PLASMA CONFINEMENT IN GENERAL ATOMIC TOROIDAL OCTOPOLE

Plasma confinement in the toroidal octopole device is described. The octopole field is produced by four current-carrying rings which are held in place by supporting rods. A toroidal field may also be applied. The plasma is injected from a coaxial gun. The density is variable from 108 cm -3 to 1011 cm-3 and the ion and electron temperature is approximately 100 and 6 eV, respectively. Measurements are performed by neutral and ion analyzers and by electrostatic and magnetic probes. The ion confinement time depends on the plasma density. At densities higher than 1011 cm-3, the high-energy ions are lost in tens of microseconds by charge exchange interactions with the gas released by plasma bombardment near the injection window. At plasma densities below 1011 cm-3 the ion confinement tune is several hundreds of microseconds at 100-eV ion energies. The decay is consistent with the loss to the supporting rods. The decay rate is at least an order of magnitude less than predicted from the Bohm diffusion coefficient.