A method of producing and accelerating an ion beam comprising the steps of: providing
a magnetic field with a cusp that opens in an outward direction along a centerline
that passes through a vertex of the cusp: providing an ionizing gas that sprays
outward through at least one capillary-like orifice in a plenum that is positioned
such that the orifice is on the centerline in the cusp, outward of the vortex of
the cusp; providing a cathode electron source, and positioning it outward of the
orifice and off of the centerline; and positively charging the plenum relative
to the cathode electron source such that the plenum functions as an anode. A hot
filament may be used as the cathode electron source, and permanent magnets may
be used to provide the magnetic field.