Self-focusing and filamentation of laser light in high Z plasmas

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Abstract
Self‐focusing and filamentation of short wavelength laser light in high Z plasmas of interest to laser fusion are discussed. It is found that self‐focusing behavior is very dependent on the details of the characteristics of the laser beam, the plasma conditions, and the energy transport processes. Laser light absorption and self‐focusing are strongly competitive processes. At. 0.26 μm wavelength the collisional absorption is often so great that there is no intensity amplification of the beam despite the fact that strong self‐focusing is present. Wide variations are found in laser light penetration, affected by several factors. Diverging optics reduce the likelihood of self‐focusing. Large scale length density gradients have little effect on focusing behavior. The self‐focusing behavior is very dependent on beam shape. Large scale hot spots can have a significant effect on whole beam self‐focusing early in the pulse. The behavior of small scale hot spots can be qualitatively different than the standard picture. The calculations indicate that small scale hot spots do not achieve a steady state in some cases. Sound waves cause chaotic interactions among neighboring hot spots. It is found that sub‐beam size structures are also generated when nonlocal thermodynamic equilibrium (non‐LTE) radiation and atomic physics are used in the calculations. The nature of the heat flux and thermoelectric magnetic field generation are examined with a kinetic model. Stimulated Raman backscattering levels in self‐focused light are significantly reduced for short wavelengths and high Z plasmas Landau damping plays an important role in determining the Raman levels. Implications for suprathermal electron production, symmetric illumination, x‐ray conversion efficiency, and laser light absorption are discussed.