JET TFM General Meeting, 15 – 16 March 2010

1. 3D effects on Stability M Gryaznevich JET TFM General Meeting, 15 – 16 March 2010

2. 3D effects • Two types of 3D effects: • - geometry (structure of fields and real wall geometry) • - Physics (helical equilibrium, local stability thresholds) • What to do: • 3D wall model (VILEN etc.) • 3D description of JET wall and inclusion of 3D effects • in modelling are already addressed (Liu, Villone) • Helical equilibrium • Stability codes to include 3D effects (from TERPSICHORE to MARS-3D)

3. 3D effects • The importance of 3D effects on stability must be considered. • Helical distortion of plasma edge @ 1kA in EFCC coils measured to be ~ 3 cm (I Chapman, NF 2008) - will be 18 cm at proposed 6kA – what about ITER relevance ??? • Helical distortion due to 2/1 mode ~ 6 – 12 cm (P Buratti, 2010) • Any local stability threshold (e.g. pressure driven core modes, core turbulence or edge modes) is strongly affected by such distortion

4. 3D effects • The importance of 3D effects on stability must be considered Edge perturbation data (lines and open symbols – predictions, filled symbols – experiment, RP - circles and ECE - squares) in Octant 5 (Reciprocating Probe) shows a linear response to the applied error field. I Chapman Internal perturbation (from Te contours, ECE) due to bursting mode with peak at q=2 and continuous kink P Buratti

5. High beta disruptions and ELMs mitigation: what is common? • High beta disruptions: • Ballooning modes: local threshold can be exceeded due to helical distortion of plasma (local increase in pressure gradient while plasma is close to beta limit) • Cause: toroidal alignment of low-n modes (H Tojo, PhD thesis), error field, LM • Can, and has been observed: TFTR, MAST • Need more data for quantitative analysis – can be done with new MAST internal saddle coils

6. High beta disruptions and ELMs mitigation: what is common? • ELMs mitigation: • ELMy regimes are always at stability threshold • Application of external helical field (EFCC) reduces “integral” stability threshold due to violation of local stability • This causes increase in ELM frequency and reduction in amplitude • In other words, we are provoking ELMs with EFCCs, not stabilising them • The first ELM happens when low-n precursor provides enough local threshold violation due to n=1 distortion • Outer mode may “shave” such local perturbation, removing local stability violation, resulting in ELM-free regime

7. 7 8009 7 8010 RFA D a n = 1 n=2 outer mode n = 2 RFA during ELM-free period, role of “outer mode” n = 1 RFA during ELM-free period before 1st ELM • Comparison of two high current pulses, with (#78010, red) and without (#78009, blue) “outer mode”: • - clear reduction of RFA during outer mode • - n = 2 outer mode causes reduction in n = 1 RFA indicating that changes in the edge stability affect core n=1 stability, confirming MARS-F predictions

8. 3D effects on ballooning and global pressure-driven kink Cooper, Todd, Hender et al, PPCF 2000 Normal curvature perturbation due to applied 3/1 external field. ST, b = 14.4% TERPSICHORE code: 3/1 externally applied perturbation, < 10%: local magnetic shear g S = y’(s)F’’(s) - F‘(s)y’’(s)- g B hs becomes locally positive, from -0.0005 going to -0.0011 – 0.0004 Pressure perturbation • Global kink stability was not affected by small 3/1 external perturbation • - eigenvalue increased from - 2.0 10-3 to - 2.5 10-3 • However, interaction of p with the normal curvature 2p’(s)g k s could destabilize local edge ballooning stability, both poloidally and toroidally • This caused changes in local pressure profile. Average normal curvature was unchanged, but local shear becomes modulated which affects global kink stability. • Ripple and RMP on JET will have strong effect on local edge stability.

9. CONCLUSIONS • Local stability thresholds (e.g. pressure driven core modes, core turbulence or edge modes) could be strongly affected by 3D local deformations caused by external or internal sources