[Paper Review] Magnetic Compression of Compact Tori Experiment and Simulation

Carl Dunlea|ArXiv.org|Jan 27, 2026

Superconducting Materials and Applications4 citations

TL;DR

This thesis reports the magnetic compression experiments of compact tori (CT) at General Fusion, and their simulation with the DELiTE finite-element MHD framework, including formation, levitation, compression, and diagnostics analysis.

ABSTRACT

The magnetic compression experiment at General Fusion was a repetitive non-destructive test to study plasma physics applicable to magnetic target fusion compression. A compact torus (CT) is formed with a co-axial gun into a containment region with an hour-glass shaped inner flux conserver, and an insulating outer wall. External coil currents keep the CT off the outer wall (radial levitation) and then rapidly compress it inwards. The optimal external coil configuration greatly improved both the levitated CT lifetime and the recurrence rate of shots with good compressional flux conservation. As confirmed by spectrometer data, the improved levitation field profile reduced plasma impurity levels by suppressing the interaction between plasma and the insulating outer wall during the formation process. Significant increases in magnetic field, electron density, and ion temperature were routinely observed at magnetic compression in the final external coil configuration tested, despite the prevalence of an instability, thought be an external kink mode, at compression. Matching the decay rate of the levitation currents to that of the CT currents resulted in a reduced level of MHD activity associated with unintentional compression by the levitation field, and a higher probability of long-lived CTs. The DELiTE (Differential Equations on Linear Triangular Elements) framework was developed for spatial discretisation of partial differential equations on an unstructured triangular grid in axisymmetric geometry. The framework is based on discrete differential operators in matrix form, which are derived using linear finite elements and mimic some of the properties of their continuous counterparts. A single-fluid two-temperature MHD model is implemented in this framework.

Motivation & Objective

• Investigate how external levitation field configurations affect CT lifetime, compression behavior, and flux conservation.
• Develop and apply a DELiTE-based axisymmetric two-temperature MHD model to simulate CT formation, levitation, and magnetic compression.
• Incorporate plasma-neutral interactions and anisotropic diffusion to understand changes in density and temperature during CT evolution.
• Assess how q-profile and stability against kink modes influence stability and performance during compression.
• Compare simulated diagnostics with experimental measurements to validate the modelling framework.

Proposed method

• Use six-coil and other coil configurations to levitate CTs and induce compression in an insulating outer wall setup.
• Implement a single-fluid two-temperature MHD model within the DELiTE framework on an unstructured triangular mesh to ensure conservation properties (energy, particle count, toroidal flux, angular momentum).
• Formulate discrete operators and finite-element mappings for axisymmetric CT dynamics and derive the continuous MHD equations and their conservation properties.
• Incorporate boundary conditions, external toroidal flux sources, and vacuum fields in insulating regions to simulate CT formation, levitation, and compression.
• Extend the model with a neutral fluid module and radiative-recombination energy terms to study plasma-neutral interactions and edge fueling effects.
• Provide simulated diagnostics that correspond to experimental measurements (e.g., CT radius, density, temperature, magnetic fields) for direct comparison.

Experimental results

Research questions

• RQ1 How does the levitation field profile impact levitated CT lifetime and recurrence of good compression with flux conservation?
• RQ2 Can a DELiTE-based MHD model reproduce CT formation, levitation, and magnetic compression, while conserving key quantities and matching experimental diagnostics?
• RQ3 What role do q-profile and kink-mode stability play in improving CT stability and compression efficiency?
• RQ4 What are the effects of plasma-neutral interactions and edge fueling on observed CT density and temperature during and after formation?
• RQ5 How well do simulated diagnostics agree with experimental measurements across different coil configurations and operating conditions?

Key findings

• External coil configurations that optimize levitation improve CT lifetime and the recurrence rate of compression shots.
• Spectrometer data indicate reduced plasma impurities during formation when levitation fields suppress plasma-wall interactions.
• During magnetic compression, significant increases in magnetic field, electron density, and ion temperature were routinely observed in optimized configurations.
• Matching levitation and CT current decay rates reduces MHD activity from unintentional compression, increasing the probability of long-lived CTs.
• The DELiTE framework enables energy, particle, toroidal flux, and angular momentum conservation in the discrete MHD formulation and supports simulation-based insights into CT formation, levitation, and compression.
• The model incorporating anisotropic thermal diffusion and a neutral fluid module helps clarify mechanisms behind post-formation electron density increases and electrode biasing effects on temperature and density.