Observation of the Parametric Two-Ion Decay Instability with Thomson Scattering

We present the first direct experimental observation of the parametric two-ion decay instability of ion-acoustic waves driven by a high intensity ($5\times {10}^{15}\mathrm{W}{\mathrm{c}\mathrm{m}}^{-2}$) laser beam in a laser produced high-$Z$ plasma. Using two separate Thomson scattering diagnostics simultaneously, we directly measure the scattering from thermal ion-acoustic fluctuations, the primary ion waves that are driven to large amplitudes by the high intensity beam, and the two-ion decay products. The decay products are shown to be present only where the interaction takes place and their $k$ spectrum is broad.