Magnetic levitation and compression of compact tori

Abstract

The magnetic compression experiment at General Fusion was a repetitive non-destructive test to study plasma physics 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 maintain the CT off the outer wall (radial levitation) and then rapidly compress it inward. The optimal external coil configuration greatly improved both the levitated CT lifetime and the rate of shots with good flux conservation during compression. 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 the magnetic field, density, and ion temperature were routinely observed at magnetic compression despite the prevalence of an instability, thought be an external kink, at compression. Matching the decay rate of the levitation coil currents to that of the internal 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. An axisymmetric finite element MHD code that conserves system energy, particle count, angular momentum, and toroidal flux was developed to study CT formation into a levitation field and magnetic compression. An overview of the principal experimental observations and comparisons between simulated and experimental diagnostics are presented.