Electron Acoustic Waves in Pure Ion Plasmas

Electron Acoustic Waves (EAW) are the low frequency branch of electrostatic plasma waves. These waves exist in neutralized plasmas, pure electron plasmas and in pure ion plasmas\footnote{F. Anderegg {\it et al.}, PRL {\bf 102}, 095001 (2009) and PoP {\bf 16}, 055705 (2009).} (where the name is deceptive). Here, we observe standing $m_\theta = 0$ $m_z = 1$ EAWs in a pure ion plasma column. At small amplitude, the EAWs have a phase velocity $\mathrm{v}_{\mathrm{ph}} \simeq 1.4 \overline{\mathrm{v}}$, and the frequencies are in close agreement with theory. At moderate amplitudes, waves can be excited over a broad range of frequencies, with observed phase velocities in the range of $1.4 \overline{\mathrm{v}} \leq \mathrm{v}_{\mathrm{ph}} \leq 2.1 \overline{\mathrm{v}}$. This frequency variability comes from the plasma adjusting its velocity distribution so as to make the EAW resonant with the drive frequency. Our wave-coherent laser-induced fluorescence diagnostic shows that particles slower than $\mathrm{v}_{\mathrm{ph}}$ oscillate in phase with the wave, while particles moving faster than $\mathrm{v}_{\mathrm{ph}}$ oscillate 180$^\circ$ out of phase with the wave. From a fluid perspective, this gives an unusual negative dynamical compressibility. That is, the wave pressure oscillations are 180$^\circ$ out of phase from the density oscillations, almost fully canceling the electrostatic restoring force, giving the low and malleable frequency.