BOSTON — ON Christmas Eve 11 years ago, a team of scientific ``wisemen'' began following a new kind of ``star'' - a miniature star held in a magnetic bottle right here on Earth. Now that star has led them to a major milestone along the road to practical nuclear-fusion power.
Ever since these scientists and engineers inaugurated the United States' largest experimental fusion reactor at the Princeton University Plasma Physics Laboratory that Christmas Eve, they have been working with a ``practice'' fusion fuel. Now they're ready to burn what deputy laboratory director Dale Meade calls ``the real stuff.''
Starting either today or early next week, the laboratory's Tokamak Fusion Test Reactor (TFTR) will be running on the kind of fuel a future commercial fusion power plant most likely will use. At first, the fuel will be a little lean. But in the new year, it will have the richness that a power-producing reactor requires. Dr. Meade likens the switch from the old fuel to the new to a switch from a low- to a high-octane rating in a gasoline engine.
A fusion reactor generates energy by fusing together the nuclei of various forms of hydrogen. The fusing hydrogen nuclei form helium nuclei and release energy in the process. For the past four decades, virtually all fusion reactors have used pure deuterium (doubly heavy hydrogen). That's the ``low octane'' fuel.
With this fuel, scientists have learned how to use magnetic forces to contain and control the star-hot gas, called a plasma, in which the reactions take place. That means working with the kind of nuclear reactions that power a star, but doing it at temperatures six to 13 times hotter than the core of our sun.
So far, physicists have done this only in bursts of a few seconds each, which uses up more power than is produced. The long-term goal is to ignite self-sustained fusion that produces net power.
Now the Princeton reactor will be using a mixture of deuterium and tritium (triply heavy hydrogen). That's the ``high octane'' fuel. D-T fusion produces more than four times the energy of pure deuterium fusion. However, it also releases energetic neutrons that make the reactor structure radioactive. Moreover, tritium is, itself, radioactive. This makes working with the ``high octane'' fuel trickier than handling pure deuterium. That's why experimenters have avoided it up to now.
The Joint European Torus (JET) reactor at Culham, England - now shut down for upgrading - briefly reached a fusion power output of two megawatts with an 11 percent tritium mixture in 1991. That's the only time up to now that a D-T fuel mixture has ever been used. The Princeton team will begin with a similarly lean fuel mix. But it expects to be using a 50-50 deuterium-tritium mix next year.
MEADE says his laboratory is aiming for five megawatts of fusion power by this Christmas Eve and 10 megawatts next year. It will still take more power to run the reactor than the reactor produces.
The radioactivity is relatively easily handled. Tritium radioactivity is not penetrating and does not require heavy shielding. The radioactivity induced in the reactor structure dies away fairly quickly once the reactor is shut down. There is no radioactive waste from the fusion reactions themselves. The experiment will be ended late next year. Meade says the reactor should be safe to dismantle within a year or so after that, to make way for a possible successor installation.
Fusion scientists see the new Princeton work in terms of three stages of fusion-power development - scientific feasibility, engineering feasibility, and commercial feasibility.
Scientific feasibility means understanding how to control fusion. That has involved learning how to use magnetic forces to contain and control a star-hot gas that is as turbulent as a pot of boiling water. Meade notes that working with the new fuel should substantially finish this phase of the development.
The joint American, European, Japanese, and Russian International Thermonuclear Reactor (ITER) project, now under way, will explore engineering feasibility. That means learning how to build a practical fusion-power reactor. It should ignite self-sustaining fusion.
Georgia Institute of Technology physicist Weston Stacey in Atlanta, who is with the ITER steering committee, says the Princeton work is ``very important for ITER.'' It will help in understanding how to make fusion self-sustaining.
As for demonstrating commercial feasibility, that's a challenge for the next century, perhaps around 2020 to 2025.