Cyclotron resonance phenomena in a non-neutral multispecies ion plasma

Cyclotron modes of a non-neutral Mg ion plasma were studied in a long cylindrical Penning–Malmberg trap. Several modes with angular dependence exp(ilθ), l≥1, are observed near the cyclotron frequencies of the various Mg ions. The l=1 modes for the majority species are downshifted from the cyclotron frequencies, with downshifts as large as four times the diocotron frequency. These large shifts are quantitatively explained by a multispecies cold-plasma theory, including the dependence on the plasma size and composition. These dependencies allow the plasma size and composition to be obtained from the measured mode frequencies. In contrast, the l=1 downshifts for minority species are generally close to twice the diocotron frequency, and remain unexplained. Cyclotron heating of the plasma ions was also observed with a surprising effect of improving the plasma confinement.