Plasma confinement in ring-current configurations

The field from one or several ring-shaped coils can be used as a magnetic bottle for plasma confinement. In stationary operation as a fusion device the coils have to be suspended. A possible method is provided by magnetically screened leads. Earlier investigations by the author on this problem are extended by the following results: In the confinement volume defined by field lines encircling one or several ring-shaped coils a static plasma equilibrium should be possible even in the presence of the magnetic field from the leads. The particle losses introduced by the leads are mainly non-adiabatic. They are partly due to deviations from the constancy of the equivalent magnetic moment in some very small regions where the field is weak, partly to deviations from the constancy of the longitudinal invariant which arise when a particle passes the lead region repeatedly and in finite steps. These effects produce a scattering which displaces a particle randomly across the magnetic field by about one Larmor radius for each complete lead passage. It is equivalent to a kind of ambipolar diffusion across the magnetic field.