Magnetic Compression of Compact Tori Experiment and Simulation

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.