For the last three years, UCI scientists have participated in magnetic fusion experiments at Princeton's Tokamak Fusion Test Reactor (TFTR). These experiments have set a new record (10.7 megawatts) for the amount of power produced by controlled nuclear fusion.In the reactor, the two hydrogen isotopes deuterium and tritium fuse to produce a neutron and a helium nucleus (alpha), releasing 17.6 MeV of energy. The fuel ions are confined in the reactor by strong 5 Tesla magnetic fields that wrap around in the donut-shaped ``tokamak'' configuration. The goal of the research is to produce a fusion power plant that is cheap, clean, and uses a virtually inexhaustible fuel.
A large team of some 150 scientists participated in the experiments. The UCI collaborators include faculty (William Heidbrink), postdoctoral scholars (Hau Duong), graduate students (Emil Ruskov and Jing Fang), and undergraduates (Transon Nguyen). The major innovation in the TFTR experiments was the use of tritium. Apart from a brief preliminary study in 1991 at the European Community's Joint European Torus, previous tokamak experiments have not used tritium fuel. (Because tritium is radioactive, expensive safety precautions are required for its use.) The motivation for tritium experiments fall into three main categories: increased power production, new effects on plasma behavior, and enhanced diagnostic capabilities. The UCI scientists have participated in experiments in each of these categories.
The deuterium-tritium reaction has a large fusion cross section. In the TFTR experiments, the combination of high fuel-ion temperatures (>30 keV or over 300 million degrees centigrade) and the injection of energetic neutral deuterium and tritium with energies of 100 keV insures that the relative velocity of colliding fuel ions coincides with the maximum of the fusion cross section. The total fusion power is determined from measurements of the number of 14 MeV neutron reaction products. The first major effort produced 6.2 megawatts of fusion power. Subsequent increases in fuel-ion temperature have produced 10.7 megawatts. The UCI team developed one of the 14 MeV neutron detectors used in these fusion-power experiments.
The second major category of experiments is the exploration of new effects associated with deuterium-tritium plasmas. One welcome (and quite unexpected) discovery was that the confinement of energy in the tokamak substantially increases when tritium is employed. There also have been extensive searches for deleterious effects. It was feared that the 3.5 MeV alpha particles produced in the fusion reactions might create new instabilities. In one study, Professor Heidbrink led a weeklong search for a particular instability that occurs when the plasma pressure is large. Fortunately, no evidence of an alpha-driven instability was uncovered.
It is easy to measure the large number of reactions produced by tritium fuel ions. The third category of experiments exploits these accurate measurements to give improved understanding of plasma behavior. For his Ph.D. thesis, Emil Ruskov used 14 MeV neutron measurements to assess the confinement of tritium that is injected as neutral beams. He found that these tritium ``beam ions'' are very well confined in the center of the plasma but that their confinement deteriorates near the outer edge of the plasma due to irregularities in the structure of the magnetic field.