OMEGA: a short-wavelength laser for fusion experiments

The OMEGA, Nd:glass laser facility was constructed for the purpose of investigating the feasibility of direct-drive laser fusion. With 24 beams producing a total energy of 4 kJ or a peak power of 12 TW, OMEGA is capable of nearly uniform illumination of spherical targets. Six of the OMEGA beams have recently been converted to short-wavelength operation (351 nm). In this paper, we discuss details of the system design and performance, with particular emphasis on the frequency-conversion system and multi-wavelength diagnostic system

Ablative Richtmyer- ⁣\!-Meshkov instability: Theory and experimental results

Rayleigh–Taylor instability, the main source of symmetry degradation in ICF experiments, is seeded at the early stage of an implosion, during the shock transit through the shell. The ablation-front nonuniformities at such a time can be amplified by an instability, which is similar to the Richtmyer–Meshkov instability. In the presence of ablation, however, the dynamic overpressure (rocket effect) significantly reduces the perturbation growth. The modes localized inside the conduction zone between the laser-absorption region and the ablation front are totally stabilized. An analytical model is presented to describe the perturbation evolution at the ablation front during the shock propagation time. The model is compared against the results of both the multidimensional simulations and a series of experiments performed on the OMEGA Laser System

Large aperture harmonic conversion experiments at LLNL - comments

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Polar direct drive – Ignition at 1 MJ

Target designs to achieve direct-drive ignition on the NIF using the x-ray-drive beam configuration are examined. This approach, known as polar direct drive (PDD), achieves the required irradiation uniformity by repointing some of the beams toward the target equator, and by increasing the laser intensity at the equator to compensate for the reduced laser coupling from oblique irradiation. Techniques to increase the equatorial intensity can include using phase plates that produce elliptical spot shapes, increasing the power in beams directed toward the equator, and using a ring offset from the equator to redirect rays toward the target normal. The requirements for beam pointing, power balance, single-beam smoothing, and inner-ice-surface roughness are examined. Designs with an incident laser energy of 1.0 MJ are presented. The simulations use the 2-D hydrocode DRACO with 3-D ray trace to model the laser irradiation and Monte Carlo alpha particle transport to model the thermonuclear burn

Polar drive on OMEGA

High-convergence polar-drive experiments are being conducted on OMEGA [T. R. Boehly et al., Opt. Commum. 133, 495 (1997)] using triple-picket laser pulses. The goal of OMEGA experiments is to validate modeling of oblique laser deposition, heat conduction in the presence of nonradial thermal gradients in the corona, and implosion energetics in the presence of laser–plasma interactions such as crossed-beam energy transfer. Simulated shock velocities near the equator, where the beams are obliquely incident, are within 5% of experimentally inferred values in warm plastic shells, well within the required accuracy for ignition. High, near-one-dimensional areal density is obtained in warm-plastic-shell implosions. Simulated backlit images of the compressing core are in good agreement with measured images. Outstanding questions that will be addressed in the future relate to the role of cross-beam transfer in polar drive irradiation and increasing the energy coupled into the target by decreasing beam obliquity