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Experiments were performed to maximize the neutron yield from a 2 kJ dense plasma focus (DPF) and characterize the amount of copper sputtered from the surface of an anode by varying the diameter of the anodes' on-axis hollow. The hollow is a void in the copper material along the longitudinal axis of the anode. All the anodes had an outer diameter of 1.2 in. and the diameter of the hollow varied from 0 in. (no hollow) to 1 in. The anodes with a hollow produced a greater number of neutrons per discharge than the anode without a hollow. Over 40 discharges, the hollow anode that yielded the most neutrons (9.1 ± 0.4 × 10 6 neutrons per discharge produced with the 0.75 in. hollow) produced > 6 times more neutrons than the anode with no hollow. A qualitative observation of the anodes after 130 discharges showed less surface damage on anodes with a larger hollow. Quantitative sputter measurements were performed by characterizing the amount of copper sputtered onto on-axis quartz targets for three newly machined anodes, each with a particular hollow diameter. The quantitative results matched the qualitative observations: the copper sputter was reduced using larger hollows. The largest hollow sputtered 17 ± 1.0 nm/sr/discharge of copper, a reduction of 69 % compared to the anode with the most damage. [ABSTRACT FROM AUTHOR]

The high implosion velocity, high adiabat BigFoot design [Casey et al., Phys. Plasmas 25, 056308 (2018)] has produced the highest neutron yield to date in an ignition hohlraum on the National Ignition Facility. It has used up to 500 TW of peak power and nearly 2 MJ of laser energy in pulses up to 8 ns in duration, with the goal of fielding controlled implosions with high coupled energy, which can suppress deleterious hydrodynamic instabilities. However, when the laser pulse exceeds 6 ns with the laser energy greater than 1.6 MJ, stimulated Brillouin scattering (SBS) reaches levels that may damage optical components in the laser. Pending development of techniques to reduce SBS, limitation of laser power, and energy to avoid damage prevents the full exploitation of this approach to ignition. In this manuscript, we present three-dimensional simulations that match the experimentally measured SBS energy, in particular, reproducing quantitatively the time in the pulse when maximum backscatter occurs, and its magnitude across ∼10 BigFoot experiments. The demonstrated robustness of the modeling, which combines LASNEX and pF3D simulations, motivates us to explore and recommend several feasible SBS mitigation strategies: modified laser pointing, different laser frequencies for each cone of beams, increased laser bandwidth on all or some of the cones, and materials with a mixture of light and heavy atoms lining the inside of the hohlraum walls. [ABSTRACT FROM AUTHOR]

We have designed experiments to study the effect of the laser wavelength (0.527 versus 0.351 μm) on the coupling efficiency in plasma conditions relevant to compression and shock ignition (SI) schemes in different intensity regimes. A difficult issue was to produce interaction conditions that are equivalent for the two wavelengths. This was obtained by using plasma preformed from a solid target with a plasma-preforming beam at the same wavelength as the interaction beam. This produced an almost exponential density profile from vacuum to the critical density of the interaction beam in which all interaction mechanisms are taken into account. The growth and saturation of stimulated Brillouin scattering (SBS) have been measured at the two wavelengths, in backward as well as in near-backward directions. We have found that the SBS intensity threshold is ∼1.5 times higher at 3ω than at 2ω in agreement with the Iλ dependence of the SBS gain. The SBS behaviour is very well reproduced by the linear calculations of the postprocessor PIRANAH, giving us confidence that we have a good control of the relevance of the experimental conditions for the study of the laser wavelength effect on laser-plasma coupling. When SBS reaches the saturation regime, same levels of reflectivity are measured at 2 and 3ω. Numerical simulations were performed with the paraxial code HERA to study the contribution of the fluid mechanisms in the saturation of SBS, showing that pump depletion and interplay with filamentation are likely to be the most important processes in SBS saturation for these conditions. This scenario also applies to the SBS of shock ignition high-intensity beams. [ABSTRACT FROM AUTHOR]