Triggering of neo-classical tearing modes by mode coupling

Abstract

The formation of magnetic islands with mode numbers (m>1,n=1) apparently triggered by sawtooth crashes has for a long time been a concern in JET discharges with auxiliary heating as it affects the peak performance, in terms of stored energy or of neutron yield (in DT experiments). The phenomenological association of the sawtooth crash with the onset of resistive modes is becoming particularly relevant in the context of Neoclassical Tearing Modes (NTM) theory that requires the formation of a "seed" island above a critical value. Relying on observations and on a critique of the assumed role of the collapse phase of the (1,1) instability we propose as physical mechanism of destabilisation of the metastable NTM a forced reconnection process due to electro-dynamic mode-mode coupling. For specific frequency matching conditions a bifurcation pattern appears in the ∆(W) plot. Modeling of a JET discharge shows that for perfectly frequency matching conditions the triggering of the (m=3, n=2) NTM could be due to either two-mode toroidal coupling or threemode non-linear coupling. The observed mode frequencies are consistent with the non-linear coupling. 1-Introduction In several experiments performed at JET, the peak performance, in terms of neutron yield and plasma stored energy, has been observed to deteriorate in coincidence with the onset of fast

Ion cyclotron resonance heating for tungsten control in various JET H-mode scenarios

“…In addition to the sawtooth instability, other MHD modes are expected to act on the tungsten transport [Nave 2003, Pütterich 2013, Angioni 2014. When the large sawtooth (∆Te=1.5keV) occurs after the 1.3s long sawtooth free period (#85376), a (m=3, n=2) neoclassical tearing mode (NTM) is destabilized for the rest of the discharge (f=9-10kHz) whereas a m=1, n=1 internal kink mode (f=8-9kHz) is present during the other pulses (#85377 &85081).…”

Nonlinear dynamics of turbulence driven magnetic islands. I. Theoretical aspects

“…In other words, the triggering of NTMs in a device is not directly linked to the b levels but to the mechanisms which generate seed islands. 4 Previous research has identified several different causes for the formation of seed islands such as an unstable tearing mode, [5][6][7] MHD events such as sawtooth 8 and fishbones, 9 error fields, 10,11 edge localized modes (ELMS), 12 nonlinear mode coupling, 13 and turbulence. 14,15 This diversity of origins poses serious challenges for experimentally anticipating their appearance and choosing appropriate control parameters for their avoidance and suppression.…”

Nonlinear dynamics of multiple neoclassical tearing modes in tokamaks

“…11 For all our runs, we have ensured that this happens by keeping the ratio to be ! 10 6 . Finally, Equation (4) is the evolution equation for the parallel velocity component.…”

“…NTMs are believed to set the upper limit on attainable normalized plasma b N 3,4 in long pulse tokamaks and have therefore been the subject of much experimental and theoretical investigations in the recent past. [5][6][7][8][9][10][11] While there has been considerable progress in the experimental characterization and control of this instability with concurrent development of a useful theoretical framework in the form of a model island evolution equation (the modified Rutherford equation) there are still many basic physics issues associated with the mode that remain poorly understood or relatively less explored. The trigger and the mechanism for the creation of the seed island, nonlinear mode coupling between NTMs of different helicities, interaction of the magnetic island with energetic particles, evolution of the island in a turbulent media [12][13][14] are among some of the basic areas that need further attention.…”

“…The triggering of a seed island at the q ¼ 2 surface due to a sawtooth crash at the q ¼ 1 surface or due to fishbones is an example of a mode coupling process operating at the onset stage. 5,6 There is also experimental evidence of mode coupling effects influencing the evolution of multiple NTMs often leading to the predominance of a single helicity over others. 6,[8][9][10] There have been some past model studies addressing the issue of the interaction between multiple helicity NTMs.…”