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

## TL;DR

This study directly measures laser-imprint-induced shock velocity nonuniformities using high-resolution velocimetry, revealing that increasing laser beam overlap reduces long-wavelength perturbations and exposing underestimations in simulations that neglect plasma microphysics.

## Contribution

It provides the first direct experimental validation of laser imprint effects on shock velocity nonuniformities and highlights the importance of plasma microphysics in accurate modeling.

## Key findings

- Long-wavelength perturbations decrease with more overlapping laser beams.
- Simulations underestimate perturbation levels by a factor of 3 when plasma microphysics are neglected.
- Experimental results align with theoretical expectations for laser imprint reduction.

## Abstract

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 um) 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.