Electrostatic reflex plasma source as a plasma bridge neutralizer

Abstract

A new type of plasma source is described, to be used as a plasma bridge neutralizer (PBN) for space and current compensation of low energy ion beams. In contrast to existing hollow cathode PBN, the present design has a discharge which operates down to a low neutral pressure (1–5×10 −3 torr), so that the plasma jet now effuses through an aperture which may be quite large (5–12 mm), according to the beam current to be compensated. Two anodes are provided: one is located within the source chamber whereas the second is outside, near the plasma emission aperture. Thus two different coupling modes of the PBN are possible. Typical performance and lifetime are reported for the neutralization of reactive beams (O 2 or CF 4). With a 6 mm emission aperture and the external anode it is possible to neutralize 230 mA ion beams, the operation parameters being: coupling voltage— 40 V; discharge current—2 A; argon flow rate—1.3 sccm; total beam chamber pressure–1×10 −4 torr. For these conditions the filament lifetime is: 60 h for O 2; 150 h for Ar and 250 h for CF 4.