Effect of a stochastic electric field on plasma confinement in FTU

TL;DR

This paper models how a stochastic electric field influences plasma confinement in a tokamak, explaining the density limit and linking the electric field's scale to micro-physics, with diffusion depending on magnetic field strength.

Contribution

It introduces a stochastic model for electron behavior in a magnetically confined plasma, explaining the density limit and quantifying the electric field's spatial scale and diffusion dependence.

Findings

The stochastic electric field scale is about a few millimeters.

Diffusion depends on magnetic field as B^{-3/2}.

Model aligns with experimental data on plasma density limits.

Abstract

We discuss a stochastic model for the behavior of electrons in a magnetically confined plasma having axial symmetry. The aim of the work is to provide an explanation for the density limit observed in the Frascati Tokamak Upgrade machine. The dynamical framework deals with an electron embedded in a stationary and uniform magnetic field and affected by an orthogonal random electric field. The behavior of the average plasma profile is determined by the appropriate Fokker-Planck equation associated to the considered model and the disruptive effects of the stochastic electric field is shown. The comparison between the addressed model and the experimental data allows to fix the relevant spatial scale of such a stochastic field. It is found to be of the order of the Tokamak micro-physics scale, i.e. few millimeters. Moreover, it is clarified how the diffusion process outlines a dependence on the magnetic field as $\sim B^{-3/2}$.