Langmuir wave self-focusing versus decay instability
Electron trapping in a finite amplitude Langmuir wave (LW) leads to a frequency shift, Δω_{TP} < 0, and reduced Landau damping. These may lead to modulational instability. Its growth rate and damping threshold, due to escape of trapped electrons at rate ν, are calculated for the first time in the short wavelength regime. If the background plasma is in thermal equilibrium, it is shown that this trapped particle modulational instability (TPMI) is not possible when k λ_D > 0.46, while for 0.33 < k λ_D < 0.46, TPMI requires that the fluctuation wavevector have a component perpendicular to k, the LW wavevector, with λ_D the electron Debye length. Its nonlinear evolution leads to self-focusing. Comparison is made with a re-evaluated LW ion acoustic decay instability (LDI): compared to classical estimates, the new LDI threshold is lowered by primary LW Δω_{TP} since frequency matching leads to wavenumber and hence damping reduction of the daughter LW. For parameters estimates relevant to a recent stimulated Raman scatter experiment (Kline et al., submitted to PRL), the LDI and TPMI thresholds cross in the range 0.28 < k λ_D < 0.34, consistent with the observed LDI regime change. However, if νexceeds a critical value, estimated to be order 1% of the electron plasma frequency, then TPMI is not possible at any wavenumber.