Theory of mass-discrimination effects in ion extraction from a continuum plasma

Mass-discrimination effects in stagnation-point ion extraction are treated for continuum conditions lambda << Rsub(p), lambda << lambdasub(D), by combining the results of the following ion flux calculations: (a) an analytic solution for arbitrarily shaped sampling electrodes in the Laplace limit lambdasub(D) >> Rsub(p), without flow, (b) numerical solutions for the front stagnation point of spheres in flow, for Rsub(p) << lambdasub(D) and Rsub(p) >> lambdasub(D), and (c) numerical solutions for spheres at finite Rsub(p)/lambdasub(D), without flow. Here lambda is ion mean free path, Rsub(p) is electrode radius or characteristic size, lambdasub(D) is Debye length. The treatment is applicable to sampling electrode Reynolds numbers Re from 0 to 103, and ion Schmidt numbers Scsub(i) from 0 to 104. Results are presented for various values of scaled electrode (extraction) potential phisub(p)=eVsub(p)/kTsub(i) relative to the plasma. For the nonflowing limit, correction formulas for electrode shape are given. A prediction is made that in the early stages of a decaying afterglow, effects of Rsub(p)/lambdasub(D) can cause ion density underestimates which may affect measurements of chemical constants. Flow effects are shown to decrease mass-discrimination. Large extraction potentials suppress flow effects and therefore increase mass-discrimination. (Auth.)