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Modulational instability of a circularly polarized electromagnetic
wave, propagation along an external constant magnetic field in
plasma is investigated. the method is based on the derivation of a
nonlinear Schrodinger equation, which contains nonlinear terms
associated with trapped electrons and also relativistic electron
quiver velocity. Among different mechanisms of trapping the
collision-less process is considered. Since we are interested in
characteristic spatial scales larger than the Debye length, the
condition of quasi-neutrality is assumed. The maximum growth rate
is calculated and the result shows that the trapped particles
prevent the spatial localization of the electron cyclotron waves.
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