ballooning stability trigger; 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  13. Predictive Modelling of ELMs (3) • Then we study the relative role of ballooning and peeling mode as an ELM trigger; 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  14. Predictive Modelling of ELMs (4) • Or with a better resolution: 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  15. Predictive Modelling of ELMs (5) • We conclude that both ballooning and peeling mode trigger ELM within the experimentally observed parameter range; • If peeling mode is used as ELM trigger the ELM duration becomes much longer than experimentally observed length telm<0.2msec; • The explanation is that the edge current should be removed in order to suppress peeling mode and this current is controlled by the skin time; 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  16. Predictive Modelling of ELMs (6) • We also found that ELM has a composite character in many cases: it is triggered by ballooning mode, which is superseded by the peeling mode: 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  17. Predictive Modelling of ELMs (7) 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  18. Predictive Modelling of ELMs (8) • It is interesting to note that since skin time is proportional to skinTe3/2, the ELM duration should depend on edge temperature; 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  19. Predictive Modelling of ELMs (9) • Now we can check how our combined peeling-ballooning model for ELM trigger will react on the current ramp: 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  20. Predictive Modelling of ELMs (10) • or, with a better resolution: 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  21. Predictive Modelling of ELMs (11) • Qualitatively (only!) it goes in line with experiment: 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  22. Predictive Modelling of ELMs (12) • Qualitatively (only!) it goes in line with experiment; • We can easily reproduce temperature and density profiles but not ELM dynamics; 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  23. Conclusions • Experimental observations and predictive modelling show that both ballooning and peeling mode can play their part in ELMs; • Recent current ramp-up experiment in JET (although not quite conclusive) indicate that peeling mode might play a crucial role in keeping plasma edge in type-III ELMy H-mode in low density plasmas; 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

  24. Conclusions 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003

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