Measurements of Plasma Wave Decay to Longer Wavelengths

We measure the decay of plasma waves to longer wavelengths, for both ``standard'' Langmuir waves with $\mathrm{v}_{\mathrm{phase}} \gg \overline{\mathrm{v}}$, and for the lower phase velocity ``EAW'' modes with $\mathrm{v}_{\mathrm{phase}} \sim \overline{\mathrm{v}}$. These are $\theta$-symmetric standing modes on pure ion or pure electron plasma columns with discrete wavenumbers $k_z = m_z ( \pi / L_p )$. A large amplitude $m_z \!=\!2$ Langmuir wave causes phase-locked exponential growth of the $m_z \!=\!1$ wave when they are near resonant, at growth rates $\Gamma_e \propto \delta n_2 /n_0$ consistent with cold fluid theory. For larger detuning $\Delta \omega\equiv 2\omega_1 -\omega_2$, mode amplitude $A_1$ is observed to ``bounce'' at rate $\Delta\omega$, with amplitude excursions $\Delta A_1\propto\delta n_2/n_0$ also consistent with cold fluid theory; but $A_1$ often exhibits a slower overall {\it growth}, as yet unexplained by theory. In contrast, a large amplitude $m_z\! =\!2$ EAW mode generally causes either strong phase-locked $m_z \!=\!1$ growth or no growth at all, apparently because the EAW ``frequency fungibility'' enables $\Delta\omega =0$, and EAW mode damping is strong until the velocity distribution $F (\mathrm{v}_{\mathrm{phase}} )$ is ``flattened.''