Negative-mass Instability in Nonlinear Plasma Waves
Authors: I.Y. Dodin, P.F. Schmit, J. Rocks and N.J. Fisch
Abstract:
The negative-mass instability (NMI), previously found in ion traps, appears as a distinct regime of
the sideband instability in nonlinear plasma waves with trapped particles. As the bounce frequency
of these particles decreases with the bounce action, bunching can occur if the action distribution
is inverted in trapping islands. In contrast to existing theories that also infer instabilities from the
anharmonicity of bounce oscillations, spatial periodicity of the islands turns out to be unimportant,
and the particle distribution can be unstable even if it is at at the resonance. An analytical model
is proposed which describes both single traps and periodic nonlinear waves and concisely generalizes
the conventional description of the sideband instability in plasma waves. The theoretical results are
supported by particle-in-cell simulations carried out for a regime accentuating the NMI effect.
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Submitted to: Physical Review Letters, (January 2013)
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