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Description
Neoclassical tearing modes (NTMs) are MHD instabilities that develop near rational surfaces, forming magnetic islands that, if unstable, can grow and ultimately degrade (or even disrupt) plasma confinement.
The nonlinear drive of the instability is associated with the reduction of the bootstrap current within the magnetic island and scales with the plasma poloidal β.
For this reason, tearing-type instabilities are expected to be particularly impactful in high-β machines such as JT-60SA.
NTMs can be controlled by injecting EC waves at the rational surface to compensate for the reduced bootstrap current, either through direct current drive (ECCD) or via local heating (ECRH).
Experiments regarding the NTM control are planned for the third experimental campaign of JT-60SA (OP3).
This work presents a predictive analysis of NTM evolution, with a particular focus on the ECRH control phase, with the goal of providing a priori guidance in preparation for OP3.
A predictive framework for NTM island dynamics based on a nonlinear model derived from the generalized Rutherford equation (GRE) has been developed. This work aims to apply the model both in a coupled framework with integrated codes (JETTO–JINTRAC) and in a stand-alone configuration, enabling fast and systematic parameter scans.
The uncontrolled evolution is first analyzed to estimate characteristic growth/saturation timescales and the saturated island width.
The stabilizing contribution of EC injection is then incorporated to assess EC-based control. The model is used to derive quantitative indications for required EC power, to compare the relative impact of heating and current-drive contributions (Δh and Δcd), and to estimate the stabilization time.
The ideal analysis is extended by assessing the impact of the misalignment of the deposited power with respect to the island O-point, arising from non-ideal conditions that affect the tracking of the resonant surface position. The impact is quantified in terms of the power required for stabilization as a function of the degree of misalignment.
Overall, this work can contribute to provide indications on the most effective suppression strategies for NTM control. For example, it addresses the optimal toroidal and poloidal injection angles to best exploit the balance between heating and current drive, the potential power savings associated with an early stabilization, and the level of alignment accuracy required to achieve complete suppression. The analysis can be further extended to account for the alignment process by comparing the energy cost of island suppression for different launcher trajectories (e.g., a linear approach toward the rational surface versus a sweep around it).