Ion Acoustic Instability Resulting from Suprathermal Electrons Generated by Stimulated Raman Scattering in...

Using space-time-resolved Thomson scattering, we investigate experimentally the development of electrostatic waves in a preformed undercritical helium plasma driven by a 1.059  μm laser pulse of ∼1.5  ps duration and ∼10^{15-16} W cm^{-2} mean intensity. We observe the excitation of intense ion plasma waves (IAWs) over a broad wave number range, distinct from what is expected from stimulated Brillouin scattering or Langmuir decay instability. These IAWs are correlated with the occurrence of backward stimulated Raman scattering (B-SRS) at earlier times but persist much longer than the B-SRS-driven electron plasma waves. Particle-in-cell modeling reveals that they result from an ion acoustic instability triggered by the bulk electrons drifting against the ions to neutralize the current carried by the B-SRS-generated hot electrons. The properties of the simulated unstable IAWs match well with the measurements.