Cutting-edge simulations run at Lawrence Berkeley National Laboratory's National Energy Research Scientific Computing Center (NERSC) over a two-year period are helping physicists better understand what influences the behavior of the plasma turbulence that is driven by the intense heating necessary to create fusion energy. This research has yielded exciting answers to long-standing questions about plasma heat loss that have previously stymied efforts to predict the performance of fusion reactors and could help pave the way for this alternative energy source. The key to making fusion work is to maintain a sufficiently high temperature and density to enable the atoms in the reactor to overcome their mutual repulsion and bind to form helium. But one side effect of this process is turbulence, which can increase the rate of plasma heat loss, significantly limiting the resulting energy output. So researchers have been working to pinpoint both what causes the turbulence and how to control or possibly eliminate it.

Because fusion reactors are extremely complex and expensive to design and build, supercomputers have been used for more than 40 years to simulate the conditions to create better reactor designs. NERSC is a Department of Energy Office of Science User Facility that has supported fusion research since 1974.

One roadblock in the quest for fusion is that, to date, computer models have often been unable to predict exactly how turbulence will behave inside the reactor. In fact, there have long been differences between predictions and experimental results in fusion experiments when studying how turbulence contributes to heat loss in the confined plasma.

Now researchers at MIT's Plasma Science and Fusion Center, in collaboration with colleagues at the University of California at San Diego (UCSD) and General Atomics, have discovered a solution to this disparity. By performing high-resolution multi-scale simulations, the team was able to simultaneously resolve multiple turbulence instabilities that have previously been treated in separate simulations. A series of these multi-scale simulations run on NERSC's Edison system found that interactions between turbulence at the tiniest scale (that of electrons) and turbulence at a scale 60 times larger (that of ions) can account for the mysterious mismatch between theoretical predictions and experimental observations of the heat loss.

The findings, published December 17, 2015 in Nuclear Fusion, could greatly improve our knowledge of what's really going on inside the current tokamak research experiments that exist around the world and in future experimental reactors under construction or planning, the researchers noted.