Ta. Thorson et al., CONVERGENCE, ELECTROSTATIC POTENTIAL, AND DENSITY-MEASUREMENTS IN A SPHERICALLY CONVERGENT ION FOCUS, Physics of plasmas, 4(1), 1997, pp. 4-15
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.