Magnetic field diffusion and flux loss within a spheromak

The magnetic confinement of a plasma within a prototype controlled-fusion experiment, the spheromak, is considered. This device has a containment vessel that is topologically spherical, offering considerable engineering advantages compared with conventional toroidal systems. The aim has been to evaluate possible designs for the flux conserver and gun magnetic field coils, taking account of flux penetration into the walls, caused by finite resistivity. The copper walls cannot remain perfect magnetic flux surfaces for the duration of the experiment, and the magnetic field penetration into the walls is calculated for a range of designs. This study is in response to recent results showing that wall conditions and flux loss are a vital element of the system´s performance, with a substantial increase in global resistance arising if field becomes embedded in the walls creating a `dead space´ that is not driven by the gun current. A model bearing general application to magnetic field interaction with resistive walls in complex geometries is developed, with particular reference to the UMIST spheromak experiment SPHEX