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

Perturbations in the velocity profile of a laser-ablation-driven shock wave seeded by speckle in the spatial beam intensity (i.e., laser imprint) have been measured. Direct measurements of these velocity perturbations were recorded using a two-dimensional high-resolution velocimeter probing plastic material shocked by a 100-ps picket laser pulse from the OMEGA laser system. The measured results for experiments with one, two, and five overlapping beams incident on the target clearly demonstrate a reduction in long-wavelength ($>25\text{−}\mu \mathrm{m}$) perturbations with an increasing number of overlapping laser beams, consistent with theoretical expectations. These experimental measurements are crucial to validate radiation-hydrodynamics simulations of laser imprint for laser direct drive inertial confinement fusion research since they highlight the significant (factor of 3) underestimation of the level of seeded perturbation when the microphysics processes for initial plasma formation, such as multiphoton ionization are neglected.

Figure 1

(a) Experimental setup: one, two, or five beams delivering 20 J in a 100-ps picket are incident on a $100\text{−}\mu \mathrm{m}$ CH polymer disk. A shock is generated and propagates through the medium where it is probed by the OHRV beam 0.9 ns later. (b) Relative timing for beams in the five-beam experiment (solid blue) and the two-beam experiment (dashed red). In the five-beam experiment, one beam was roughly 20 ps early compared to the others.

Figure 2

Two-dimensional velocity modulation maps taken by OHRV over a $315\times 315\text{−}\mu \mathrm{m}$ region in the uniformly driven portion of the target plane for (a) one- and (b) two-beam configurations. Qualitatively one can see significantly more perturbations in the one-beam case across all modes. (c) Radially averaged velocity spectral information inferred from OHRV measurements for (a), (b), and five beams. (Ninety-five percent) confidence intervals are represented by the shaded region for each spectrum. The single-beam case, indeed, shows more energy in perturbations across all modes when compared to the two- or five-beam cases. (d) ${\sigma }_{\mathrm{rms}}$ for modes of interest in all shots for each experimental case demonstrate repeatability of the data.

Figure 3

(a) A cartoon of the 2D cylindrical geometry created to approximate a planar solution for the draco simulations. (b) A zoomed in image at the large sphere radius where the beam interacts with the target. One can see that the target surface (light blue) approximates a planar surface very closely as the beam along the $z$ axis pushes into it.

Figure 4

(a) Example laser intensity profile input to draco for the one-beam case. (b) Simulated position of the shock front as time progresses. (c) The calculated velocity of the shock front. (d) The normalized velocity spectrum is calculated from this velocity profile.

Figure 5

[(a) and (c)] Line out and extrapolated 2D velocity modulation map of the draco-calculated shock front for the one-beam case and [(b) and (d)] one beam with multiphoton ionization (MPI) accounted for. One can see that the simulation without MPI drastically underestimates the degree of imprint, especially when compared to Fig. 2. (e) A comparison of the velocity spectra from draco (red and black) to the experimental data (blue) for the one-beam case.