The physics of laser-plasma interaction is studied on the Shenguang III prototype laser facility under conditions relevant to inertial confinement fusion designs. A sub-millimeter-size underdense hot plasma is created by ionization of a low-density plastic foam by four high-energy (3.2 kJ) laser beams. An interaction beam is fired with a delay permitting evaluation of the excitation of parametric instabilities at different stages of plasma evolution. Multiple diagnostics are used for plasma characterization, scattered radiation, and accelerated electrons. The experimental results are analyzed with radiation hydrodynamic simulations that take account of foam ionization and homogenization. The measured level of stimulated Raman scattering is almost one order of magnitude larger than that measured in experiments with gasbags and hohlraums on the same installation, possibly because of a greater plasma density. Notable amplification is achieved in high-intensity speckles, indicating the importance of implementing laser temporal smoothing techniques with a large bandwidth for controlling laser propagation and absorption.
Photoconductive transients and responsivity in a GaN p-n UV photodetector under different applied voltages are investigated at room temperature. The electron mobility of minority carriers in the p-GaN epilayer has been measured by a diffusion time-of-flight technique, and was found to be about 0.12 cm 2 V −1 s −1 with the bias between 1 V and 12 V. The difference of the electron mobilities between the minority carriers in p-GaN and the majority carriers in n-GaN is explained by different scattering mechanisms. The neutral impurity and phonon scattering mechanisms dominate the minority electron mobility in p-GaN. The photoconductive responsivity increases nearly linearly at low voltage and saturates at about 10 V, corresponding to a saturation field of approximately 3.7 × 10 4 V cm −1 . The implication of these results for applications of GaN UV detectors is also discussed.
The physical properties of basic minerals such as magnesium silicates, oxides, and silica at extreme conditions, up to 1000 s of GPa, are crucial to understand the behaviors of magma oceans and melting in Super-Earths discovered to data. Their sound velocity at the conditions relevant to the Super-Earth’s mantle is a key parameter for melting process in determining the physical and chemical evolution of planetary interiors. In this article, we used laser indirectly driven shock compression for quartz to document the sound velocity of quartz at pressures of 270 GPa to 870 GPa during lateral unloadings in a high-power laser facility in China. These measurements demonstrate and improve the technique proposed by Li et al. [PRL 120, 215703 (2018)] to determine the sound velocity. The results compare favorably to the SESAME EoS table and previous data. The Grüneisen parameter at extreme conditions was also calculated from sound velocity data. The data presented in our experiment also provide new information on sound velocity to support the dissociation and metallization for liquid quartz at extreme conditions.
The first laser–plasma interaction experiment using lasers of eight beams grouped into one octad has been conducted on the Shenguang Octopus facility. Although each beam intensity is below its individual threshold for stimulated Brillouin backscattering (SBS), collective behaviors are excited to enhance the octad SBS. In particular, when two-color/cone lasers with wavelength separation 0.3 nm are used, the backward SBS reflectivities show novel behavior in which beams of longer wavelength achieve higher SBS gain. This property of SBS can be attributed to the rotation of the wave vectors of common ion acoustic waves due to the competition of detunings between geometrical angle and wavelength separation. This mechanism is confirmed using massively parallel supercomputer simulations with the three-dimensional laser–plasma interaction code LAP3D.
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