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