Representation and modeling of charged particle distributions in tokamaks

TL;DR

This paper presents a new method for representing and modeling charged particle distributions in tokamaks, accommodating large magnetic drifts and enabling conversions, visualization, and analysis within the ITER IMAS platform.

Contribution

It introduces an orbit database approach that accurately maps particle distributions to equilibrium states using guiding center orbits in axisymmetric tokamak geometry.

Findings

Successfully reconstructs drift-induced features of alpha particle distributions.

Enables conversions between various coordinate systems for particle data.

Demonstrates applicability to fusion-born alpha particles in large tokamaks.

Abstract

Experimental diagnostics, analysis tools and simulations represent particle distributions in various forms and coordinates. Algorithms to manage these data are needed on platforms like the ITER Integrated Modelling & Analysis Suite (IMAS), performing tasks such as archiving, modeling, conversion and visualization. A method that accomplishes some of the required tasks for distributions of charged particles with arbitrarily large magnetic drifts in axisymmetric tokamak geometry is described here. Given a magnetic configuration, we first construct a database of guiding center orbits, which serves as a basis for representing particle distributions. The orbit database contains the geometric information needed to perform conversions between arbitrary coordinates, modeling tasks, and resonance analyses. Using that database, an imported or newly modeled distribution is mapped to an exact equilibrium, where the dimensionality is reduced to three constants of motion (CoM). The orbit weight is uniquely given when the input is a true distribution: one that measures the number of physical particles per unit of phase space volume. Less ideal inputs, such as distributions estimated without drifts, or models of particle sources, can also be processed. As an application example, we reconstruct the drift-induced features of a distribution of fusion-born alpha particles in a large tokamak, given only a birth profile, which is not a function of the alpha's CoM. Repeated back-and-forth transformations between CoM space and energy-pitch-cylinder coordinates are performed for verification and as a proof of principle for IMAS.