Abstract
Laser drive ramp compression is an important method to achieve an extremely high-pressure but relatively low-temperature material state. In this study, we found VISAR streaks disappearing under high pressure. To solve this problem, the mechanism of probe-laser absorption in laser drive ramp compression experiments has been studied, and X-ray preheat is found to be the main factor in this mechanism. Subsequently, target configuration optimization and code simulation were performed based on previous experiments, and laser drive ramp compression experiments were performed on the SHENGGUANG prototype facility. In the experiments, the probe-laser reflectivity was above 90% at 450 GPa, which is much higher than those in previous experiments, demonstrating that the X-ray preheat was shielded well with the optimized target configuration.
Highlights
The study of material properties at extremely high pressures and relatively low temperatures is an important topic in inertial confinement fusion and astrophysics
X-ray preheat is the cause of decrease in the intensity of velocity interferometer system for any reflector (VISAR) streaks, the underlying reason is that the CH ablation layer and the studied aluminum have low opacity; a part of the X-rays can propagate to the CH/Al interface, Al/Lithium fluoride (LiF) interface, and even the LiF window, and electrons produced by X-ray ionization in LiF and on the Al/LiF interface absorb the probe laser
To solve the problem of disappearance of VISAR streaks in laser direct-drive ramp compression experiments, a theoretical analysis was performed and a new target was designed by adding a gold interlayer
Summary
The study of material properties at extremely high pressures and relatively low temperatures is an important topic in inertial confinement fusion and astrophysics. Laser direct-drive ramp compression is a method to achieve such a material state in the laboratory, and it can yield a higher pressure and strain rate compared to conventional methods. Theoretical studies and experiments on laser direct-drive ramp compression have been performed in recent years, and important experimental results such as particle velocities, a pressure of the order of hundreds of gigapascals in the material, and the stress-density curve have been obtained. These studies have shown that laser direct-drive ramp compression is a useful technique to achieve an extremely high-pressure but relatively lowtemperature material state. The observation window became opaque under high pressure or the metal interface in targets stopped reflecting the probe laser This phenomenon limits the pressure range in experiments. LiF is a frequently-used observation window in ramp compression experiments and shock compression experiments for its transparency at high pressure It has been observed transforming from transparent insulators into partially degenerate liquid semiconductors with an optical reflectivity of several percent above 5 Mbar loaded by shocks.. X-ray preheat is the cause of decrease in the intensity of VISAR streaks, the underlying reason is that the CH ablation layer and the studied aluminum have low opacity; a part of the X-rays can propagate to the CH/Al interface, Al/LiF interface, and even the LiF window, and electrons produced by X-ray ionization in LiF and on the Al/LiF interface absorb the probe laser.
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