The Laboratory for Laser Energetics (LLE) at the University of Rochester is the biggest university-based U.S. Department of Energy (DoE) program in the nation. The OMEGA laser is located at the LLE. It is the most powerful laser at any academic institution in the U.S. The LLE is one of leading research laboratory in the U.S. exploring the laser direct-drive approach to generating energy from nuclear fusion.
In this approach, spherical deuterium-tritium pellets of fuel are bombarded with sixty laser beams which hit the surface of the pellets from all directions at once. Under the intense heat of the laser beams, the pellets implode and turn into a plasma. If they can concentrate enough heat and pressure at the exact center of the implosion, a thermonuclear burn wave will travel outward in all directions in the plasma. This would produce energy from nuclear fusion far in excess of the energy used to drive the lasers. Much more powerful lasers than OMEGA would be needed to achieve this goal.
One of the biggest problems with advancing nuclear fusion research is the lack of models which could predict accurately which specifications for target shape and laser pulse shape would yield the best results. The LLE has been able to triple the yield of their fusion experiments by utilizing the latest data science techniques to make use of previously collected data and earlier computer simulations.
The Rochester team is made up of Varchas Gopalaswamy and Dhrumir Patel, PhD students and their supervisor Riccardo Betti, chief scientist and Robert L. McCrory Professor at LLE. The team applied sophisticated analytical techniques to data gathered from one hundred previous fusion experiments with the OMEGA laser.
Gopalaswamy said, “We were inspired from advances in machine learning and data science over the last decade.” Betti said “This approach bridges the gap between experiments and simulations to improve the predictive capability of the computer programs used in the design of experiments.” The results of their research allowed the team to optimize the specifications for the exact shape and size of the fuel pellets and the temporal shape of the laser pulse to best trigger fusion.
The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory has lasers that are about seventy times as powerful as the OMEGA laser. If the models developed by the Rochester team can be extrapolated for these more powerful lasers, the result should be about one thousand times as many fusion reactions per test run. A modest improvement in target compression on the OMERA laser system should be enough to approach breakeven conditions at the level of power available to the NIF laser systems. This extrapolation of modeling is not a simple thing. Betti said, “Extrapolating the results from OMEGA to NIF is a tricky business. It is not just a size and energy issue. There are also qualitative differences that need to be assessed.”
Parallel to the work at Rochester, scientists at the NIF are working to see if the results from Rochester can be applied successfully at the NIF. Unlike the direct drive fusion approach at Rochester, at the NIF they use an indirect drive approach. The fuel pellet is enclosed in a metal can called a hohlraum. Lasers are fired into both ends of the can along the axis to heat the hohlraum and its contents. X-rays are generated in the hohlraum which causes the fuel pellet to explode and produce fusion reactions in the plasma. This approach has been making progress towards breakeven.