Propagation of intense ion beams across a plasma-filled magnetic cusp

Abstract

Preionized plasma has been compared with vacuum and with neutral gas as a medium for the propagation of intense ion beams across magnetic fields. Two cusp-injection ion ring experiments have been used to study the effectiveness of these three cusp-fill media for space-charge neutralization, as shown by the subsequent spreading of the injected rings. In the ion ring experiment a ≲100-ns ion beam was injected into ∼20-eV plasma fills of ≲1012/cm3, giving much better propagation than vacuum, but not as good as 100-mTorr H2 gas. In the long-pulse ion ring experiment with ≳200-ns beam rise time, plasma fill and vacuum gave similar propagation. The results suggest that for complete neutralization of space charge in ion beams propagating across magnetic fields, background media must be provided to meet certain minimum requirements of conductivity and collisionality which depend upon the beam current and rise time.