The magnetic coherent mode with shifted Alfvén gap frequency destabilized by the thermal trapped electron resonance in the pedestal of high-confinement tokamak plasmas

Abstract

The magnetic coherent modes (MCM) with toroidal mode number n about 1 (Chen R. et al 2018 Nucl. Fusion 58 112004) frequently appear in the edge pedestal of high-confinement tokamak plasmas on EAST in the absence of energetic particles. Although these modes are experimentally compatible with the steady-state operation of the pedestal, the driving mechanism without energetic particles of MCM is a long-standing mystery. To reveal the excitation mechanism, a fluid-drift kinetic hybrid local linear model has been developed. It is found that MCM is a new Alfvén eigenmode with a gap frequency much lower than the ideal Toroidal Alfvén Eigenmodes (TAEs) with two significant properties: (1) due to the unique steep pressure gradient in the pedestal region, the diamagnetic frequency becomes comparable to the ideal TAE frequency, which makes the Alfvén continuum in this region move significantly in the ion diamagnetic direction and form a gap of lower frequency; (2) due to the bounce frequencies of thermal electrons becoming also comparable to the ideal TAE frequency in the pedestal region, the free energy of the pressure gradient can be fed into the MCM through the thermal electron bounce resonance excitation, which is essentially the coupling between the shifted TAEs and low- n trapped electron modes. The low- n MCM is proved to be a shifted Alfvén gap mode in the pedestal region, which is anticipated to exist in low collisional plasmas of future fusion reactors. It is of great significance to carry out relevant physical model research to enhance the understanding of pedestal physics.