Active control of 2/1 magnetic islands in a tokamak

Abstract

A new sawtooth control mechanism relying on toroidally propagating ion cyclotron resonance frequency waves: Theory and Joint European Torus tokamak experimental evidencea) Phys. Plasmas 17, 056118 (2010); 10.1063/1.3363201 Sawtooth control using beam ions accelerated by fast waves in the DIII-D tokamakClosed and open loop control techniques were applied to growing m/nϭ2/1 rotating islands in wall-stabilized plasmas in the High Beta Tokamak-Extended Pulse ͑HBT-EP͒ ͓J. Fusion Energy 12, 303 ͑1993͔͒. HBT-EP combines an adjustable, segmented conducting wall ͑which slows the growth or stabilizes ideal external kinks͒ with a number of small ͑6°wide toroidally͒ driven saddle coils located between the gaps of the conducting wall. Two-phase driven magnetic island rotation control from 5 to 15 kHz has been demonstrated powered by two 10 MW linear amplifiers. The phase instability has been observed and is well modeled by the single-helicity predictions of nonlinear Rutherford island dynamics for 2/1 tearing modes including important effects of ion inertia and finite Larmor radius, which appear as a damping term in the model equations. The closed loop response of active feedback control of the 2/1 mode at moderate gain was observed to be in good agreement with the theory. Suppression of the 2/1 island growth has been demonstrated using an asynchronous frequency modulation drive which maintains the inertial flow damping of the island by application of rotating control fields with frequencies alternating above and below the natural mode frequency. This frequency modulation control technique was also able to prevent disruptions normally observed to follow giant sawtooth crashes in the plasma core.

First observation of rotation acceleration of magnetic island by using rotating resonant magnetic perturbation on the J-TEXT tokamak

“…When the current of the DRMP coil decreases, the island unlocks. There is an apparent decrease of the island width during the locking and unlocking procedure, which was also observed in [15]. It should be noted that the magnetic field of DRMP contributes little to the Mirnov signals because the Mirnov array locates relatively far from the DRMP coils.…”

“…Early experiments to study effects of rotating RMP on the tearing mode were conducted on ATC [12], TO-1 [13] and DITE tokamak [14]. On HBT-EP, the frequency modulation of RMP is found to be helpful for tearing mode stabilization, in which the ion flow damping was found to play an important role [15]. Numerous findings on rotating RMP on TEXTOR in the fields of plasma rotation, tearing mode suppression, edge radial electric field and edge plasma transport are reported over the past several years [16,17].…”

Role of the shear flow profile on the stability of magnetic islands

“…12,13 One can readily see that such an explanation is equivalent to the occurrence of plasma flow inside the magnetic islands. The excitation of strongly sheared plasma flow and, as a result, the excitation of the internal circulation can also be expected in the experiments on active control of magnetic islands in a tokamak 8 where the frequency of the external control field was strongly modulated. Such a frequency modulation appears to be crucial for a successful suppression of the otherwise unstable magnetic islands.…”

“…If stabilizing, this term may provide a threshold for the modes driven by external sources such as neoclassical tearing modes ͑NTM͒ 6 or the modes driven by the external resonant perturbations. 5,8 It has been realized recently [9][10][11] that in absence of ion drift effects the ion polarization current is destabilizing for a certain class of the velocity profiles near the magnetic island. In this note we analyze how do more general plasma velocity profiles with a flow inside a magnetic island change the effect of the ion polarization current.…”

Control and diagnostic uses of feedback

“…Interesting and promising results using magnetic coil suppressors for a single MHD mode have been reported. [12][13][14][15] However, neutral beam suppressors may have some advantages, especially when the modes are located deep in the plasma, e.g., a mϭ1, nϭ1 MHD mode. The stabilization of a single ideal MHD mode (mϭ2,nϭ1) 16 or a single tearing mode (mϭ1,nϭ1) 17 via a neutral beam suppressor has been discussed before.…”