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FIR-NTMs on ASDEX Upgrade and JET Active Control of (2,1) NTMs on ASDEX Upgrade

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

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FIR-NTMs on ASDEX Upgrade and JET Active Control of (2,1) NTMs on ASDEX Upgrade

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  1. FIR-NTMs on ASDEX Upgrade and JET Active Control of (2,1) NTMs on ASDEX Upgrade S. Günter1, M. Maraschek1, M. de Baar2, D.F. Howell3, E. Strumberger1, C. Tichmann1, H. Zohm1 ASDEX Upgrade Team, contributors to the JET-EFDA work programme 1Max-Planck-Institut für Plasmaphysik Garching, Germany 2FOM instituut voor plasmafysica, Rijnhuizen, Nieuwegein, The Netherlands 3UKAEA, 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 ususal NTMs FIR-NTMs JET: full symbols ASDEX Upgrade: open symbols Remarkable agreement between ASDEX Upgrade and JET results!

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

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

  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) p’=2.5x expt. s=0.8 p’=1.6x 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 (LHCD ctr-CD in start-up phase) Despite (3,2) NTM excellent confinement: H98y=1.4, N = 3.3

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

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

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

  12. AC DC 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 wCD • 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 / wCD AC/DC stabilization efficiency  >> 1/c Modulation required for ITER?

  13. Investigate the influence of wsat/wdep 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

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