Space charge frequency shifts of the cyclotron modes in multi-species ion plasmas.

Abstract

Shifts of the cyclotron frequency away from the "bare" cyclotron frequency are observed to be proportional to the total ion density through the E × B rotation frequency, and to the relative concentration of each ion species, in quantitative agreement with analytic theory. These shifts are measured at small excitation amplitudes on the typical center-of-mass m = 1 mode, and also on cyclotron modes with m = 0 and m = 2 azimuthal dependence. The frequency spacing between these modes is proportional to the rotation frequency of the ion cloud, which is controlled and measured using a "rotating wall" and laser-induced fluorescence. These cylindrical ion plasmas consist of Mg(+) isotopes, with H3 O (+) and O2 (+) impurities. It is observed that the shift in the m = 1 cyclotron frequency is larger for the minority species (25)Mg(+) and (26)Mg(+), than for the majority species (24)Mg(+). A simple center-of-mass model is presented, which is in quantitative agreement with these results. It is also shown that this model interprets and expands the intensity dependent calibration equation, (M/q) = A/f + B/f (2) + CI/f (2).