On the Requirements to Control Neoclassical Tearing Modes in Burning Plasmas

Neoclassical tearing modes (NTMs) are magnetic islands which increase locally the radial transport and therefore degrade the plasma performance. They are self-sustained by the bootstrap current perturbed by the enhanced radial transport. The confinement degradation is proportional to the island width and to the position of the resonant surface. The q=2 NTMs are much more detrimental to the confinement than the 3/2 modes due to their larger radii. NTMs are metastable in typical scenarios with N \supeq 1 and in the region where the safety factor is increasing with radius. This is due to the fact that the local perturbed pressure gradient is sufficient to self-sustain an existing magnetic island. The main questions for burning plasmas are whether there is a trigger mechanism which will destabilize NTMs, and what is the best strategy to control/avoid the modes. The latter has to take into account the main aim which is to maximize the Q factor, but also the controllability of the scenario. Standardized and simplified equations are proposed to enable easier prediction of NTM control in burning plasmas from present experimental results. The present expected requirements for NTM control with localised ECCD (electron cyclotron current drive) in ITER are discussed in detail. Other aspects of the above questions are also discussed, in particular the role of partial stabilisation of NTMs, the possibility to control NTMs at small size with little ECH power and the differences between controlling NTMs at the resonant surface or controlling the main trigger source, for the standard scenario namely the sawteeth. It is shown that there is no unique best strategy, but several tools are needed to most efficiently reduce the impact of NTMs on burning plasmas

Power requirements for electron cyclotron current drive and ion cyclotron resonance heating for sawtooth control in ITER

13MW of electron cyclotron current drive (ECCD) power deposited inside the q = 1 surface is likely to reduce the sawtooth period in ITER baseline scenario below the level empirically predicted to trigger neo-classical tearing modes (NTMs). However, since the ECCD control scheme is solely predicated upon changing the local magnetic shear, it is prudent to plan to use a complementary scheme which directly decreases the potential energy of the kink mode in order to reduce the sawtooth period. In the event that the natural sawtooth period is longer than expected, due to enhanced alpha particle stabilisation for instance, this ancillary sawtooth control can be provided from > 10MW of ion cyclotron resonance heating (ICRH) power with a resonance just inside the q = 1 surface. Both ECCD and ICRH control schemes would benefit greatly from active feedback of the deposition with respect to the rational surface. If the q = 1 surface can be maintained closer to the magnetic axis, the efficacy of ECCD and ICRH schemes significantly increases, the negative effect on the fusion gain is reduced, and off-axis negative-ion neutral beam injection (NNBI) can also be considered for sawtooth control. Consequently, schemes to reduce the q = 1 radius are highly desirable, such as early heating to delay the current penetration and, of course, active sawtooth destabilisation to mediate small frequent sawteeth and retain a small q = 1 radius.Comment: 29 pages, 16 figure

Modulation of electron transport during Swing ECCD discharges in TCV

Generation of a swing electron cyclotron current drive (swing ECCD), i.e. driving alternated, symmetric, positive or negative local ECCD, during a single discharge and at constant total input EC power, was performed at the Tokamak a Configuration Variable (TCV). The electron temperature is observed to be modulated inside the deposition radius, implying modulation of the electron transport properties. The modulation of ECCD is the only actuator for the observed modifications in the electron transport properties. These exhibit inverted behaviors depending on the deposition location of the co- and counter-ECCD. At more on-axis depositions, swing ECCD results in a higher electron temperature during the co- ECCD phase, whereas at more off-axis depositions, the electron temperature is higher during the counter-ECCD phase. Transport modeling of these discharges shows that the local electron tranport behavior depends on the value of the modulated magnetic shear. The results are transport model independent, confirming the robustness of the magnetic shear modeling, and indicating that the main contribution is due to the ECCD. Moreover, the results are consistent with predictions from gyrokinetic simulations, that the local electron confinement is proportional to the magnetic shear at low shear and inversely at high shear values, s greater than or similar to 1

Studies of Electron Transport and Current Diffusion in Switched ECCD experiments on TCV

The aim of the present work is to provide a better insight on the magnetic shear profile modification in the Switched ECCD experiments. Modelling of the plasma current density is carried on by the ASTRA transport code employed in both predictive and interpretative modes, with two shear-dependent models for the calculation of the electron energy diffusion coefficient. In this context, the modulation of ECCD is the only actuator for the transport properties modifications. This study confirms the synergy between electron transport and magnetic shear, both of which are modulated around the EC deposition region. It also allows to completely decouple the effects of the current profile modification from those of slight plasma heating misbalance or non-constant plasma elongation, which are key concepts at the basis of Switched ECCD and can be a rather delicate experimental issue. The numerical results moreover validate a previous rough model based on electrodynamics calculations