Description
Disruptions remain a major concern for the reliable operation of tokamaks. Impurity injection is therefore employed to radiate thermal energy prior to the thermal quench, yet achieving sufficiently deep and uniform impurity deposition remains challenging. On the EAST tokamak, we report a rapid event in which impurity penetration is abruptly intensified. The inferred inward impurity pinch increases by approximately an order of magnitude, nearly coincident with a sharp transition in the poloidal propagation of a 2/1 magnetic island on a sub-millisecond timescale.
Based on measurements from toroidal Mirnov coil arrays, following impurity injection the 2/1 mode evolves toward rotation in the electron-diamagnetic direction within <0.5 ms. In NBI-heated plasmas, the mode frequency switches from +2.5 kHz to −3 kHz, while in RF-heated plasmas it shifts from −5 kHz to −11 kHz. Closely synchronized with this transition, the impurity inward pinch inferred from bolometer measurements increases from ~60 m/s to ~300 m/s, allowing impurities to penetrate inside the q = 2 surface.
These coupled observations can be consistently interpreted as arising from the rapid development of a negative radial electric field of order −10 kV/m, which simultaneously modifies the poloidal rotation of the 2/1 island and enhances inward impurity transport. Within a neoclassical quasi-balance framework with time-scale separation, the toroidal rotation can be treated as quasi-stationary over the observed interval. Enhanced ion–impurity collisions modify the parallel main-ion viscosity and friction, requiring a more negative radial electric field to maintain instantaneous force balance. Owing to their higher charge state, impurities experience a preferentially strengthened inward convection.
Overall, these results advance the understanding of impurity penetration physics and suggest new pathways to optimize impurity injection strategies for disruption mitigation in future fusion devices.