A plasma 10 times as hot as the sun.

A plasma 10 times as hot as the sun

"Tokamak" is a word composed of thefirst syllables of the Russian words for "toroidal magnetic chamber." a Russian invention, the tokamak has been voted by many physicists most likely to succeed in achieving controlled thermonuclear fusion. In the toroidal chamber, magnetic fields confine a plasma -- an

ionized gas -- while its temperature and density are raised to the point where nuclei in it should fuse and produce energy. In experiments conducted during July, one of the world's biggest tokamaks achieved a plasma temperature of 200 million kelvin (200 MK).

This temperature, achieved by theTokamak Fusion Test Reactor (TFTR) at the Princeton (N.J.) Plasma Physics Laboratory, is the highest ever reached in a laboratory. It is 10 times the temperature in the center fo the sun, but more important, it is more than enough for breakeven, the point where fusions produce as much energy as has to be expended to ignite them.

Besides temperature, break-even requiresanother criterion: the product of plasma density and confinement time, usually called the LAwson criterion. In April, TFTR experiments at lower temperatures produced a Lawson criterion of 1.5 X 10.sup.14 seconds per cubic centimeter, which is close to the goal for a practical reactor and five to seven times what is needed for break-even. However, the 200-MK experiments had a Lawson criterion of 10.sup.13., two or three times too small for break-even.

The next step is to put the high valuestogether and get break-even. Donald Grove, TFTR project manager, says they expect to echieve that in 1987 using the hydrogen isotope deuterium, with which they have been working so far. Then they intend to introduce another hydrogen isotope, tritium. Deuterium-tritium fusion, which most controlled fusion experiments today are trying to achieve, produces energetic neutrons, from which energy can easily be harvested and converted to useful things like steam or electric power. They hope for deuterium-tritium break-even in 1989.

One reason tokamaks are expected tosucceed is that they should help confine themselves by producing a "bootstrap current," a current flowing through the plasma around the torus. This current will generate a magnetic field that will help the external magnets confine the plasma. In these experiments, Grove says, a current not due to external driving seemed to arise in the plasma. The physicists concluded that this was probbably the bootstrap current, Grove says, although supporting evidence for that conclusion so far is nil.