Infra-red thermographic inversion in ST40

TL;DR

This paper introduces FAHF, a new Python-based numerical tool for IR thermographic inversion in the ST40 spherical tokamak, enabling precise heat flux calculations on divertor surfaces for edge plasma analysis.

Contribution

The paper presents FAHF, a novel numerical inversion tool that solves the heat conduction equation for IR thermography in spherical tokamaks, validated against simulations and energy balance checks.

Findings

FAHF accurately recovers heat flux with high spatial resolution.

The tool's numerical methods are verified through convergence and energy balance.

FAHF's model agrees within 10% with COMSOL simulations.

Abstract

Infra-red (IR) thermography is an essential diagnostic tool for understanding the edge plasma behavior in fusion devices. In this work, we present a new in-house numerical tool, Functional Analysis of Heat Flux (FAHF), for IR thermographic inversion on Tokamak Energy's spherical tokamak (ST40). FAHF, written in Python, is designed for multi-2D thermographic inversions by solving the heat conduction equation within the divertor tiles using the finite difference method, and an explicit time stepping scheme. Utilising IR camera data with the highest available effective spatial resolution, FAHF calculates the plasma perpendicular heat flux density on the divertor tile surfaces -- a crucial quantity for edge plasma analysis. The tool's internal numerics is first verified through formal time and space convergence analyses, and further corroborated by an energy balance assessment. Although FAHF demonstrates significant sensitivity to user-selected spatial resolution, precise heat flux values are recoverable by ensuring a sufficiently high resolution. Implications for the optimal resolution of both the code and the diagnostic system are discussed. Finally, FAHF's model and geometry simplifications are confirmed to be accurate within 10%, based on comparison with COMSOL Multiphysics simulations. As such, FAHF is proven to be a precise and accurate tool for IR thermographic inversions in ST40.