Fusion: A Big Win For ORNL

ORNL's selection as a U.S. project manager for ITER means the Laboratory's multi-pronged fusion research will likely influence the international fusion reactor's design and research program.

U.S. participation in designing and operating an international fusion reactor is the top priority in the Department of Energy's 20-year plan for scientific research facilities. So it was not surprising that ORNL staff were elated by a major announcement in July 2004 by Ray Orbach, director of DOE's Office of Science. Orbach said that ORNL, in partnership with the Princeton Plasma Physics Laboratory, will lead the U.S. contribution to the International Thermonuclear Experimental Reactor project.

Artist's rendering of the ITER device

Construction of ITER, a $5 billion international fusion experiment, is scheduled to begin in 2006 with initial operations in 2013.

The U.S. project office for ITER will be hosted by PPPL, located in New Jersey. The Princeton-ORNL team will oversee the office, provide staff and facilities, and support construction of ITER at a site in either France or Japan. ORNL has traditionally been a partner on fusion-related projects with PPPL, a national collaborative center for plasma and fusion science.

The potential of fusion energy is extraordinary in the context of the world's energy and environmental challenges. A fusion power plant would produce no greenhouse gas emissions, use widely available fuel, require no fissionable materials, produce heat continuously to meet demand for electricity, shut down easily, and produce manageable radioactive waste. In a fusion power plant, the charged particles of the plasma fuel—heavy hydrogen nuclei such as deuterium extracted from seawater and tritium bred in the reactor—would be heated to 100 million degrees and held close together by magnetic fields for a sufficient time for heat-producing fusion reactions to occur. The heat would make steam to produce electricity.

Heating and Fueling the Plasma

Because of ORNL's outstanding research contributions to fusion physics and technology since the 1970s, DOE has selected the Laboratory to play an important role in the international fusion project. ORNL will be involved in designing and building systems for heating and refueling ITER's plasma.

"Our approach to ITER is to provide an integrated package of research and development to maximize the impact of our contribution," says Stan Milora, director of ORNL's Fusion Energy Division. "As such we have targeted key plasma control technologies for heating, fueling, and diagnosing the plasma, and we will complement these contributions with theory and simulation, technology developments, and experiments on existing fusion facilities."

ORNL's David Swain, who works part-time for the ITER International Team, is responsible for development of ITER's ion cyclotron heating (ICH) system. ICH will boost the temperature of the ITER plasma.

"My job involves leading a team of U.S. and European researchers to design, build, and assemble an ICH system," Swain says. "ORNL researchers, led by Dave Rasmussen, have been doing most of the U.S. research and development on the antenna, the critical component of the ICH system and its biggest technical challenge."

The antenna will deliver 20 million watts of high-frequency radio waves to the ions in the plasma, causing the plasma to heat up. One ORNL improvement in the antenna is the location of capacitors right behind the antenna's beryllium surface at the vessel's inner wall. The capacitors would be used to tune the radiofrequency waves, enabling the waves to control, as well as heat, the plasma.

Rick Goulding and other members of Rasmussen's group, in another partnership with PPPL, are developing a prototype of the ICH antenna that will be installed and tested on the Joint European Torus (JET) in England. A one-strap prototype has been built and tested at ORNL in vacuum at high voltage.

"We saw some problems in the original design," Swain says. "We found hot spots in the antenna, so we modified the design and plan to test it again in February 2005. If successful, the Europeans will then build a four-strap antenna. ORNL will work with them on its final design and operation at JET, probably in 2006. If this ICH design works well, then it should be a strong contender as the plasma heating system concept for ITER."

How effectively RF waves of different frequencies heat and control the plasma is being determined by three-dimensional models run on a supercomputer at ORNL. Don Batchelor, theory group leader, is leading this effort as part of a Scientific Discovery through Advanced Computing (SciDAC) project in collaboration with PPPL, the Massachusetts Institute of Technology Plasma Science and Fusion Center, and three small businesses.

A team led by ORNL Corporate Fellow Steve Zinkle is studying how well the materials in the ICH antenna, high heat flux regions of the vessel, tritium test breeding modules, and plasma diagnostics will hold up under neutron irradiation and other stresses.

Pellet fueling, pioneered at ORNL by Stan Milora and Chris Foster in the late 1970s and further developed by Steve Combs and Larry Baylor, has been used successfully to refuel the plasmas of fusion research devices in England, France, Germany, and Japan. The pellets of frozen deuterium at a temperature of about 10 degrees Celsius above absolute zero must be accelerated to high speeds to penetrate to the hot fusion plasma. Milora and Combs developed a gas-powered pellet injection gun used in U.S. and European fusion experiments, and Foster invented a centrifugal-type mechanical arm to fling pellets into the plasma. This technology, employed in Europe and Japan, likely will be used on ITER.

The challenge for ORNL researchers is to build a device that produces frozen deuterium and tritium pellets, the size of pills, and injects them into ITER's plasma at 300 meters per second, a speed equivalent to that of a Boeing 747 jet at altitude. The pellet system must produce 10 times more deuterium ice than is made at fusion devices today. ITER requirements call for the system to accelerate 16 pellets per second for an hour, well beyond the typical systems that now produce 10 pellets per second for 10 seconds.

Combs and Baylor have demonstrated that they can transport pellets through a long, winding ITER-like tube, and that the pellets can survive this roller-coaster ride. How well frozen pellets refuel an ITER-like plasma is being studied by ORNL researchers at the DIII-D fusion device in San Diego, in collaboration with General Atomics.

Probing Plasma Physics

ORNL researchers are engaged in a wide range of physics research activities that contribute to the ITER program. At DIII-D Mickey Wade and Masanori Murakami are leading a study on the hybrid scenario for operating ITER to achieve high fusion output.

Schematic of the DIII-D pellet injection system showing the tokamak with plasma (left) and the injector (right), which produces and accelerates frozen hydrogen pellets that are directed into the plasma.

In this scenario a current generated by the plasma itself minimizes the need for a transformer, making the fusion power plant more attractive to electric utilities.

Donald Hillis is involved in developing a key diagnostic tool for JET that is a strong contender for use on ITER. JET is the only tokamak capable of producing an ITER-like, deuterium-tritium (D-T) plasma and the helium "ash" resulting from fusion reactions. In future burning fusion devices, spectrometers will continuously monitor helium ash as it is produced and removed from the plasma core to prevent D-T fuel dilution and quenching of the burn.

Hillis has fabricated two high-efficiency spectrometers for measuring the helium ash concentration produced in JET. He also uses spectroscopy to measure variations in the intensity of the light emitted from JET's plasma to determine its ion temperature and ion flow velocities, key indicators of fusion device operation.

This wide spectrum of capabilities contributes to making ORNL a leader in fusion research. "Our goal at ORNL is to have a lead role in ITER's experimental program, and that's why we are seeking to support the ITER project in these critical areas," Baylor says. "In that way, ORNL would be a key player in demonstrating to the world the feasibility of fusion as part of a long-term answer to our energy needs."

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