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Physicists discuss fusion breakthrough

Calling last month's nuclear fusion breakthrough at Princeton University an historic accomplishment that shows the scientific feasibility of nuclear fusion as a controllable source of energy, two MIT physicists who are leaders in the field recently explained the importance of the Princeton work and described ongoing MIT contributions to the effort.

Dieter J. Sigmar, acting director of MIT's Plasma Fusion Center (PFC), and Miklos Porkolab, associate director of the PFC and a professor of physics, also compared the Princeton Tokamak Fusion Test Reactor (TFTR) that made the world-record amount of fusion power to the experimental reactor here at MIT. Among other things, the MIT scientists noted that when the Princeton reactor is shut down in about a year (it will have fulfilled its mission), MIT's recently completed Alcator C-Mod "will be the main fusion research reactor on the East Coast." (Princeton hopes to build another reactor by the year 2000; the other existing advanced reactor in the United States is in San Diego.)

The Princeton research, which to date has resulted in a peak power of 6.1 million watts (compared to the previous record of 1.8 million watts produced two years ago in the Joint European Tokamak, or JET, in England), is important for two reasons, said Dr. Sigmar.

First, the record amounts of power were achieved using a fuel composed of a fifty-fifty mixture of deuterium and tritium, two heavy forms of hydrogen. This fuel mixture is the most likely to be used in the first practical fusion power plants. (The JET experiment used a deuterium/tritium fuel containing only about 10 percent tritium.) Commenting on the significance of this achievement, Professor Porkolab, who has worked in the field for 30 years, said "When I started research in the '60s I never thought I'd see D-T [deuterium-tritium] fusion power produced in my lifetime."

Second, the Princeton tests, which will continue through September, will allow the first studies of the alpha particles produced by D-T fusion. These particles are critical to fusion's Holy Grail: ignition, a self-sustaining nuclear reaction that requires no additional energy after the reactor is started up. (Fusion reactors including Princeton's have yet to reach the "breakeven" point when they produce as much power as they consume to achieve fusion. "This is a matter of scaling up to `critical size,'" Dr. Sigmar said. )

Princeton's TFTR consists primarily of a donut-shaped vacuum chamber about 30 feet in diameter surrounded by powerful magnets. To achieve nuclear fusion, deuterium and tritium are injected at very high energies into the vacuum chamber where they lose their electrons and become an electrically charged gas, or plasma. Strong magnetic fields then confine the plasma at temperatures many times that of the core of the sun, until the deuterium and tritium nuclei fuse.

This reaction results in an alpha particle (or helium nucleus) and a neutron, both of which have enormous energies. In a commercial fusion power reactor the neutrons, which contain most of the energy from the reaction, would leave the vacuum chamber and be captured in a lithium blanket surrounding the chamber, where they would give up their energy. That energy could be used to generate electricity.

But the alphas also have high energy, and that energy could be used to reach ignition. Because the alphas are positively charged, they remain confined within the vacuum chamber by the magnetic field. As a result physicists are hopeful that they will transfer their energy back to the plasma fuel, maintaining the reaction indefinitely. Until now, scientists had not been able to study the behavior of alpha particles under D-T fusion conditions (too few were produced in the first D-T test two years ago in the JET reactor).


MIT contributions to the TFTR experiments include a diagnostic device that will measure the density distribution of alpha particles in the plasma. The microwave-scattering device, which was developed and installed by two PFC scientists-principal research engineer Paul P. Woskov and research scientist John S. Machuzak-is expected to be operational this month.

"I can't think of a more fundamental diagnostic device," Dr. Sigmar said, noting the importance of learning more about the behavior of alpha particles.

Also key to the TFTR work is a plasma conditioning device that injects pellets of lithium into the plasma. "The lithium, which eventually gets deposited on the wall of the vacuum chamber, controls the plasma wall interaction and improves plasma performance," Dr. Sigmar explained. The lithium pellet injector was developed by senior research scientist Earl S. Marmar and Research Scientist James L. Terry, both of the PFC. Joseph A. Snipes, another research scientist at the PFC, also participated in this project in previous years.

In addition, Dr. Sigmar said, senior research scientist Jay Kesner of the PFC has proposed a series of experiments for the TFTR that will explore "what happens when the plasma is maintained at a high pressure but the plasma current [that helps confine the plasma] is reduced." The idea is to "maintain the plasma at as low a current as possible, which would result in a more economical reactor."


MIT's fusion research reactor, the Alcator C-Mod, operates under the same general principles as the TFTR (both belong to a class of reactors called tokamaks), but it has some significant differences. For example, C-Mod was not designed to use a deuterium-tritium fuel mix; it burns only pure deuterium.

"We're in the game to improve the tokamak concept, which means we're investigating such things as the confinement, heating and stability of the plasma," Dr. Sigmar said. "And for those types of investigations you don't need tritium." He noted, however, that MIT's Professor Bruno Coppi of physics, who initiated the high-magnetic-field Alcator tokamak approach, is spear-heading a project to develop a closely related tokamak device in Italy (called Ignitor) that will use deuterium-tritium fuel.

C-Mod is the latest tokamak to be built in the US (it commenced operation in May 1993), and represents the cutting edge in fusion research reactors. For example, while the TFTR plasma has a circular cross-section, C-Mod's plasma cross-section is shaped more like a D. "It has been found by researchers around the world that this new geometry produces much better confinement of the plasma," Professor Porkolab said. In addition, C-Mod includes an advanced "exhaust channel," or divertor, for controlling the exhaust of particles and power from the tokamak.

These and other technical features, including C-Mod's high magnetic field and compact size, make C-Mod very efficient and cost-effective. While C-Mod, which would fit in a garage-albeit a large garage-is much smaller than the five-story TFTR, "the plasma parameters (such as plasma pressure and current) that can be achieved in it may ultimately be similar to those in TFTR," Dr. Sigmar said.

Underlying the C-Mod program is a strong commitment to education. "The Plasma Fusion Center is the leading laboratory in the United States in terms of number of students trained in fusion plasma science and engineering," Dr. Sigmar said. "Cross-generational training is critical in a long-term research program such as the quest for fusion energy."

A version of this article appeared in the January 5, 1994 issue of MIT Tech Talk (Volume 38, Number 18).

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