LLE–CEA: Ten Years of French–American Collaboration at the Omega Laser Facility

September, 2010

If science and society are best served by cooperation and collaboration, the results from using the OMEGA Laser have demonstrated remarkable success during the past ten years, leading to fruitful interactions between the French Commissariat à l’Énergie Atomique et aux Énergies Alternatives (CEA) – Atomic and Alternative Energies Commission, the U.S. Department of Energy (DOE) and the National Nuclear Security Administration (NNSA). The U.S.–French collaboration in the field of experimental laser–matter interaction physics reaches back more than 20 years with the first shots on the Nova Laser at the Lawrence Livermore National Laboratory (LLNL), and the Phebus laser at the Limeil–Valenton CEA Centre in France. Experiments on Nova, in close cooperation with LLNL and Los Alamos National Laboratory (LANL), produced results that contributed to showing that ignition was feasible in megajoule-class lasers and, therefore, the National Ignition Facility (NIF) and the Megajoule Laser (LMJ) programs could begin.

2000–2001: OMEGA experimental operations group pays tribute to a team from CEA who have worked on-site.
2000–2001: OMEGA experimental operations group pays tribute to a team from CEA who have worked on-site.

With NIF and LMJ in preparation, the shutdown of Phebus and Nova in 1999 raised the issue of maintaining competent teams and facilities aimed at studying the design of future targets to reach ignition. In the U.S., the response was to use the Omega Laser Facility. DOE decided to continue its collaboration with CEA beyond the planned shutdown of Nova through joint experiments at the Laboratory for Laser Energetics (LLE).

Since the first shot on OMEGA over a decade ago, more than 500 successful joint laser shots have been performed, thanks to the use of resources specific to CEA (targets, diagnostics, and principal investigators) and LLE teams. The CEA Experimental Program at LLE has extended the topics started on Nova, such as laser-plasma interaction, x-ray conversion, implosion symmetry, and hydrodynamic instabilities. The first experiments were performed with U.S. targets and diagnostics, making it possible for CEA, after the closing of Phebus, to continue to train their teams in target fabrication, validation of diagnostics, and the control of experiments at a large laser facility.

Because modeling the relevant phenomena responsible for the conversion of laser power to x-ray power is complex and yields uncertainties in the simulations, the first CEA experiments carried out on OMEGA had the following goals: (i) x-ray conversion experiments in open geometry to benchmark numerical simulations and (ii) indirect-drive experiments to assess the performance of hohlraums that were used on subsequent experiments to characterize implosion symmetry or hydrodynamic instability growth. The Omega Facility, which delivers very uniform irradiation on spherical targets, makes benchmarking numerical simulations easier and more relevant by minimizing 2-D effects, and minimizes the influence of self-generated magnetic fields that may affect electron transport. The first indirect-drive experiments in 2001 tested cylindrical hohlraums for implosion symmetry studies. They were followed in 2003 by comparisons between cylindrical and “rugby-shaped” hohlraums. The influences of gas fill on drive and wall blow-off motion was investigated later.

2009: Collaboration focused on optics in a site visit with CEA and LMJ.
2009: CEA scientists on-site during a “Neutron Derby”.

Recent experiments have validated hohlraum performance at radiation temperatures above 250 eV, approaching drive temperatures for LMJ ignition target designs.

For the past decade, CEA has developed, produced, and delivered more than 500 types of targets to the Omega facility. Many of the laser experiments have allowed for the development of important expertise in target component fabrication, assembly, and implementation. For example, x-ray hohlraum conversion, part of many experiments on OMEGA, is often composed of two layers (plastic-coated gold) or sometimes of very small thicknesses (a few micrometers) of other elements (titanium, copper, and germanium). A succession of specific techniques (polymerization, machining, vacuum deposition, and mandrel removal) has been successfully implemented in manufacturing these hohlraums. These technologies and the expertise developed for more than ten years at CEA, particularly for the experimental studies carried out on OMEGA, have paved the way for future targets for the LMJ program.

You can download a copy of CEA’s June 2010 CHOCS FOCUS (38.8 MB PDF), a bilingual edition dedicated to this exceptional international collaboration.