Unconventional Ballooning Structures for Toroidal Drift Waves

TL;DR

This paper reveals that in high confinement mode (H-mode) fusion plasmas, the most unstable drift wave modes exhibit unconventional ballooning structures, which differ from traditional modes and may enhance plasma confinement.

Contribution

It introduces the existence of unconventional ballooning mode structures in H-mode plasmas and links them to different eigen states, expanding understanding of plasma behavior at strong gradients.

Findings

Unconventional mode structures localize at arbitrary poloidal positions.

Most unstable modes in H-mode are not the ground eigen state.

Unconventional modes may lead to improved plasma confinement.

Abstract

With strong gradients in the pedestal of high confinement mode (H-mode) fusion plasmas, gyrokinetic simulations are carried out for the trapped electron and ion temperature gradient modes. A broad class of unconventional mode structures is found to localize at arbitrary poloidal positions or with multiple peaks. It is found that these unconventional ballooning structures are associated with different eigen states for the most unstable mode. At weak gradient (low confinement mode or L-mode), the most unstable mode is usually in the ground eigen state, which corresponds to a conventional ballooning mode structure peaking in the outboard mid-plane of tokamaks. However, at strong gradient (H-mode), the most unstable mode is usually not the ground eigen state and the ballooning mode structure becomes unconventional. This result implies that the pedestal of H-mode could have better confinement than L-mode.