Electron Acoustic Waves in Pure Ion Plasmas

Electron Acoustic Waves (EAWs) are the low-frequency branch of near-linear Langmuir (plasma) waves: the frequency is such that the complex dielectric function $(D_r , D_i )$ has $D_r = 0$; and ``flattening'' of $ f(v)$ near the wave phase velocity $v_{ph}$ gives $D_i=0$ and eliminates Landau damping. Here, we observe standing axisymmetric EAWs in a pure ion column.\footnote{F. Anderegg, {\it et al.}, Phys. Rev. Lett. {\bf 102}, 095001 (2009).} At low excitation amplitudes, the EAWs have $v_{ph} \simeq 1.4 \bar{v}$, in close agreement with near-linear theory. At moderate excitation strengths, EAW waves are observed over a range of frequencies, with $1.3 \bar{v} < {v}_{ph} < 2.1 \bar{v}$. Here, the final wave frequency may differ from the excitation frequency since the excitation modifies $f (v)$; and recent theory analyzes frequency shifts from ``corners'' of a plateau at $v_{ph}$.\footnote{F. Valentini et al., arXiv:1206.3500v1.} Large amplitude EAWs have strong phase-locked harmonic content, and experiments will be compared to same-geometry simulations, and to simulations of KEEN\footnote{B. Afeyan et al., Proc. Inertial Fusion Sci. and Applications 2003, A.N.S. Monterey (2004), p. 213.} waves in HEDLP geometries.