Saturated helical mode in EAST high β hybrid plasmas

Abstract

A hybrid scenario with the minimum safety factor above unity and a safety factor profile with a wide range of low magnetic shear in the core is one of the candidates for ITER operations. Theory predicts that such equilibria are prone to quasi-interchange mode, which was confirmed by observations from several devices. The presence of those 3D quasi-interchange helical modes has both positive and negative effects on plasma performance. On the one hand, they can avoid low mode number instabilities such as sawteeth. On the other hand, they would lead to confinement degradation, significant fast ion losses and toroidal rotation damping. Such modes are observed for the first time in EAST high βp hybrid plasmas achieved with off-axis neutral beam injection (NBI) heating. The mode manifested itself as long-lived saturated helical instability. Notably, when the mode was destabilized, the toroidal rotation profile in the core was completely flattened. The extent of the flattened rotation profile reaches up to half of the minor radius. As the minimum safety factor qmin approaches unity, strong n = 2 and n = 3 harmonics are successively driven unstable as nonlinear consequences of the n = 1 mode. A 3D resistive nonlinear simulation with a realistic tokamak configuration has been applied to reproduce the evolution of long-lived helical instability with the M3D code. The simulated magnetic perturbation shows a strong n = 2 component. As the mode grows, the amplitude of the n = 2 harmonic grows immensely with respect to that of the n = 1 harmonic. The n = 2 harmonic gradually becomes dominant as the mode saturates. In saturation, the initial weak shear safety factor profile in the core becomes completely flat. This change of safety factor profile in the core corroborates a self-regulating magnetic flux pumping mechanism which is considered responsible for maintaining stationary non-sawtoothing hybrid discharges.