Virtual anode in Hall source of ions

Abstract

Summary form only given. The influence of ion beam electrostatic instability on discharge characteristics of a Hall accelerator has been investigated in this work. It was shown that the existence of positive feedback between decompensation value and ion beam density accomplished by a reflected ion beam leads to the formation of a virtual anode. The virtual anode is the azimuthal-symmetric pulsating hump of a positive potential localized in the exit slit of the Hall accelerator. Under weak amplitude of pulsations (/spl phi//spl Lt/U, where /spl phi/-potential of virtual anode, U-accelerating voltage) the beam cross-section increases. The size of the beam cross-section is more than the width of the exit slit. The appearance of the virtual anode is accompanied by strong modulation discharge current and ion beam current. The modulation depth reaches 30/spl divide/70%. However, the current efficiency is high. Under /spl phi//spl les/U the strong deformation of the time average distribution of the beam current density takes place: the current density decreases on the exit slit and increases on the periphery. The quasi-periodical locking of the ion beam occurs by virtual anode potential. These results indicate the determining role of the virtual anode for forming of the ion beam profile. We suggest a theoretical model describing obtained experimental results.