New radiographic image processing for fusion experiments

In inertial confinement fusion (ICF), a promising approach to controlled thermonuclear fusion, a high-powered laser compresses a fuel pellet. The laser beam rapidly heats the surface of the pellet and forces ablation, leading to fast inward compression and ignition of fusion reactions.

Physicists from the French Alternative Energies and Atomic Energy Commission (CEA) recently performed a laser shock-tube experiment to better understand the ablation fronts and shocks critical to the ICF process. They tested on the OMEGA laser facility at the University of Rochester and report the results in Physics of Plasmas.

According to author Olivier Poujade, radiographs of most HEDP experiments tend to have low quality because of non-uniformity of the x-ray backlighter. In this study, a new radiographic image processing method allowed removal of the backlighter background imprint making possible quantitative comparisons between measurement and hydrodynamic simulations.

The researchers’ imaging achievement required step-by-step processing. Images selected for background removal required visibility of the fiducial grid and axis of the tube along with a reasonable signal-to-noise ratio. To infer the backlighter profile, the authors then employed an optimization procedure for best fit using the raw experimental profile of intensity — specifically, points in the ablated zone (rescaled with a multiplier) and the un-shocked zone.

The removal of the backlighter contribution allowed for quantitative comparison of experimental intensity profiles with their radiative hydrocode simulations. To their surprise, relative transmission varied rapidly in the ablated region in radiographic images but remained constant in the shocked region. The group plans to design a similar high energy density physics (HEDP) experiment at CEA’s Megajoule Laser facility (LMJ) to study materials under extreme conditions similar to those reached during the operation of nuclear weapons.

Source: “New radiographic image processing tested on the simple and double-flux platform at OMEGA,” by Olivier Poujade, Michel Ferri, and Isabelle Geoffray, Physics of Plasmas (2017). The article can be accessed at https://doi.org/10.1063/1.5002697 .