The available energy of trapped electrons and its relation to turbulent transport

TL;DR

This paper calculates the available energy of trapped electrons in a plasma and demonstrates its strong correlation with turbulent energy fluxes in gyrokinetic simulations, revealing a proportional relationship.

Contribution

It introduces a fast numerical method to compute available energy of trapped electrons and links it quantitatively to turbulent transport in fusion devices.

Findings

Available energy correlates strongly with turbulent energy flux.

The flux scales as the available energy to the power of approximately 3/2.

The numerical calculation method is computationally efficient.

Abstract

A collisionless plasma possesses a certain amount of "available energy", which is that part of the thermal energy that can be converted into field energy. Here, a calculation is presented of the available energy carried by trapped electrons in a slender non-omnigenous flux tube of plasma. This quantity is compared with gyrokinetic simulations of the nonlinear saturated radial energy flux resulting from turbulence driven by collisionless trapped-electron modes in various stellarator and a tokamak. The numerical calculation of available energy is extremely fast and shows a strong correlation with the turbulent energy fluxes found in the gyrokinetic simulations. Indeed, the energy flux is found to be proportional to the available energy to the power of approximately 3/2, as one would expect from a simple phenomenological model.