CONVERGENCE, ELECTROSTATIC POTENTIAL, AND DENSITY-MEASUREMENTS IN A SPHERICALLY CONVERGENT ION FOCUS

Unique measurements of the basic plasma-flow characteristics in a low pressure (less than or equal to 53 mPa H-2) spherically convergent ion focus are obtained using high-voltage (less than or equal to 5 kV) em issive and double probes. The radial plasma potential distribution agr ees with a collisionless, recirculating, space-charge-limited current model. Flow convergence increases with voltage and neutral pressure an d decreases with cathode grid wire spacing and current. Core radii wit hin 4-5 times the ideal geometric limit are measured, and the observed core sizes are consistent with predictions from a multipass orbit mod el which includes asymmetries in the accelerating potential well. A vi rtual anode is observed in the converged core region, and no evidence for multiple potential well structures in the core is found. Measureme nts of the core ion density (n(ic)similar to 10(15) m(-3)) are consist ent with simple flow convergence models. (C) 1997 American Institute o f Physics.