Cyclotron Mode Frequency Shifts in Multi-Species Ion Plasmas*

We observe cyclotron modes varying as $\cos \left( {m_{\theta } \theta -\omega_{m} t} \right)$ on laser-diagnosed multi-species ion plasma columns. These mode frequencies are shifted away from the ``bare'' cyclotron frequency $\Omega_{s} =\left( {q_{s} /M_{s} } \right)B$ for each species s, due to electric fields and plasma rotation. For radially uniform ion species, we observe resonances at frequencies $\omega_{s} =\Omega_{s} +\left[ {m_{\theta } -2+\delta_{s} \left( {1-R_{m} } \right)} \right]\omega _{ExB} $, in close agreement with theory [1]. Here, $\omega_{ExB} $ is the measured ExB rotation frequency, $\delta_{s} $ is the relative fraction of the resonant species, and $R_{m} $ is a small image charge correction. These frequency shifts are measured on the $m_{\theta } =1$ center-of-mass mode, on the $m_{\theta } =2$ ``elliptical'' mode, and on the novel $m_{\theta } =0$ radial ``breathing'' mode. Absent laser diagnostics, these mode frequencies could be used to estimate the plasma composition $\delta_{s} $ and rotation $\omega_{ExB} $. When the plasma is laser cooled to $10^{-5}10^{-1}$ eV, the cyclotron resonances develop a multiple peak structure with temperature-dependent frequency spacing, and a connection to radially standing Bernstein modes [1] will be pursued. \\[4pt] [1] D.H.E. Dubin, Phys. Plasmas \textbf{20}, 042120 (2013).