Reduction of time-averaged irradiation speckle nonuniformity in laser-driven plasmas due to target ablation

Abstract

In inertial confinement fusion (ICF) experiments, irradiation uniformity is improved by passing laser beams through distributed phase plates (DPPs), which produce focused intensity profiles with well-controlled, reproducible envelopes modulated by fine random speckle. [C. B. Burckhardt, Appl. Opt. 9, 695 (1970); Y. Kato and K. Mima, Appl. Phys. B 29, 186 (1982); Y. Kato et al., Phys. Rev. Lett. 53, 1057 (1984); Laboratory for Laser Energetics LLE Review 33, NTIS Document No. DOE/DP/40200-65, 1987 (unpublished), p. 1; Laboratory for Laser Energetics LLE Review 63, NTIS Document No. DOE/SF/19460-91, 1995 (unpublished), p. 1.] A uniformly ablating plasma atmosphere acts to reduce the contribution of the speckle to the time-averaged irradiation nonuniformity by causing the intensity distribution to move relative to the absorption layer of the plasma. This occurs most directly as the absorption layer in the plasma moves with the ablation-driven flow, but it is shown that the effect of the accumulating ablated plasma on the phase of the laser light also makes a quantitatively significant contribution. Analytical results are obtained using the paraxial approximation applied to the beam propagation, and a simple statistical model is assumed for the properties of DPPs. The reduction in the time-averaged spatial spectrum of the speckle due to these effects is shown to be quantitatively significant within time intervals characteristic of atmospheric hydrodynamics under typical ICF irradiation intensities.

Direct-drive measurements of laser-imprint-induced shock velocity nonuniformities

“…The laser-intensity profile was constructed using a sum of sinusoidal modes whose amplitudes are determined from the measured intensity profile for a beam using an 850-µm DPP on the OMEGA Laser System [13,47]. A random numerical seed is applied to these modes in order to randomize their spatial distribution in the simulation.…”

Time-dependent filamentation and stimulated Brillouin forward scattering in inertial confinement fusion plasmas

“…Of particular interest in this study is the effect of axial coupling in the plasma, which allows the light field at one axial position to affect the light field at other axial positions through a hydrodynamic, slow time-scale conduit. This interest is prompted in part by earlier studies that found qualitative differences in the behavior of light due to such hydrodynamic coupling [15,16,3].…”

“…The instability is expected to occur in indirectly-driven inertial confinement fusion (ICF) hohlraums [1] and might also occur in the coronal interaction region of directly-driven ICF targets [2,3]. It is a fundamentally important nonlinear laser-plasma interaction for two reasons.…”

Application of optical diffraction method in designing phase plates

“…In high power laser facilities, the uniformity of laser irradiation on the target has extremely strict requirements. [1] In order to meet such requirements, various laser beam smoothing techniques have been put forward, including two-dimensional smoothing by spectral dispersion (SSD), [2,3] polarization smoothing (PS), [4,5] segmented wedge array (SWA), [6] distributed phase plates (DPP), [7,8] and continuous phase plates (CPP). [9][10][11] All of these measures have their unique performances.…”