# Equilibrium reconstruction with 3D eddy currents in the Lithium Tokamak eXperiment

## TL;DR

This paper presents a method for axisymmetric equilibrium reconstruction of tokamak plasmas in LTX, accounting for 3D eddy currents from conducting structures, leading to improved diagnostic agreement and magnetic field predictions.

## Contribution

It introduces a novel approach to include 3D eddy currents in equilibrium reconstructions using fixed poloidal sources and simplified filament models.

## Key findings

- Reconstructed equilibria agree well with diagnostic signals.
- Predicted 3D eddy current-induced magnetic perturbations.
- Method effectively incorporates non-axisymmetric effects into axisymmetric models.

## Abstract

Axisymmetric free-boundary equilibrium reconstructions of tokamak plasmas in the Lithium Tokamak eXperiment (LTX) are performed using the PSI-Tri equilibrium code. Reconstructions in LTX are complicated by the presence of long-lived non-axisymmetric eddy currents generated by vacuum vessel and first wall structures. To account for this effect, reconstructions are performed with additional toroidal current sources in these conducting regions. The source distributions are fixed poloidally, but their scale is adjusted as part of the full reconstruction. Eddy distributions are computed by toroidally averaging currents, generated by coupling to vacuum field coils, from a simplified 3D filament model of important conducting structures. The full 3D eddy current fields are also used to enable the inclusion of local magnetic field measurements, which have strong 3D eddy current pick-up, as reconstruction constraints. Using this method, equilibrium reconstruction yields good agreement with all available diagnostic signals. An accompanying field perturbation produced by 3D eddy currents on the plasma surface with primarily n=2, m=1 character is also predicted for these equilibria.