Confinement properties of a low-beta discharge in a spindle cusp magnetic field

An experimental study of the confinement properties of a low-(average) beta discharge plasma in a spindle cusp magnetic field is described. Electron and ion densities, space potentials, and plasma flow velocities were measured in the ring and point cusps. The leak width of the escaping plasma was measured over a large range of magnetic field strengths and neutral pressures. Leak widths varying between the hybrid gyrodiameter and the ion gyrodiameter were observed. The dependence of the leak width on neutral pressure and magnetic field is accounted for by a simple model in which plasma diffuses across the magnetic field as a result of neutral particle collisions while streaming out of the cusps along the magnetic field lines. At the lowest neutral pressures investigated, the scaling of the leak width with magnetic field strength suggests that noncollisional mechanisms (e.g., Bohm diffusion) determine the leak width