FIR-NTMs on ASDEX Upgrade and JETActive 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, Germany2FOM instituut voor plasmafysica, Rijnhuizen, Nieuwegein, The Netherlands3UKAEA, Culham, UK

• FIR (Frequently Interrupted Regime)-NTMs• (2,1) NTM stabilization• Modulated vs. non-modulated CD for ITER?

FIR-NTMs cause only benign confinement degradation

JET: full symbolsASDEX Upgrade: open symbols

FIR-NTMsususal NTMs

Remarkable agreement between ASDEX Upgrade and JET results!

FIR-NTMsususal NTMs

FIR-NTMs cause only benign confinement degradation

S. Günter et al., PRL 2001

0.030.012

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

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.

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

Active triggering of FIR-NTMs possible? YES!

=const. (power control)

higher s

lower s

Low global magnetic shear in the plasma center

Despite (3,2) NTM excellent confinement: H98y=1.4, N = 3.3

(LHCD ctr-CD in start-up phase)

Confinement improvement for low global central shear

strong FIR character

small NTM amplitude already without FIR character

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

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

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

ACDC >> 1/c

Modulation required for ITER?

Investigate the influence of wsat/wdep

Current drive efficiency

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

Stabilisation of (2,1) NTMs with ECCD