IN ITS FIRST test, shortly before dawn on the day before Christmas, the new Tokamak Reactor at Princeton University produced a plasma of superheated gas. It means that the machine works. That brief test, in itself, was a technological triumph of a very high order. But it is chiefly significant as another important step toward the production of energy by nuclear fusion. Now comes a long series of further tests and refinements that will gradually increase the temperature and duration of the plasma. The next great goal is the break-even point, at which the fusion process generates as much energy as it requires. The Princeton physicists hope to be there by 1986.

The fuel for this machine is hydrogen, as common as water. The principal product of the reaction is helium. While this line of experiment has immense interest for physical theory, its substantial costs are borne by a government that--like others around the world--is looking for an alternative to the uranium- plutonium fuel cycle of the other nuclear technology, the fission process by which the present reactors operate. Americans, Russians, Europeans and Japanese are all working hard on fusion as the route by which to escape the day when uranium might be scarce and its desperately dangerous fission product, plutonium, is all too plentiful.

For the past several years there has been much quarreling here in Washington over the proper purposes of federal aid for scientific research and development, particularly in the area of power generation. The fusion reactor is the leading example of the kind of work for which the case for federal support is beyond argument. It is expensive--the government will put nearly half a billion dollars into the field this year --and commercial application is utterly unlikely for half a century. But there is a great deal of real value to be learned from the experimentation now successfully launched. And it now seems highly probable that a point will come, sometime in the next century, when this technology will be worth any price as the alternative to other and far more dangerous technologies now in prospect.

That reference to dangerous technologies includes not only the plutonium breeder reactor, but the conventional coal-fired generator. The implications of rapidly expanding use of coal, both for human health and for the global climate, are now clear enough to invite real doubts about the wisdom of encouraging it indefinitely. But surely the most dangerous of all these competing methods is the breeder reactor, with the production of plutonium on a commercial scale. A divided Congress, by the narrowest possible margin, has just appropriated money to begin building the prototype Clinch River Breeder Reactor. Congress was wrong. Going ahead with the breeder is an intolerably risky path to follow. Does not the highly promising test of the Tokamak machine provide still another compelling reason to rule out those risks and, at last, to kill Clinch River?