The wave of the future for controlling fusion energythe process that powers the sunmay include more effective harnessing of a fusion device heating process, using electromagnetic, or radio, waves. An understanding of how to get "radio-controlled fusion" may come from calculations done using supercomputers at ORNL.
Controlled fusion energy, when achieved, could be a favored energy source someday. It would draw upon an unlimited source of fuelhydrogen isotopes from seawaterand would produce no greenhouse gases that could adversely affect climate.
The first key to achieving fusion energy is to heat charged particles (nuclei of hydrogen isotopes) to very high temperatures such that the electrical repulsion of the nuclei is overcome during collisions, allowing nuclear fusion reactions to occur. At these temperatures, the electrons are completely stripped from the atomic nuclei, yielding an electrically conducting gas called plasma. The second key is to hold, or confine, the particles and their energy with magnetic fields long enough for many collisions and reactions to occur. Such fusion reactions would release enormous amounts of energy that can be converted to electricity.
In the quest for controlled fusion, scientists have attained several important milestones. They have achieved plasma temperatures as high as 520 million degrees, more than 20 times the temperature at the center of the sun. More than 16 million watts of fusion power have been produced in the laboratory. The unsolved problem is how to control fusion plasmas to get sustained fusion reactions. The goal is to prevent the loss of heat from the plasma center to the edge as a result of irregular fluctuations in plasma velocity and pressure (turbulence) brought on by the plasma current and other causes.
"Besides heating the plasma in the way that a microwave oven heats food, experiments show that radio waves can drive electric currents through the plasma and force the plasma fluid to flow," says Don Batchelor, head of the Plasma Theory Group in ORNL's Fusion Energy Division (FED). "These waves have even been seen to improve the ability of the applied magnetic field to hold the energetic particles and plasma energy inside the device."
"Radio waves give us the best 'knob' for precision control of the plasma," says Mark Carter of FED. "With radio waves we can control where the power goes, because these waves resonate with the motion of the plasma particles as they orbit around magnetic field lines. Unfortunately, because the orbiting plasma particles move at nearly the speed of light, it has been impossible to calculate how they will respond to radio waves and how much electric current they will produce."
To address this problem, Fred Jaeger and Lee Berry of FED, working with Ed D'Azevedo of ORNL's Computer Science and Mathematics Division, developed a computer program for the IBM supercomputer of the Department of Energy's Center for Computational Sciences at ORNL to compute plasma waves across the entire cross section of a fusion plasma. The program solves an enormous set of equations, providing the first two-dimensional (2D), high-definition picture of radio waves injected from an antenna into the plasma of a doughnut-shaped tokamak. Using 576 processors at speeds of up to 650 billion operations per second, the program shows that, at certain locations, the waves shift from a long-wavelength to a short-wavelength structure (mode conversion) and become rapidly absorbed by the plasma. The group has recently created a 3D code for this modeling that could lead to a method of fine tuning the injection of the waves to maximize control of the plasma.
Another area in which FED scientists are using supercomputers to advance fusion research is in the analysis of very complex, nonsymmetric magnetic systems for plasma containment, called stellarators. These are shaped like a cruller wrapped with twisting magnetic coils. The ORNL supercomputers were used in the analysis and design of a new type of magnetic fusion device called the Quasi-Poloidal Stellarator (QPS). QPS will use a much smaller plasma current and rely more heavily on external coils to provide the needed magnetic fields for plasma confinement. This device may result in a much smaller and more economically attractive fusion reactor than existing stellarators and would eliminate the potentially damaging plasma disruptions that plague conventional research tokamaks. It is hoped that QPS will be built at ORNL, using DOE funds, starting in 2003.
"We are employing a Levenberg-Marquardt algorithm on the IBM supercomputer to calculate how to modify the plasma shape to optimize energy transport and plasma stability," says Don Spong of FED. "Once we have determined the best shape, then we will infer the design of external magnetic coils that can be engineered cost effectively to achieve that shape."
"We need supercomputers to model as many as 40 variables that interact with each other to describe the plasma," Batchelor says. "We are twiddling 40 knobs at the same time computationally to get six or more competing physical properties simultaneously as good as they can be."
The beauty of the new technique developed to study waves is that it can be extended to 3D plasmas such as those in stellarators. These are significantly more complicated in shape than the tokamak, the present state of the art for plasma wave computations.
With the help of ORNL's supercomputers and new funding from DOE's Scientific Discovery through Advanced Computation (SciDAC) initiative, fusion researchers at ORNL are likely to make waves in this important energy research field.
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