Experimental results on trapping a gun plasma in a toroidal magnetic cusp experiment

A start-up method for producing a plasma in the bi-cusp field configuration of a toroidal magnetic cusp (TORMAC) is described. The method uses the radial injection and trapping of a toroidal gun plasma. Measurements of an injected plasma with a velocity of 17 μsec^-1 and 4·5 × 10¹⁸ particles is presented. The plasma was observed to be stopped and trapped in an equilibrium position. A well-defined outer boundary remained stationary for 20 μsec. Particle flux distribution emanating from the cusp field lines defined a sheath having a width of 1-1·5 ion gyroradii in the poloidial field. This translates to a narrow outer boundary and a broad inner boundary based on the gradient of the poloidial field at the two radial positions. Measurements of Thomson scattering and interferometry give a T_e of 15eV, a 15μsec density decay time, and a 5μsec energy decay time. These results show that this injection and trapping method is successful, and thus a higher gun plasma energy combined with a flux conserving barrier may lead to higher temperatures for testing containment in TORMAC.