MASS SPECTROGRAPH FOR IMAGING LOW-ENERGY NEUTRAL ATOMS

We describe an instrument concept for measuring low-energy neutral H a nd O atoms with kinetic energies ranging from about 10 eV to several h undred. The instrument makes use of a low work function surface to con vert neutral atoms to negative ions. These ions are then accelerated a way from the surface and brought to an intermediate focus by a large a perture lens. After deflection in a spherical electrostatic analyzer, the ions are postaccelerated to similar to 25-keV final energy into a carbon-foil time-of-flight mass analyzer. Mass resolution is adequate to resolve H, D, He, and O. Energy and azimuth angle information is ob tained by means of position imaging the secondary electrons produced a t the carbon foil. A large geometric factor combined with simultaneous angle-energy-mass imaging that eliminates the need for duty cycles pr ovide the necessary high sensitivity. From a spinning spacecraft this instrument is capable of producing a 2-D map of low-energy neutral ato m fluxes.