Nontrapping arrest of Langmuir wave damping near the threshold amplitude

Evolution of a Langmuir wave is studied numerically for finite amplitudes slightly above the threshold which separates damping from nondamping cases. Arrest of linear damping is found to be a second-order effect due to ballistic evolution of perturbations, resonant power transfer between field and particles, and organization of phase space into a positive slope for the average distribution function $f_{av}$ around the resonant wave phase speed $v_\phi$. Near the threshold trapping in the wave potential does not arrest damping or saturate the subsequent growth phase.