Prevention of electron cyclotron current drive triggering explosive bursts in reversed magnetic shear tokamak plasmas for disruption avoidance

Abstract

The explosive burst excited by neoclassical tearing mode (NTM) is one of the possible candidates of disruptive terminations in reversed magnetic shear (RMS) tokamak plasmas. For the purpose of disruption avoidance, numerical investigations have been implemented on the prevention of explosive burst triggered by the ill-advised application of electron cyclotron current drive (ECCD) in RMS configuration. Under the situation of controlling NTMs by ECCD in RMS tokamak plasmas, a threshold in EC driven current has been found. Below the threshold, not only are the NTM islands not effectively suppressed, but also a deleterious explosive burst could be triggered, which might contribute to the major disruption of tokamak plasmas. In order to prevent this ECCD triggering explosive burst, three control strategies have been attempted in this work and two of them have been recognized to be effective. One is to apply differential poloidal plasma rotation in the proximity of outer rational surface during the ECCD control process; The other is to apply two ECCDs to control NTM islands on both rational surfaces at the same time. In the former strategy, the threshold is diminished due to the modification of classical TM index. In the latter strategy, the prevention is accomplished as a consequence of the reduction of the coupling strength between the two rational surfaces via the stabilization of inner islands. Moreover, the physical mechanism behind the excitation of the explosive burst and the control processes by different control strategies have all been discussed in detail.

Enhancement of ECCD by the current condensation effect for stabilizing large magnetic islands caused by neoclassical tearing modes in tokamak plasmas

“…Considering the larger the island is, the effectiveness and efficiency of ECCD is more enhanced, the condensation effect is expected to play important roles in dealing with off-normal events in future largescale devices. For example, the explosive burst triggered by ill-advised ECCD reported in [59] may be mitigated on some level while considering the condensation effect. Moreover, the synergistic application of RMP and ECCD [60] may bring other more effective control strategies, which requires more complicated design and will be investigated in future work.…”

Influence of electron cyclotron current drive on the transition between normal sawteeth and the stationary state

“…Although the mechanism of sawteeth, especially for incomplete-reconnection sawteeth, has yet to be fully understood, several methods have been implemented for sawtooth control in experiments. Electron cyclotron current drive (ECCD) is widely used and proven effective for sawtooth control [26][27][28][29][30][31][32][33][34][35]. This is because the localized current driven by Electron cyclotron waves can modify the local magnetic shear and then change the magnetohydrodynamics (MHD) stability condition [24].…”

Effects of counter-current driven by electron cyclotron waves on neoclassical tearing mode suppression

“…[14] However, the most successful approach is still ECCD. Two ECCDs are applied in tokamak plasma with negative magnetic shear configuration to control magnetic islands on two resonant magnetic surfaces to reduce the coupling strength between the two modes, [15] hence suppressing NTMs. ECCD will also be used in the future International Thermonuclear Fusion Reactor (ITER) to control these important neoclassical tearing modes.…”