Driven response of a trapped particle beam

The dynamics of a metastable beam of particles ``injected'' into artificially excited resonance islands in the Fermilab Tevatron is studied. Evolution of the captured beam distribution is measured by an externally detectable coherent signal. Dynamical partitions observed in the control space of amplitude and tune of a focusing modulation are consistent with an equivalent mode of an ensemble of driven gravity pendula. The model's single parameter, the island tune ${\mathit{Q}}_{\mathit{I}}$ of small oscillations, is inferred as the central frequency of resonant detrapping under this weak drive.