On the stabilisation of locked tearing modes in ITER and other large tokamaks

TL;DR

This paper suggests that allowing tearing modes to lock before stabilization in large tokamaks like ITER can be more effective than stabilizing them during rotation, potentially improving fusion performance.

Contribution

It introduces a novel approach of stabilizing locked tearing modes after they form, challenging the traditional focus on stabilizing during rotation in large tokamaks.

Findings

Locked mode stabilization can be more efficient in large devices.

Allowing modes to lock before stabilization improves control.

Potential positive impact on ITER's fusion gain and disruptivity.

Abstract

Tearing modes in tokamaks typically rotate while small and then lock at a fixed location when larger. Research on present-day devices has focused almost exclusively on stabilisation of rotating modes, as it has been considered imperative to avoid locked modes. However, in larger devices, stabilisation during the rotating phase is made difficult by fast locking at small island widths, and large broadening of the stabilising wave-driven current profile. In contrast, the smaller island width at locking not only mitigates the deleterious consequences of locked modes, but also permits their efficient stabilisation. On large devices, it thus becomes surprisingly advantageous to allow the mode to grow and lock naturally before stabilising it, challenging the mainstream strategy of neoclassical tearing mode stabilisation during the rotating phase. Calculations indicate that a locked island stabilisation strategy should be adopted in the ITER tokamak, with a large potential impact on the fusion gain and disruptivity.