1. FIR-NTMs on ASDEX Upgrade and JET Active Control of (2,1) NTMs on ASDEX Upgrade S. Günter 1, M. Maraschek 1, M. de Baar 2, D.F. Howell 3, E. Strumberger 1, C. Tichmann 1, H. Zohm 1 ASDEX Upgrade Team, contributors to the JET-EFDA work programme 1 Max-Planck-Institut für Plasmaphysik Garching, Germany 2 FOM instituut voor plasmafysica, Rijnhuizen, Nieuwegein, The Netherlands 3 UKAEA, Culham, UK FIR (Frequently Interrupted Regime)-NTMs (2,1) NTM stabilization Modulated vs. non-modulated CD for ITER?

2. FIR-NTMs cause only benign confinement degradation JET: full symbols ASDEX Upgrade: open symbols FIR-NTMsususal NTMs Remarkable agreement between ASDEX Upgrade and JET results!

3. FIR-NTMs ususal NTMs FIR-NTMs cause only benign confinement degradation S. Günter et al., PRL 2001 0.03 0.012

4. Amplitude drops caused by non-linear mode coupling A. Gude et al., NF 2002 … of (3,2) NTM to (4,3) and (1,1) mode activity

5. Why large  N values needed? Short bursts of (4,3) activity, small growth time (< 300  s)  ideal (4,3) mode activity? Ideal (4,3) mode driven unstable by large pressure gradient (s=const)low magnetic shear (p’=const) s=0.8 p’=1.6x expt. p’=2.5x expt.

6. Active triggering of FIR-NTMs possible? Ideal (4,3) mode triggered when ECCD lowers magnetic shear at q=4/3 surface (discharge without NTMs) (4,3) mode

7. Active triggering of FIR-NTMs possible? YES!  =const. (power control) higher s lower s

8. Low global magnetic shear in the plasma center Despite (3,2) NTM excellent confinement: H 98y =1.4,  N = 3.3 (LHCD ctr-CD in start-up phase)

9. Confinement improvement for low global central shear strong FIR character small NTM amplitude already without FIR character

10. In ASDEX Upgrade, (2,1) NTM usually occurs at high   and locks to wall target plasma has power step-down to obtain rotating (2,1) at lower   Stabilisation of (2,1) NTMs with ECCD

11. At  N = 1.9, ECCD power of 2.0 MW just sufficient for stabilisation higher power requirement than for (3,2) NTM (effect of lower  CD ?) Stabilisation of (2,1) NTMs with ECCD

12. Modulated versus Non-Modulated CD Experiments on ASDEX Upgrade did not show any difference in stabilization efficiency between AC and DC current drive, in agreement with theory for w  w CD Reason: - Current driven at X-point is not very effective as flux surface averaged current density is small -   for co-ECCD at rational surface - Modulation more effective for small w / w CD AC/DC stabilization efficiency AC DC  >> 1/  c Modulation required for ITER?

13. Investigate the influence of w sat /w dep Current drive efficiency

14. Summary Good agreement in requirements for (3,2) FIR-NTMs on ASDEX Upgrade and JET (  Nonset > 2.3) Triggering transition to FIR-NTMs demonstrated by destabilizing linear (4,3) mode (lower magnetic shear) close to L-H transition also large ELMs cause amplitude drops of (3,2) NTMs with corresponding confinement improvement  (3,2) NTMs would not be a great danger for ITER if sufficient high  N values and/or low central magnetic shear are reached (e.g. improved H-mode) Complete NTM stabilization on ITER might require modulated current drive, corresponding experiments on the way on ASDEX Upgrade

15. Stabilisation of (2,1) NTMs with ECCD