Plasma confinement by localized magnetic cusps

Magnetic confinement of a plasma is studied in a thin walled 24 liter chamber surrounded by external movable walls of magnetic dipoles. Full-line cusps were found to be the most effective arrangement, with an optimum spacing of 8 cm for the 24 liter vessel. Such a magnetic array improves the confinement of primary ionizing electrons by two orders of magnitude over a chamber without confinement, and improves ion confinement by a factor between 2 and 3. The details of cusp confinement were studied using a picket-fence (current produced line cusps). Results give additional evidence that the primary electrons are confined very efficiently by line cusps, with weaker confinement of plasma ions. Probe scans behind the picket-fence with helium, argon and xenon plasmas show that the half-width of the leakage aperture of plasma electrons through the line cusp is twice the hybrid gyroradius (r/sub e/r/sub i/)/sup 1/2/ with r/sub e/ and r/sub i/ as the electron and ion gyroradii. This dependence differs from existing theories. The primary electrons are found to have leakage half-widths approximately equal to their gyroradii. Time evolution of the leakage widths are given. Steady state widths are reached in a time less than the ion gyro-period