MASS-DISCRIMINATION IN ION AND NEUTRAL EXTRACTION BY MASS-SPECTROMETERS UNDER SPACECRAFT CONDITIONS

A calculation of extraction mass-discrimination effects on absolute ion and neutral density determination in a space plasma, is presented. In the presence of mass-discrimination, the ion composition is given by the expression N1∞8= N∞8 1+ I2 I1G12 + I3 I1G13+ I4 I1G14+⋯, for ion species 1, where N∞8 is the total ambient ion number density obtained by other measurement techniques, I is the ion signal in the mass-spectrometer, Gjk= δjkHjk is a total mass-discrimination factor, δjk is the mass-discrimination factor for the ion extraction process, Hjk is the mass-discrimination factor for the mass-spectrometer including its inlet orifice, and the subscripts j,k = 1,2,3 ... refer to the ion species. Determination of Hjk is usually experimental. We have obtained theoretical predictions of δjk for unipotential spherical spacecraft in collisionless flowing plasmas. The treatment has been done for orbit-limited conditions (Debye length sufficiently large), for ion speed ratios Si = U ( 2kTi mi) 1 2 from 0 to 20, and for θ{symbol}p = eVp kTi from 0 to -10, where Vpis sampling electrode(i.e. spacecraft) potential. For two ion species of masses mj and mk, with mk > mj, the numerical results show that: (1) for front-axial-point sampling, δjk may be a non-monotonic function of Sj(or Sk) and increases with increasing θ{symbol}p; (2) for rear-axial-point sampling, δjk increases with increasing Sj and decreases with increasing θ{symbol}p. © 1979.