Ion-Acoustic Wave Instability Driven by Laser-Driven Return Currents

Thomson-scattering measurements of the amplitude and frequency of ion-acoustic waves show an instability when the ion-wave damping is reduced. Experimental results from the OMEGA Laser use simultaneous measurements of the electron-plasma wave and ion-acoustic wave features to characterize the plasma (Te, Ti, Z, Ne) and to directly probe the amplitude of the ion-acoustic waves. The ion Landau damping was varied by changing the target material: CH, V, Ag, Au. The amplitude of the plasma wave increased as the ion Landau damping was reduced and became unstable for ZTe/Ti $>$ 50. As the waves grow to wave-breaking amplitudes, their frequency shifts, and turbulence is expected. These results confirm the speculation that heat-flux--driven ion-acoustic fluctuations exist in laser-produced plasmas, which was previously invoked to increase the collision rate and account for anomalous absorption.\footnote{S. H. Glenzer \textit{et al}., Phys. Rev. Lett. \textbf{88}, 235002 (2002)} This work was supported by Laser Basic Science and the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.