Progress continues on the quest for a lasting source of clean energy.
As the international scientific community accelerates the effort to develop a prototype fusion reactor, they confront a fundamental challenge. How can the reactor generate plasma at temperatures ten times hotter than the sun and then contain the plasma safely? For the first time, simulations made possible with extraordinary computing power may provide a portion of the answer.
In 2007 a fusion research team led by Fred Jaeger and Lee Berry of Oak Ridge National Laboratory achieved a performance of more than 87 trillion calculations per second, or teraflops, on the Cray XT4 Jaguar supercomputer at the National Center for Computational Sciences. The simulation provided insight into how best to heat an experimental reactor scheduled to begin operating in 2016 in Cadarache, France. The ambitious project, named ITER, is a coalition comprising the United States, the European Union, Russia, India, South Korea, China and Japan formed to provide the collective funding and scientific expertise needed to develop commercial fusion power plants.
ITER will use antennas to launch radio waves carrying 20 megawatts of power into the reactor, the equivalent of a million compact fluorescent light bulbs. The waves will heat the deuterium and tritium fuel to fusion temperatures—or more than 400 million degrees Fahrenheit. The deuterium and tritium form plasma, a state of matter created when gases become so hot that electrons get energized and fly off their atoms. As conceived, the radio waves would drive currents that help confine the plasma. Jaeger's simulations will contribute to understanding how to make the most of the wave power in both heating and controlling the plasma.
"The 2007 run was the first two-dimensional simulation of mode conversion in ITER," said Jaeger, who used the simulation to explore the conversion of fast electromagnetic waves to slow electrostatic waves. Before the run, mode conversion in ITER was simulated in only one dimension, although scientists could simulate mode conversion in two dimensions for smaller tokamaks. "We need to know which types of waves are present because different waves can interact differently with the plasma."
Jaeger's team uses AORSA, a software code that solves Maxwell's equations for the electromagnetic wave fields in the plasma. The AORSA team is part of a Scientific Discovery through Advanced Computing project known as the SciDAC Center for Simulation of Wave-Plasma Interactions. The team includes plasma scientists, computer scientists, and applied mathematicians from ORNL, the Massachusetts Institute of Technology, Princeton Plasma Physics Laboratory, General Atomics, CompX, Inc., Tech-X Corporation and Lodestar Research Corporation.
For the 2007 simulation, the code employed 22,500 processor cores—98 percent of the machine's capacity—to calculate the interplay between radio waves and particles in the plasma as well as the current produced by the interaction. A mesh of 500 by 500 cells, or 250,000 individual cells—more than triple the resolution of earlier simulations—gave the team the ability to examine interactions in fine-grained detail.
Upon analyzing the energy distributions of the very-high-energy ions created when radio waves heat the plasma, the scientists found that, in some cases, the ions increased the fusion reaction rate. They learned the optimal frequency for driving current in the ITER plasma and identified the heat-loss channels that limit the current. Comparing the ITER model with current tokamaks, they also found stronger central focusing of radio waves in ITER.
These findings at ORNL, made possible by the new capabilities of supercomputing, bode well for the goal of keeping plasma hot enough for fusion, and for the world's quest for a lasting source of clean energy.—Dawn Levy and Leo Williams
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