An improved low-pressure dc discharge

We describe a scheme in which the primary beam electrons are electrostatically confined in a low-pressure dc discharge plasma which can substantially improve the density (300 or 400% in our case). The electrons are heated to 2 eV with a fraction thermalized to ∼10 eV by the extended presence of the primary beam electrons. Ion noise observed was not changed substantially from that without confinement. (It was slightly better than unconfined.) The plasma noise is strongly dependent on filament geometry. We also describe a second electrostatic confinement scheme in which both the primary electrons and the plasma are confined, resulting in a further density increase.