Feedforward and feedback control of locked mode phase and rotation in DIII-D with application to modulated ECCD experiments

Abstract

Abstract. The toroidal phase and rotation of otherwise locked magnetic islands of toroidal mode number n=1 are controlled in the DIII-D tokamak by means of applied magnetic perturbations of n=1. Pre-emptive perturbations were applied in feedforward to "catch" the mode as it slowed down and entrain it to the rotating field before complete locking, thus avoiding the associated major confinement degradation. Additionally, for the first time, the phase of the perturbation was optimized in real-time, in feedback with magnetic measurements, in order for the mode's phase to closely match a prescribed phase, as a function of time. Experimental results confirm the capability to hold the mode in a given fixed-phase or to rotate it at up to 20 Hz with good uniformity. The controlcoil currents utilized in the experiments agree with the requirements estimated by an electromechanical model. Moreover, controlled rotation at 20 Hz was combined with Electron Cyclotron Current Drive (ECCD) modulated at the same frequency. This is simpler than regulating the ECCD modulation in feedback with spontaneous mode rotation, and enables repetitive, reproducible ECCD deposition at or near the island O-point, X-point and locations in between, for careful studies of how this affects the island stability. Current drive was found to be radially misaligned relative to the island, and resulting growth and shrinkage of islands matched expectations of the Modified Rutherford Equation for some discharges presented here. Finally, simulations predict the as designed ITER 3D coils can entrain a small island at sub-10 Hz frequencies.Page 1 of 15 AUTHOR SUBMITTED MANUSCRIPT -NF-102117. R2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Control of locked mode phase and rotation with application to modulated ECCD 2 Figure 1. Perturbed current pattern, sinusoidal in helical angle mθ − nφ, of a m/n = 2/1 magnetic island mapped onto a thin surface. Also depicted are the internal I-coils (green) and external C-coils (red) used to generate 3D fields on DIII-D.

Progress in disruption prevention for ITER

“…A reduced MHD simulation of island dynamics shows good qualitative agreement with experiment [63]. A simple 0-D model indicates that in ITER, the internal non-axisymmetric coils could entrain a locked island of 6-8 cm width at sub-10 Hz frequencies [58].…”

“…Forced rotation of magnetic islands by applied electromagnetic torque prevents locking to the wall, and reduces the island size. Several experiments [56], [57], [58], have demonstrated entrainment of a locked island by a rotating resonant magnetic perturbation (RMP) at frequencies in the 5-50 Hz range, limited in frequency by close coupling of the coils to the conducting vacuum vessel wall (Fig. 9).…”

Screening of resonant magnetic perturbation penetration by flows in tokamak plasmas based on two-fluid model

“…As for the dynamic RMP, it can be utilized to unlock the magnetic island and maintain a stable toroidal and poloidal rotation [18]. Lately, experimental and numerical results show that the synergetic application of RMP and electron cyclotron current drive (ECCD) is a promising and effective method to control the NTM [19][20][21]. The RMP can be used as an auxiliary method to lock and locate the phase of the NTM, and then to enhance the accuracy and effectiveness of the ECCD.…”

Disruption avoidance through the prevention of NTM destabilization in TCV

“…Studies of NTM stabilization through temporal power modulation to target the Opoint have been conducted on a range of machines [23,10,24]. Experiments on Asdex Upgrade (AUG) and JT-60 have shown improved suppression efficiency when depositing ECCD only into the O-point of the island [25,26].…”

“…Experiments on Asdex Upgrade (AUG) and JT-60 have shown improved suppression efficiency when depositing ECCD only into the O-point of the island [25,26]. Recent experiments on DIII-D used the most advanced NTM stabilization techniques currently available; resistive magnetic perturbations to entrain a mode, preventing locking to the vessel and establishing a target for modulated ECCD deposition into the O-point [10]. These experiments were not able to achieve the desired result due to technical difficulties.…”