Understanding effects of radiation from radiative shock on Richtmyer-Meshkov instability

Abstract

<sec>Radiative shocks are ubiquitous in stellar environments and are characterized by high temperature plasma emitting a considerable fraction of their energy as radiation. Radiative shocks occur commonly in nature, especially in astronomical systems and inertial confinement fusion. The study of the effects of radiation on Richtmyer-Meshkov (RM) instability will improve our ability to understand and predict the evolution of RM instability under high energy density conditions.</sec><sec>A few experiments have been performed to compare the radiative case with the non-radiative case in Rayleigh-Taylor (RT) instability, thereby studying how the radiative effects change the evolution of RT instability, but the interplay between RM instability and radiative shock has been studied rarely. </sec><sec>This paper reports mainly the role of radiation in the changing of the RM instability. Two experiments are performed at Shenguang III prototype laser facility, the RM instability growth data are obtained by varying the laser intensity. The laser intensity for high-drive experiment is approximately 60% greater than that for low-drive experiment. The target consists of a multiple layer in the axial direction, in which the first layer is a 15μm-thick CH sample serving as an ablator, followed by a 10 μm-thick aluminum used as a shield layer to prevent the preheat effect. The next layer is a 350-μm-thick SiO₂ foam, which is used as a material to produce a radiative shock. The last layer is the CH perturbed sample. There is a sinusoidal perturbation on the surface of CH sample which is adjacent to the SiO₂ foam. The target is irradiated by four overlapping laser beams, and the laser beams produce a large pressure that drives a shock wave, whose velocity can be changed by varying the laser intensity, into the target package.</sec><sec>In the experiments, shock-generated radiative fluxes first ablate the unstable interface which the shock has not passed through, then the shock transmits the unstable interface to produce the RM instability. The images of unstable interface are captured using side-on x-ray radiography, and the experimental results show that the RM growth is suppressed in the experiment for the higher laser intensity. Radiation hydrodynamic code Multi1D is used to evaluate the electron temperature, shock velocity, and electron density. The simulations show that the foam temperature in the high-drive case can reach 80 eV in the front of shock, this energy flows away from the shock front, generating a radiative precursor ahead of the shock. The radiative precursor velocity of 270 km/s is much larger than the shock velocity of 170 km/s, the radiative precursor arrives at the unstable interface before the shock and ablates the unstable interface, so the radiative flux changes the initial conditions of unstable interface. When the shock propagates through the unstable interface, the ablation increases the density gradient length scale and reduces the Atwood number of the unstable interface, so the RM growth is suppressed in the high-drive case because of the ablation of the radiative precursor.</sec>