William L Chisholm

US Patent:

47422327, May 3, 1988

Filed:

Mar 3, 1987

Appl. No.:

7/021100

Inventors:

Alan P. Biddle - Euless TX
Nobie H. Stone - Huntsville AL
David L. Reasoner - Huntsville AL
William L. Chisholm - Huntsville AL
John M. Reynolds - Huntsville AL

Assignee:

United States of America as represented by the Adminstrator, National
Aeronautics and Space Administration - Washington DC

International Classification:

H01J 2720

US Classification:

250427

Abstract:

A vacuum chamber (12) within which an instrument (22) to be calibrated or tested is placed, is fitted with an ion gun (14) having an ion source (48). The source (48) has an electron emitting filament (60) positioned adjacent one end of an ionization chamber (71), with a negatively biased grid (72) located behind the filament (60). Gas is injected into the source (48) by a gas flow regulator (54) in one end (52) of the source (48). The chamber (71) is surrounded by a plurality of independently energizable coils (82, 84, 86, 88, 90), with the last coil (90) being operated at the highest current level, thus producing the highest magnetic flux. This presents a region (160) of magnetic repulsion to the electrons produced by the filament (60) and causes them to be confined between the grid (72) and the region (160), greatly increasing the chances that an ionization collision will occur between the electrons and atoms of injected gas. Ions are extracted from the ionization chamber (71) by a negatively biased extractor grid (106) positioned adjacent an opposite end (80) of the chamber (71) and are collimated by a negatively biased shield grid (110) positioned adjacent the extractor grid (106).