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Numerical model for calculating convective gains and absolute thresholds of stimulated Raman scattering (SRS) in inhomogeneous plasmas is constructed based on the Fourier-space method. The model is valid for arbitrary density profiles and scattering geometries, including both backscattering and side scattering. It is shown that 90 deg side scatter has a lower absolute threshold than other scattering geometries. Backscatter, on the other hand, has a relatively large absolute threshold under conventional direct-drive ignition conditions. For a parabolic density profile, the absolute threshold of backscatter decreases dramatically at the peak of parabola, but is still much larger than that of side scatter. We also discuss the absolute thresholds of side scatter under different density profiles, showing not big differences with the linear density profile as well as the analytic formulas. Convective gains, however, are sensitive to the density profiles and collisional damping. The k-space numerical model is verified via analytic formulas and real-space envelope model, and it offers us new perspective on the scattering angles compared with previous models for SRS. [ABSTRACT FROM AUTHOR]

The physical image of the confined electrical explosion in the source region is depicted. Metallic plasma/vapor dynamics and its fragmentation effect (on a confining structure) under μs-timescale are diagnosed via high-speed photography, electrophysical, and spectral measurements. When adding a 1-mm-thick Teflon tube outside the exploding wire, the growth of spatial heterogeneity via electro-thermal instability is largely compressed, and the deposited energy almost doubled from about 85 to 150 J. During the short period after breakdown, considerable energy depositing into the confined space, e.g., 100 J for 0.1 cm3, drives the fast inflation and burst of the 0.5 g confining tube to ∼500 m/s (kinetic energy of ∼62.5 J). Intense plasma jet eruption with a supersonic speed >1.5 km/s and induced shock waves of 2–3 km/s are observed from cracks of the inflated tube. In addition, the erupted plasma jets gradually evolve Rayleigh–Taylor instability and finally cause turbulent mixing with the ambient medium. This mechanism is very likely to explain the plasma cavity evolution in underwater explosion. Interestingly, although the confining effect of water is stronger than a Teflon tube, the latter has a better response to the high-rate impulse loading and absorbs more deposited energy by deformation, phase transition, and acceleration. [ABSTRACT FROM AUTHOR]

An electrical wire explosion is a complicated process consisting of several physical phenomena such as acoustic, optical, mechanical, electromagnetic, and thermal phenomena. The shock wave from such an explosion not only has a wide range of practical applications but it also plays an important role in analyzing the process of the explosion itself. In this paper, we propose an empirical approach to estimate the pressure and energy of the shock wave in an underwater electrical wire explosion. First, the discharge process is divided into several typical phases, and the deposited electrical energy, power, and time interval between adjacent phases are calculated. Then, the shock wave peak pressure is measured and the total mechanical energy is calculated, respectively. Finally, a multiparameter fitting method is adopted to deduce an empirical formula for peak pressure and shock wave energy, and the formula was the exponential function of the deposited electrical energy, power, and the discharge time interval. In this way, the shock wave pressure and energy are quickly and reliably estimated for given discharge parameters. As for 155 shots of the discharge experiment, the average relative error and the standard deviation of peak pressure are 8.45% and 5.47%, respectively, and those of the shock wave energy of are 12.2% and 11%, respectively. This empirical approach would be very useful in pulsed power device design and in the parameters involved in electrical wire matching. [ABSTRACT FROM AUTHOR]

The discharge channel development of microsecond electrical exploding tungsten in air was investigated. A microsecond pulsed current source was used to trigger the electrical explosion of tungsten wires with a length of 40 cm and a diameter of 0.3 mm. Charged with a 26 kV voltage, a 6 μF capacitor possessed a stored energy of 2028 J, which was not enough to vaporize the tungsten wire. We proposed two parameters, the total cross-sectional area and the granular border area, to quantitatively describe the discharge channel development. We also adopted an image processing method to study the entire discharge channel development throughout the wire explosion. Evident results suggested that the trend of the electrical explosion was layer by layer from the surface to the core, and the outward expanding speed of the channel was gradually decreasing. The results also demonstrated clear distinction between the discharge channel developments at upper and lower electrodes. During the electrical explosion, the granular border area increased in an approximate exponential way, while the total cross-sectional area increased linearly until 1.43 ms and then decreased, which was caused by hollow regions formed in the discharge channel. [ABSTRACT FROM AUTHOR]