Convergence, electrostatic potential, and density measurements in a spherically convergent ion focus

Abstract

Unique measurements of the basic plasma-flow characteristics in a low pressure (≤53 mPa H2) spherically convergent ion focus are obtained using high-voltage (≤55 kV) emissive and double probes. The radial plasma potential distribution agrees with a collisionless, recirculating, space-charge-limited current model. Flow convergence increases with voltage and neutral pressure and decreases with cathode grid wire spacing and current. Core radii within 4-5 times the ideal geometric limit are measured, and the observed core sizes are consistent with predictions from a multipass orbit model which includes asymmetries in the accelerating potential well. A virtual anode is observed in the converged core region, and no evidence for multiple potential well structures in the core is found. Measurements of the core ion density (nic ∼ 1015 m-3) are consistent with simple flow convergence models. © 1997 American Institute of Physics.