Record fusion yield for direct-drive layered DT implosions achieved on OMEGA
Connor Williams, a second year Ph.D. student in the department of Physics and Astronomy at the University of Rochester, developed a novel laser-fusion target design to achieve the highest fusion yields to date on the OMEGA laser. Fusion energy is unleashed when two lighter elements such as deuterium and tritium (DT) are fused together to produce heavier elements. Connor’s target uses an ~1.01-mm-diam, 7.5-µm-thick plastic shell enclosing a cryogenic DT ice layer of several tens of microns. The target is imploded by direct laser illumination (direct drive) from the 60 beams of the OMEGA Laser System.
The target size is critical to achieving the highest coupling of laser energy to target kinetic energy, driving the imploding shell to velocities exceeding 500 km/s. At such high velocities, the implosion produces a very hot plasma with temperatures of about 50,000,000°C upon convergence and copious amounts of fusion reactions. The main challenge in achieving these extreme velocities is that the imploding shell can break up in-flight due to hydrodynamic instabilities before converging at the center. To maintain target integrity, Connor designed a temporally shaped laser pulse delivering 30 kJ of ultraviolet light that maximizes the stability properties of the shell while in flight.
On 17 June 2021, Connor was assigned one shot on the OMEGA Laser System to test his design. His target yielded 220 trillion fusion reactions—a new facility record fusion yield! “It is a wonderful result showing that after 20 years of implosions and a thousandfold increase in fusion yields, we have not yet reached the limits of what can be achieved on OMEGA. That the shot was designed by a graduate student shows the unique educational opportunity that the University provides,” says Mike Campbell, Director of the Laboratory for Laser Energetics (LLE).
While Connor’s target achieved the highest fusion yield to date, it did not make use of the most-optimized target specifications. “For reasons of compatibility with the other targets shot on that day, we could not use the most-optimized designs. I am confident that Connor’s designs will break his own record when tested again in the fall using the best optimization. Our new goal is to first exceed 250 and then 300 trillion fusion reactions,” says Connor’s Ph.D. supervisor, Prof. Riccardo Betti. Indeed, Connor’s designs are expected to achieve even higher velocities above 600 km/s when thinner DT ice layers will be employed during an upcoming shot day in the fall of 2021, fully allocated to test Connor’s most-optimized targets.
“This is exactly the type of opportunity that drew me to Rochester for graduate school,” says Connor. “There are few lasers in the world that can do what OMEGA can do, and the chance to design fusion experiments as a student is really quite unique to LLE. We’re positioning ourselves to accomplish something very special in November. As a young scientist, you can’t help but be excited to contribute to a project like this, it’s what you live for.”
If successful in achieving the highest yields, Connor’s designs are good candidates for direct-drive laser-fusion experiments on the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory, the world’s largest laser facility. Because of their good stability properties and relatively low convergence, Connor’s targets do not require major upgrades for direct drive on the NIF and can be used for producing high neutron fluxes for experiments relevant to the Department of Energy Nuclear Stockpile Stewardship Program.