On the requirements to control neoclassical tearing modes in burning plasmas

Abstract

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 β N 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 localized electron cyclotron current drive (ECCD) in ITER are discussed in detail. Other aspects of the above questions are also discussed, in particular the role of partial stabilization 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.

On recent results in the modelling of neoclassical-tearing-mode stabilization via electron cyclotron current drive and their impact on the design of the upper EC launcher for ITER

“…A conclusion common to several studies about EC stabilization of NTMs in ITER [9,11,12] is that these terms are those involving the classical stability parameter ∆ ′ [22], the bootstrap current destabilization [23,24] (possibly reduced on account of geodesic curvature effects [25,26]) and the effect of the helical component of the EC current on the island evolution [27]. Small-island effects are usually included as a transport threshold on the bootstrap term [28] or through the polarization-current term [29].…”

“…On the other hand, ray/beam tracing calculations indicate that the high focusing capabilities of a launcher based on front steering could allow EC deposition profiles below the marginal island width. In this respect, it was noted by Sauter et al [11] that, once the beams deposit their power well inside the island, further focusing does not result in significantly higher stabilization efficiency (making the process in fact more sensitive to a possible misalignment between beam and island). For this reason, it was proposed to optimize in this case the total driven current, adding the constraint that the EC deposition width should not exceed 5 cm.…”

Assessment of the ITER EC Upper Launcher Performance

“…The power required for NTM stabilization has been estimated making use of two stabilization criteria [6][7][8]. The first criterion is expressed in terms of the ratio between the peak value of the current density J CD driven by the in- Figure 5.…”

Progress of the ECRH Upper Launcher design for ITER

“…Two criteria [5], derived from this equation, have been proposed to describe the requirements for local current drive and hence the UL performance. First the locally driven current density should exceed the unperturbed bootstrap current density by a factor of 1.2 at the flux surface where the mode develops.…”