Fusion Fuel Finally Yields Excess Energy

The two main purposes of the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory are to explore fusion energy and enable experiments to ensure the safety and viability of the U.S. nuclear weapons arsenal. Completed in 2009, the $3.5 billion facility missed a 2012 deadline to ignite a self-sustaining fusion reaction that would yield more energy than was put in to start it. NIF researchers now report reaching an important milestone toward ignition: Experiments now produce more energy than went into the deuterium-tritium fuel (Nature 2014, DOI: 10.1038/nature13008). NIF experimental “shots” involve aiming 192 lasers with a combined 1.8 MJ of energy at a pencil-eraser-sized gold cylinder called a hohlraum. The hohlraum converts the laser light into X-rays, which interact with a 2-mm-diameter spherical plastic shell called an ablator. The deuterium-tritium fuel inside the ablator then implodes and turns into helium nuclei and neutrons, releasing energy. The challenge for NIF scientists has been to keep the implosion from ripping itself apart. The best shot reported so far produced 17.3 kJ, or about double the amount of energy that made it from the lasers into the fuel. Possible approaches to further improve the yield include changing the hohlraum geometry and the ablator material, says LLNL physicist Debra A. Callahan.