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Producing National Ignition Facility (NIF)-quality beams on the Nova and Beamlet Lasers
Abstract
The Nova and Beamlet Lasers were used to simulate the beam propagation conditions that will be encountered during the National Ignition Facility operation. Perturbation theory predicts that there is a 5 mm scale length propagation mode that experiences large nonlinear power growth. This mode was observed in the tests. Further tests have confirmed that this mode can be suppressed with improved spatial filtering.
A new cylindrical-lens spatial filter is proposed to solve the blocking pinhole problem of a spatial filter. Compared with the conventional spherical-lens spatial filter, the new filter is constituted with cylindrical lens and slit. The transmission characteristics of high power laser beam in the new cylindrical-lens spatial filter are simulated using linear transfer theory. The results show that the area of the focal spot of the new filter is bigger than that of the conventional spherical-lens spatial filter, and the power density of the focal spot is smaller, while the filtering effect and near field uniformity keep the same. The new filter can effectively inhibit the blocking effect of the filtering pinhole. At the same time, the new filter can also improve the overall price of the laser device by shortening the focal length of lens. The new cylindrical-lens spatial filter to replace the conventional spherical-lens spatial filter is feasible.
We propose a new approach to imaging astrometry, the uniaxial crystal interferometer (UCI). It uses the extremely sensitive dependence of the polarization phase change on the incident angle in uniaxial crystals. This is further combined with very sensitive polarization measurements. The polarization-coding is used for a fine-scale angle determination, and is simultaneously combined with a crude measurement through a standard telescope. Further, achromatic anamorphic prisms will amplify five to tenfold the resolution which is estimated to reach the scale ratio of large interferometers. Because the fine angular information is superimposed on the light at an early stage the optical system tolerances are relieved to the level of a standard low-weight optical imaging system. We also suggest solutions to make the system achromatic. Overall the system may cover sky segments of the order of fractions of a degree square and reach a resolution of tens of microas, even for faint stars.
Nova is the Lawrence Livermore National Laboratory's 10-beam Nd:glass laser, which is used for inertial confinement fusion research. It operates in the high power, high energy regime, where several interesting nonlinear optical phenomena can occur. These phenomena and their interactions limit the power and energy that can presently be delivered to targets for laser-target interaction experiments. Understanding and controlling these processes will allow us to increase the energy available for such experiments.
The growth of a radiation pulse traversing a medium with an inverted population is described by nonlinear, time‐dependent photon transport equations, which account for the effect of the radiation on the medium as well as vice versa. The equations are solved in closed form for an arbitrary input pulse and an arbitrary initial distribution of inverted population. The solutions are discussed in detail for the particular cases of a square pulse and a Lorentzian pulse, both with a uniform initial population inversion.
For laser beams of large aperture with power far above the critical power for self‐focusing, nonlinear propagation instabilities lead to the growth of small‐scale variations in intensity and phase. We report measurements of the growth rate for interference fringes in unpumped ED‐2 laser glass as a function of fringe spacing and intensity. Calculations based on the simplest form of linearized small‐scale instability theory agree approximately with the measured rates, and a more complete linearized treatment predicts the growth rates within experimental error.
Experimental data and the results of detailed numerical calculations are presented for small scale beam breakup due to non-linear self-focusing in Cyclops, a high power Nd-glass laser amplifier. Both perturbed plane wave and nonlinear whole beam calculations were carried out. The whole beam calculations were carried out using a dust particle model of beam perturbations.