  We present a calculation of the natural oscillation spectrum of inhomogeneous
non-neutral ultracold plasma. The collective modes of these plasma oscillations
are recorded in experiments as absorption resonances of radio-frequency
electric field. It is shown that in the presence of friction of the electronic
component, a discrete spectrum of plasma eigenoscillations is formed.
Dependence of the frequency of these resonances on the charge imbalance and on
the expansion time is obtained. Good agreement with the experiments of
different authors is noted.
