Effect of resonant magnetic perturbations on COMPASS-C tokamak discharges

Abstract

Experimental results from the COMPASS-C tokamak reveal a sharp threshold in amplitude above which externally applied static resonant magnetic perturbations (RMPs) induce stationary magnetic islands. Such islands (in particular, m = 2, n = 1 islands) give rise to a significant degradation in energy and particle confinement, suppression of the sawtooth oscillation and a large change in the impurity ion toroidal velocity. The observed threshold for inducing stationary (2,l) islands is consistent with a phenomenological resistive MHD model which takes into account plasma rotation (including poloidal flow damping) and externally applied resonant fields. Broadly similar results are found for applied fields other than m = 2, n = 1. Other results from RMP experiments are also discussed, such as the stabilization of rotating MHD activity, stimulated disruptions and extensions to the disruptive density limit. Finally, the likely effect of field errors on large tokamaks is briefly examined in the light of the COMPASS-C results.

Effect of resonant helical magnetic fields on plasma rotation

“…(a) Helical field penetration: For the plasma being originally stable to tearing modes, an applied resonant helical field (or error fields of experimental devices) can penetrate through the resonant surface and generate a magnetic island there [1][2][3][4][5][6][7][8][9]. Recently it was shown on TEXTOR that the relative frequency between the mode and the helical field is important in determining the field penetration [5,6], being in agreement with theoretical results [7][8][9].…”

“…(b) Mode locking: The locking of large magnetic islands by error fields is often observed in experiments, leading to severe confinement degradation in tokamak plasmas or even to disruptions [1,10,11]. The mode locking threshold is predicted to be much lower in a fusion reactor than in existing tokamaks due to the stronger magnetic field and lower plasma rotation speed [12].…”

In-vessel saddle coils for MHD control in ASDEX Upgrade

“…Techniques to avoid large ELMs are therefore being investigated, for example ELM loss reduction by injection of cryogenic pellets [3], small ELM regimes [4][5][6] and stationary ELM-free regimes [7,8]. It has been observed early on in COMPASS-D that non-axisymmetric error fields can reduce the size of ELMs [9]. More recently, ELM mitigation is studied in DIII-D with more edge-localised n = 3 magnetic perturbations produced by a set of 2 × 6 in-vessel saddle coils [10].…”

Shielding of external magnetic perturbations by torque in rotating tokamak plasmas

“…These perturbations can lead to a significant degradation of confinement [2][3][4], but also can be used to improve the performance [5,6]. A tokamak plasma responds to a nonaxisymmetric magnetic perturbation by producing perturbed plasma currents.…”