Observation of hydrodynamic phenomena of plasma interaction in hohlraums

Abstract

In indirect-drive inertial confinement fusion (ICF), laser beams are injected into a high-Z hohlraum and the laser energy is converted into intense X-ray radiation, which ablates a capsule located in the center of the hohlraum, and thus making it implode. To achieve high implosion efficiency, it is required that the hohlraum inner wall plasma movement, which will block further laser injection through the laser entrance hole (LEH), be suppressed. Evolution of hohlraum radiation nonuniformity caused by the plasma movement will result in implosion asymmetry which will prevent the ignition from happening. Therefore it is very important to study the hydrodynamic movement of high-Z plasma in ICF experiment.<br/>In ICF hohlraum, various plasmas of laser spots, corona, radiation ablation and jets move in different ways driven by laser ablation and X-ray radiation ablation, which is hard to observe and study. An X-ray dual spectral band time-resolved imaging method is developed to clearly observe the motion of various plasmas in hohlraum. Based on the time-resolved X-ray framing camera, using the typical gold plasma emission spectrum, the gold microstrip MCP response spectrum, and the 1.5 μm Al or 3 μm Ti filter transmittance spectrum, the two narrow-band X-ray peaks at 0.8 keV and 2.5 keV are highlighted. The 0.8 keV X-ray shows the Planck spectrum of gold plasma, and 2.5 keV X-ray indicates the M-band of gold plasma.<br/>In the vacuum hohlraum, jets are observed clearly, which are verified to be 4 times the sound speed experimentally. The generation mechanism of gold plasma jets in the ICF hohlraum is mainly due to collision rather than magnetic field, because it is estimated that thermal pressure is much bigger than magnetic pressure. In the gas-filled hohlraum, low-Z C₅H₁₂ gas can effectively eliminate high-Z gold jets and suppress the high-Z gold coronal plasma movement. The interface between the low-Z and high-Z substance is observed clearly, and gold plasma is accumulated obviously in the later period at the interface. Moreover, spike and filamentous structure occur at the interface between the two substances, which is probably caused by the hydrodynamic instability. The 0.8 keV rather than 2.5 keV X-ray is observed around inner wall, which originates from the low-temperature plasma driven by radiation ablation and is predicted by simulation code. Furthermore, the pressure balance between the two substances and the density steepness at the interface are also analyzed.