Effect of density changes on tokamak plasma confinement

TL;DR

This paper investigates how changes in particle density during tokamak operation induce radial currents and torques, affecting plasma confinement, with derivations from physical, numerical, and kinetic models showing significant impact on reactor performance.

Contribution

It introduces a comprehensive analysis of ionization-induced radial currents and their effects on plasma rotation and confinement in tokamaks, supported by three consistent modeling approaches.

Findings

Radial currents from ionization can be substantial.

Ionization-induced torque influences plasma rotation.

Particle fueling can significantly modify confinement.

Abstract

A change of the particle density (by gas puff, pellets or impurity seeding) during the plasma discharge in tokamak produces a radial current and implicitly a torque and rotation that can modify the state of confinement. After ionization the newly born ions will evolve toward the periodic neoclassical orbits (trapped or circulating) but the first part of their excursion, which precedes the periodicity, is an effective radial current. It is short, spatially finite and unique for each new ion, but multiplied by the rate of ionization and it can produce a substantial total radial current. The associated torque induces rotation which modify the transport processes. We derive the magnitude of the radial current induced by ionization by three methods: the analysis of a simple physical picture, a numerical model and the neoclassical drift-kinetic treatment. The results of the three approaches are in agreement and show that the current can indeed be substantial. Many well known experimental observations can be reconsidered under this perspective. In reactor-grade plasma the confinement can be strongly influenced by adequate particle fuelling.