Observation of an Energetic-Particle-Driven Instability in the Wall-Stabilized High-βPlasmas in the JT-60U Tokamak

Abstract

We have observed a bursting mode in the high-$\beta$ plasmas above the ideal $\beta$ limit without a conducting wall. The mode frequency is chirping down as the mode amplitude increases, and its initial value is close to the precession frequency of the trapped energetic particle from the perpendicular neutral beams. The mode structure is radially extended with a peak around the $q=2$ surface. This mode can finally trigger the resistive wall mode (RWM) despite enough plasma rotation for RWM stabilization. It is concluded that the mode is driven by trapped energetic particles. The mode is attributed to the interaction between the trapped energetic particles and a marginally stable mode in the wall-stabilized high-${\beta }_N$ region.