A new diagnostic technique to simultaneously measure the electron temperature, ionic charge state, and plasma density near the critical surface in laser-plasma interaction experiments

We have studied the dependence of the second harmonic signature of the ion acoustic decay instability on the target material in laser produced plasmas. Well-defined Stokes peaks were observed. The Stokes peaks were especially sharp for high Z targets. The threshold of the IADI was quite low even in high Z plasma. It increased only weakly with increasing the atomic number. On the other hand, the Stokes intensity decreased strongly with increasing atomic number. Linear theory, combined with the results of hydrodynamic-computer-simulation (LASNEX) calculation, explained our experimental results fairly well. The effective laser intensity is strongly reduced by the increased collisionaldamping of electron plasma wave in high Z plasma, so that the Stokes intensity was reduced strongly. Hence, strong electron heating due to the IADI is less likely in high Z plasma. On the other hand, in the context of the threshold, the collisional damping effect of the electron plasma wave is canceled due to the opposite trend of the damping of the ion acoustic wave which decreases with ZTe/Ti. The low-threshold, and the sharp and steady spectrum make the IADI a good diagnostic of local plasma conditions near the critical surface for high Z plasma. The ionic charge state Z is estimated by measuring the ion acoustic wave frequency of the most unstable wave excited by the IADI. For Mo plasma, we had measured value of Z=30. We can also estimate the plasma density of the instability region using Bohm-Gross dispersion relation of the plasma wave. We have obtained plasma density Ne/Nc = 0.86. We have estimated plasma flow effects on the LADI by measuring the angular distribution of the IADI emission. Our results indicate that the flow effect is not important for the IADI in our experiments