Non-Linear Physics Modeled with the Roadrunner

Scientists at the US Department of Energy's (DOE) Los Alamos National Laboratory (LANL) are currently using the world's most advanced supercomputer to assist colleague physicists in determining the best way to go about obtaining nuclear fusion. The first machine to break the petaflop barrier is now being used to study the nonlinear physics of laser backscatter energy transfers, and also issues related to plasma instabilities in future fusion reactors.

The main goal of the new simulations is to assist colleagues at the Lawrence Livermore National Laboratory (LLNL) in their quest of obtaining fusion for the first time in 2010, at the National Ignition Facility (NIF). The scientists here essentially want to create a star in the lab, a tiny area of confinement in which temperatures will reach never-before-achieved heights. It is believed that these conditions would help tritium and deuterium nuclei fuse together, to produce helium and vast amounts of energy. The process will be considered successful if the amount of energy that goes into obtaining fusion is lower than the amount of energy that comes out of the reaction.

An adapted version of a particle-in-cell plasma physics code known as VPIC is being used to probe the possible behavior of high-energy lasers. The leaders of the investigation team are LANL experts Lin Yin and Brian Albright, both from the Applied Science and Method Development section, and Kevin Bowers, a guest scientist at the lab. “These are the largest plasma simulations ever done, looking at 0.4 trillion particles on the whole system. It would not be possible to do this without a petascale computer like Roadrunner, but even so, we are still only looking at a tiny segment of a laser beam,” Lin says.

A large number of high-energy lasers will be used to heat the mix inside the test reactor to the enormous temperature it needs to achieve fusion. This physical process is the same one that powers up stars, where temperatures and pressures are well beyond anything that can be obtained on Earth. “We can use this physics understanding to infer energy loss from the whole beam and aid the experimental design. Using Roadrunner, we now understand why it's happening in the first place, how laser energy couples to this instability, and what limits the backscatter. We believe that this work will help ensure the success of NIF,” Lin adds.

The Roadrunner supercomputer, developed by IBM in partnership with the Laboratory and the National Nuclear Security Administration, is used to perform advanced physics and predictive simulations in a classified mode to assure the safety, security, and reliability of the US nuclear deterrent. The system is used by scientists at the Los Alamos, Sandia, and Lawrence Livermore national laboratories.

The secret to its record-breaking performance is a unique, hybrid design. Each compute node in this cluster consists of two AMD Opteron™ dual-core processors plus four PowerXCell 8i™ processors used as computational accelerators. The accelerators employed in Roadrunner are a special IBM-developed variant of the Cell processor used in the Sony PlayStation 3®. The node-attached Cell accelerators are what make Roadrunner different from typical clusters.