338 / 2019-02-26 19:42:55

Observations of the hydrodynamic phenomena of plasma interaction in hohlraums

hohlraum, plasma, hydrodynamic phenomena, X-ray dual spectral band

全体主题 > Laser and Particle Beam Fusion

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 at the center of the hohlraum and makes 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 ignition. Therefore it is very important to study hydrodynamic movement of high-Z plasma in ICF experiments.
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 was developed to clearly observe the motions 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 and 3 μm Ti filter transmittance spectrum, the two narrow-band X-ray peaks at 0.8 keV and 2.5 keV were highlighted. 0.8 keV indicated Planck spectrum of gold plasma and 2.5 keV indicated M-band of gold plasma respectively.
In the vacuum hohlraum, jets were observed clearly, which was verified as 4 times sound speed by experiment. The generation mechanism of gold plasma jets in the ICF hohlraum was mainly due to collision rather than magnetic field, because it estimated that thermal pressure was 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 of the low-Z and high-Z substances was observed clearly, and gold plasma was accumulated obviously in the later period at the interface. Moreover, spike and filamentous structure occurred at the interface of the two substances, which was probably caused by the hydrodynamic instability. 0.8 keV rather than 2.5 keV X-ray was observed around inner wall, which was from low-temperature plasma driven by radiation ablation and predicted by simulation code. Furthermore, the pressure balance relation of the two substances and the density steepness at the interface were analyzed.