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PPPL-led researchers seek to demonstrate a novel design for a key diagnostic tool for ITER

Scientists working under the leadership of the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) have developed and are preparing to test a novel design for a key diagnostic instrument for ITER, a $20 billion experimental fusion facility, or tokamak, that represents the next major step in harnessing fusion power. If proven successful, the design could replace the more conventional, bulkier instrument now planned for ITER.
The new diagnostic design marks a nationwide effort by U.S. researchers in support of U.S. contributions to ITER (whose name is Latin for “the way”), which is under construction in the south of France by the European Union, China, India, Japan, South Korea, Russia and the United States. Scientists at the University of California at Los Angeles (UCLA) and the DOE’s Oak Ridge National Laboratory (ORNL) developed the prototype instrument, which is being tested on the DIII-D tokamak operated by General Atomics in San Diego for DOE. “This is a good example of U.S. fusion experts working together to support the conceptual design,” said PPPL physicist Dave Johnson, who heads the development of the diagnostic tools that the U.S. will deliver to ITER.
The prototype instrument, called a reflectometer, measures the electron density profile of the hot, electrically charged plasma gas that fuels fusion reactions. The profile shows changes in density from the volatile edge of the plasma to the center of the plasma core, and must be maintained at an optimal level for a stable self-sustaining reaction, or burning plasma, to take place. Plans call for ITER to produce a burning plasma for at least 500 seconds during the 2020s.
Radical Departure
The prototype represents a sharp departure from standard “bistatic” reflectometers, which use dual antenna systems: one to launch radar-like microwaves towards the plasma through waveguides, and a second one to carry back the reflected signal for analysis. The dual system aims to prevent any reflection off mirrors, windows or other parts of the first antenna/waveguide system from interfering with the signal coming back from the plasma.
By contrast, the new design features a single, or “monostatic,” antenna/waveguide system to both deliver and return the microwave signal from the plasma. The designers seek to solve the interference problem by increasing the distance from the microwave source to the plasma, giving the system time to filter out spurious radar images. “The goal of the DIII-D test is to see whether you can launch and receive the reflected power on the same antenna,” said Tony Peebles, head of the UCLA Plasma Diagnostics Group that designed the monostatic system together with ORNL engineer Greg Hanson, who created the waveguides that carry the microwave signal.
The single antenna/waveguide system will capitalize on the vast size of ITER, which will be three-to-five times larger than today’s experimental fusion facilities. The vacuum window for the ITER antenna will be many meters from the plasma, for example. This extended propagation distance “will make it significantly easier to filter out spurious radar images,” said Peebles. If the tests on DIII-D are successful, he noted, the prospects for a monostatic system look promising for ITER.
The UCLA design anticipates the layout of the ITER system on a smaller scale. The prototype antenna/waveguide system measures 39 feet long by 3.5 inches in diameter, compared with the 147-foot-long system that is contemplated for ITER. The prototype resembles a pipeline that runs through a series of right-angle bends between the microwave source and the plasma.
Single Antenna Advantages
Benefits of the monostatic system could range from increased diagnostic capability to potential cost savings. Six monostatic transmission systems could perform the same measurements as the twelve bistatic systems currently planned for ITER. This “monostatic advantage” would allow a potential cost-savings related to construction, installation and maintenance.
Alternatively—and perhaps preferably—increased measurement capabilities could be introduced. For example, additional monostatic transmission systems could be installed to perform highly desirable measurements related to the study of waves and instabilities that degrade plasma confinement and stability.
Researchers at DIII-D will be led by UCLA in testing the monostatic prototype on the tokamak starting in May and running throughout the summer. Findings will be compared with those previously obtained from the bistatic antenna system that has long been on the tokamak. “This is a case where the whole U.S. community is working together for ITER,” said Rejean Boivin, director of the DIII-D Computer and Diagnostic Systems division. “It’s good science and very valuable engineering tests.”
Results will help PPPL’s Johnson determine whether the monostatic system warrants further consideration for the final antenna design. A contractor will be picked later this year and ITER is to review plans for the chosen system in 2014. “We hope to learn enough from the DIII-D tests to assess the feasibility of the monostatic design,” said Johnson. “Based on these results we will possibly make a recommendation to modify the reflectometer to be monostatic.”

U.S. Department of Energy
Princeton Plasma Physics Laboratory is a U.S. Department of Energy national laboratory managed by Princeton University.

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