Further study on a magnetically filtered multicusp ion source

Abstract

An opti.mized permanent magnet filter geometry has been determined for a multi cusp ion source opera ted with hydrogen gas. By applying a negative bias potentia 1 to the plasma grid, energetic secondary electrons are produced which can generate molecular hydrogen ions in the extraction chamber and thus reduce the H+ ion fraction in the extracted beam.Both the atomic species percentage and the ion current density can be improved by the addition of 16 eV electrons into the extraction chamber.It is also observed that once started, the discharge can be maintained with only the extraction chamber and the plasma grid acting as the anode. Thus the total discharge current can pass through the filter in the form of cold electrons.

Relevance of Volume and Surface Plasma Generation of Negative Ions in Gas Discharges

“…At low electron temperature the rate of negative ions destruction decreases and they can be extracted from a larger generation area. Features of these sources were discussed in many publications, references to which are presented in reviews [1,6,7,8,9,13,20,31,32]. It is considered that the increase of proton components in tandem sources is caused by effective dissociation of molecular ions in the area of the magnetic filter by cold electrons.…”

New Developments with H− Sources

“…This work led to the suggestion that vibrationally excited H~( v ) molecules were fundamental to the enhanced H' production, which led to furlher experimenta1 [22,23] and theoretical [24] work confirming a two-step process in which cold electrons disassociate vibrationally excited H~( v ) molecules to Ji-and atomic hydrogen. This early work provided the basis for the two-chamber H-volume source where a driver region with high electron temperature is separated from the H-production region with a low electron temperature by a magnetic filter [25,26], A recent article [2'7] has summarized cross section data for electron-impact inelastic processes on vibrationally excited H2 molecules. Multicusp magnetic fields are usually used for plasnia confinement, and optimization of the multicusp field with a magnetic filter is described in [28].…”

Volume H− ion production experiments at LBL

“…15,16 In order to increase the geometric transparency and to provide adequate cooling, the filter was constructed by inserting square permanent magnet rods into copper tubes through which a square broach had been passed. Since the magnets rested on the four broached grooves, their orientation was fi xed and water was then run through the remaining space to provide cooling.…”