Stan Milora, director of ORNL’s Fusion Energy Division (FED), has many reasons to be optimistic about the future of fusion energy research at ORNL and in the United States. “Fusion is an attractive long-term energy option,” he says. “Fusion energy will rely on virtually unlimited fuel available in the United States, not imported fuels.
“Fuel from 50 cups of water contains the energy equivalent of 2 tons of coal. Fusion can be environmentally acceptable because it releases no air pollutants, including greenhouse gas emissions. Compared with fission its radioactive wastes are short-lived, with a more manageable disposal problem. Fusion is expected to be economically competitive with other power sources when the costs for waste disposal and carbon management are included. The fusion community believes that if fusion research were to receive sufficient support, commercial fusion power plants could start supplying electricity by the middle of this century.”
A fusion energy device is fueled by a plasma, an extremely hot state of matter consisting of charged heavy hydrogen ions (e.g., deuterium from seawater and tritium bred in the device) and electrons. If magnetic fields confine the plasma long enough at a sufficiently high density and temperature, sustained production of fusion reactions will result, providing the heat needed to generate electrical power.
What is ORNL doing to advance fusion technology? ORNL’s long-time involvement with the Princeton Plasma Physics Laboratory (PPPL) has been strengthened with a focus on developing innovative plasma confinement devices, such as the National Spherical Torus Experiment (NSTX) and a new class of compact stellarators.
The NSTX, which began operation at Princeton in 1999, is based on a very compact high-performance toroidal confinement concept that was advocated by Martin Peng in the mid-1980s at ORNL. Partly inspired by this success, ORNL’s fusion theory group began investigating a compact hybrid of the stellarator (pioneered in 1951 at PPPL) and the tokamak. This device relies on external coils (the stellarator) and on a plasma current (the tokamak) for magnetic confinement. The tokamak is currently the front runner in magnetic fusion energy. “Ben Carreras and Don Batchelor of our theory group first started working on exploring the compact stellarator in 1995,” Milora says. “PPPL became interested in pursuing the concept in 1998.”
ORNL management enthusiastically supports the collaboration between Oak Ridge and Princeton as co-developers of the proposed National Compact Stellarator Experiment (NCSX) at PPPL, the main element of the U.S. Compact Stellarator Program. The smaller Quasi-Poloidal Stellarator (QPS) at ORNL is a principal support element.
The NCSX is in President Bush’s fiscal 2003 budget request, and construction is scheduled for completion at PPPL in 2006. If all goes as planned, the compact NCSX will exhibit the attractive performance levels of the tokamak without some of its engineering drawbacks. ORNL will have several leadership roles through the design, construction, and operation stages. FED’s Jim Lyon is deputy project manager, and an engineering team led by FED’s Brad Nelson is responsible for design of the stellarator core, including the com-plex magnet coil system. FED’s Steve Hirshman has led the effort to develop coil configurations that are buildable and at the same time deliver the requisite plasma shaping for equilibrium, stability, and confinement at high plasma pressures.
The very-low-aspect ratio QPS was designed by Steve Hirshman, Dennis Strickler, Lee Berry, and Don Spong, all of ORNL, and Andrew Ware of the University of Montana and their PPPL colleagues, using the IBM SP supercomputer at the Department of Energy’s Center for Computational Sciences at ORNL. Unlike the tokamak and NSTX, which are shaped like a doughnut, the QPS plasma resembles two linked sausages. This highly shaped configuration is expected to eliminate the violent plasma disruptions common in conventional research tokamaks at high plasma pressures because it will have only a fraction of the plasma current.
“We are using advanced computational tools and terascale computing to design the magnetic field shapes that will ensure plasma equilibrium and stability with good confinement of plasma particles,” Milora says. “A new ‘optimizer’ simultaneously targets these physics goals and the engineering constraints needed to provide feasible, affordable coils. This integration was unimaginable just a few years ago.”
The physics and engineering properties of the proposed QPS were validated in 2001 by a technical review committee from the fusion community. QPS is now in the conceptual design phase. If approved by DOE next year, the experiment will be built and ready for operation by 2007 at ORNL. The QPS may be small but it could have a big impact on fusion device design.
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