Phase control of a wall-locked tearing mode in ITER-like cylindrical tokamak

Abstract The phase-control of a wall-locked tearing mode by application of external magnetic perturbations is examined with analytical and numerical means. The analyses are applied to a cylindrical proxy of ITER, as in other recent studies. An error field (EF), similar in amplitude to that estimated in ITER, is also included in a dedicated study. The standard feedforward method is compared to two different non-zero radial field reference feedback (FB) algorithms. The first exploits a fixed amplitude reference. The second one can be viewed as a pure phase control, since the amplitude reference is not assigned. Generally speaking, FB is more efficient than feedforward, because it aligns the mode phase to the target value with a smaller current request, implying also a smaller edge radial field. Nonetheless, the pure FB phase control exhibits limit-cycle oscillations when the EF is in phase opposition to the target. On the contrary, these oscillations do not occur with the fixed amplitude reference FB, which therefore emerges from this study as the preferable control technique.