Shielded supports in toroidal multipoles

A simple model for the motion of particles near an X-type neutral line in a magnetic field is described, and is applied to evaluate the particle loss rate along the field zero lines associated with a magnetically shielded support. The same model gives an estimate of the electrical conductivity of a field zero line. Using this estimate the effect of a shielded support on plasma rotating azimuthally around a toroidal machine is discussed; it is shown that the rotating plasma will be brought to best and an estimate of the slowing-down time is given. On the basis of these estimates it is suggested that shielded supports can in principle improve containment in existing experiments. It does not seem likely that they could be used to support internal conductors in a thermonuclear reactor based on a multipole configuration, but in other situations where the plasma layer is thicker their use may be possible.