Abstract
We report results of direct-drive laser imprint experiments measuring velocity perturbation profiles of shock waves produced by the Nike krypton fluoride laser. A new high-resolution two-dimensional velocimeter system was successfully implemented on the Nike laser facility and used for sensitive optical measurements of the velocity perturbations. Planar polystyrene targets with and without a thin high-Z overcoat (400 Å Au or 600 Å Pd) were irradiated by four, eight, and sixteen Nike laser beams to examine laser imprint and its mitigation. The results from the uncoated targets showed that the shock velocity perturbations decreased with an increasing number of laser beams overlapped on target, precisely as anticipated by the beam averaging effect on laser imprint. In the experiment on the shocks driven in the high-Z coated targets, the shock velocity perturbations were further reduced by a factor of 2–6 compared to their counterparts in the uncoated experiment, with the amplitude of the velocity fluctuations measured as small as 20 m/s rms for shock velocities of 20 km/s. These experiments allowed more direct measurements of laser imprint effects without relying on the Rayleigh–Taylor hydrodynamic amplification, providing valuable quantitative data for calibrating radiation-hydrodynamic simulations of laser imprint.
Highlights
The target perturbation produced by laser nonuniformity, called laser imprint,[1,2,3] has been a major issue for laser direct drive (LDD)inertial confinement fusion (ICF) due to its capability of disrupting shell integrity during implosion
Modern beam smoothing techniques can produce high power laser beams with overall nonuniformity levels below 1%–2% rms time averaged over nanosecond time scales that is generally considered to be necessary for LDD ICF
This trend is consistent with the anticipation that smoother target irradiation and less laser imprint should occur with the beam overlapping
Summary
The target perturbation produced by laser nonuniformity, called laser imprint,[1,2,3] has been a major issue for laser direct drive (LDD). The experiments conducted at Nike have shown that the multi-beam-overlapped laser drive provides target irradiation with nonuniformity as low as 0.15% rms for the wavelengths most important to seeding hydrodynamic instability. The x-ray backlighting radiography has been extensively used to investigate the laser imprint effects in ICF experiments This diagnostic measures areal density modulations integrated across the whole target thickness. A new high-resolution 2D VISAR, called NHRV (Nike high resolution velocimeter), has been implemented on the Nike laser facility to study the ICF laser imprint and its mitigation using the unique laser capabilities of Nike.[23] In the present paper, we report the initial experiment with NHRV measuring velocity fluctuations on the shock front launched in planar polystyrene (CH) targets with and without a thin high-Z overcoat.
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