How Zonal Flow Affects Trapped-Electron-Driven Turbulence in Tokamak Plasmas

The role of self-generated zonal flows in the collisionless trapped-electron-mode (CTEM) turbulence is a long-standing open issue in tokamak plasmas. Here, we show, for the first time, that the zonal flow excitation in the CTEM turbulence is formally isomorphic to that in the ion temperature gradient turbulence. Trapped electrons contribute implicitly only via linear dynamics. Theoretical analyses further suggest that, for short wavelength CTEMs, the zonal flow excitation is weak and, more importantly, not an effective saturation mechanism. Corresponding controlling parameters are also identified theoretically. These findings not only offer a plausible explanation for previous seemingly contradictory simulation results, but can also facilitate controlling the CTEM instability and transport with experimentally accessible parameters.

Automated summary

This study reveals, for the first time, the formal similarity between zonal flow excitation in collisionless trapped-electron-mode (CTEM) turbulence and ion temperature gradient turbulence, with trapped electrons contributing implicitly only through linear dynamics. The study further finds that for short wavelength CTEMs, the excitation of zonal flow is weak and not an effective saturation mechanism, and identifies corresponding controlling parameters. These findings not only explain seemingly contradictory simulation results, but also have implications for controlling CTEM instability and transport using experimentally accessible parameters.