1. 4th Transport/ITB IPTA Meeting, St Petersburg, 8-12 April 2003 1 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 1 UKAEA Fusion Association, Culham, UK; 2 Kurchatov Institute, Moscow, Russia, 3 Association Euratom-Tekes, Finland; 4 Association 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

2. 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

3. 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 ( n e, T e,i, j II, + 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

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

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

6. 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:

7. 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;

8. 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;

9. 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

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

11. 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 stability trigger with JET #53298

12. 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;

13. 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;

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

15. 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  t elm <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;

16. 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:

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

18. 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  T e 3/2, the ELM duration should depend on edge temperature;

19. 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:

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

21. 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:

22. 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;

23. 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;

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