Spatio-temporal focal spot characterization and modeling of the NIF ARC kilojoule picosecond laser.

The advanced radiographic capability (ARC) laser system, part of the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory, is a short-pulse laser capability integrated into the NIF. The ARC is designed to provide adjustable pulse lengths of ∼1-38ps in four independent beamlets, each with energies up to 1 kJ (depending on pulse duration). A detailed model of the ARC lasers has been developed that predicts the time- and space-resolved focal spots on target for each shot. Measurements made to characterize static and dynamic wavefront characteristics of the ARC are important inputs to the code. Modeling has been validated with measurements of the time-integrated focal spot at the target chamber center (TCC) at low power, and the space-integrated pulse duration at high power, using currently available diagnostics. These simulations indicate that each of the four ARC beamlets achieves a peak intensity on target of up to a few 1018W/cm2.

Revisiting an airgap split-optics mitigation for beam filamentation in high power lasers.

We extend the split-optic approach for mitigating filamentation in a thick optical component previously proposed for small beams to conditions relevant to high-power lasers. The split-optic approach divides a thick optic into two thinner optics separated by an airgap to reduce filamentation through diffraction management. Our numerical study focuses on filamentation of a flat-top beam with intensity modulation noise sources passing through a split-optic system. The improvement in the distance to collapse in glass is shown to be potentially substantial (>30%), yet has limited increase with the airgap size, unlike the common understanding when considering a collapse of a whole beam or a sole perturbation on a beam. The improvement in the collapse distance in glass asymptotes to an upper bound value that depends mainly on the beam mean intensity and its contrast for any airgap size above some value that depends mainly on the shortest spatial periods comprising the excitation noise source. Examining the difference in the simulation results for a periodic versus a randomly generated perturbation source-term suggests that the observed effect is governed by the statistical interference dynamics of the beam while propagating through the airgap.

Revisiting beam filamentation formation conditions in high power lasers.

The Bespalov-Talanov gain (BT-gain) and IL-rule (i.e., the product of input intensity and self-focusing length is constant) expressions are examined and generalized for filamentation under realistic conditions associated with high power lasers: filamentation seeded by both amplitude and phase perturbations on a large, flat-top beam, and the impact of cross-phase modulation from unconverted light in UV frequency-converted lasers. The validity of these models is examined with NLSE numerical calculations, which show that there are parameters beyond the commonly-used IL rule, such as the perturbation amplitude and period content. The BT-gain model presents a fair description of the tendency of spatial periods to filament, but not of the quantitative self-focusing length. Spatial filtering of short periods is shown to suppress filamentation, due to both, the removal of the more prone to filament periods, as well as the reduction of the spatial intensity amplitude root-mean-square. At the edge of a top hat beam we find that the IL product reduces in the roll-off regions, even though the self-focusing length increases. When adding a co-propagating harmonic, we find that the cross-phase modulation (XPM) could enhance or inhibit the filamentation formation, depending on the perturbation period.

Imprinting high-gradient topographical structures onto optical surfaces using magnetorheological finishing: manufacturing corrective optical elements for high-power laser applications.

Corrective optical elements form an important part of high-precision optical systems. We have developed a method to manufacture high-gradient corrective optical elements for high-power laser systems using deterministic magnetorheological finishing (MRF) imprinting technology. Several process factors need to be considered for polishing ultraprecise topographical structures onto optical surfaces using MRF. They include proper selection of MRF removal function and wheel sizes, detailed MRF tool and interferometry alignment, and optimized MRF polishing schedules. Dependable interferometry also is a key factor in high-gradient component manufacture. A wavefront attenuating cell, which enables reliable measurement of gradients beyond what is attainable using conventional interferometry, is discussed. The results of MRF imprinting a 23 µm deep structure containing gradients over 1.6 µm / mm onto a fused-silica window are presented as an example of the technique's capabilities. This high-gradient element serves as a thermal correction plate in the high-repetition-rate advanced petawatt laser system currently being built at Lawrence Livermore National Laboratory.