Changes to neoclassical flow and bootstrap current in a tokamak pedestal

TL;DR

This paper develops a non-local neoclassical model for tokamak pedestals that accounts for strong radial density variations, finite orbit widths, and electric fields, improving the accuracy of flow and bootstrap current predictions.

Contribution

It introduces a new global delta-f continuum code implementing a non-local approach with the full linearized Fokker-Planck operator, allowing for arbitrary collisionality and aspect ratio.

Findings

Non-local effects significantly modify flow and current profiles.

Strong radial electric fields are incorporated, affecting ion confinement.

The model enhances comparison between experimental measurements and theoretical predictions.

Abstract

In a tokamak pedestal, radial scale lengths can become comparable to the ion orbit width, invalidating conventional neoclassical calculations of flow and bootstrap current. In this work we illustrate a non-local approach that allows strong radial density variation while maintaining small departures from a Maxwellian distribution. Non-local effects alter the magnitude and poloidal variation of the flow and current. The approach is implemented in a new global delta-f continuum code using the full linearized Fokker-Planck collision operator. Arbitrary collisionality and aspect ratio are allowed as long as the poloidal magnetic field is small compared to the total magnetic field. Strong radial electric fields, sufficient to electrostatically confine the ions, are also included. These effects may be important to consider in any comparison between experimental pedestal flow measurements and theory.