The attainment of a temperature of 60 million degrees for a half second in a Princeton University laboratory was described yesterday as the first step toward a limitless supply of electricity for the entire world.

"We're going to make it, we're going to demonstrate the scientific feasibility of fusion," Dr. Melvin B. Gottlieb, director of Princeton's Plasma Physics Laboratory, said at a press conference held at the Department of Energy. "We're on schedule and we're very confident we will demonstrate fusion . . ."

Gottlieb said the demonstration of fusion could come as early as 1981 or 1982, when the Tokomak Fusion Test Reactor under construction at Princeton will begin to operate. This is a machine twice the size of the laboratory device used at Princeton to achieve fusion temperatures of 60 million degrees.

"We need to reach temperatures 100 million degrees to demonstrate fusion," Gottlieb said. "There is no reason we cannot reach those temperatures in the next machine, which is a larger machine than we're now operating."

He said the attainment of a 60-million-degree temperature in the present Princeton machine came sooner than he or anybody else expected.

"It took us seven years to go from 5 million degrees to 25 million degrees," he said. "It has taken us six months to go the last 35 million degrees and reach the 60-million-degree mark. The results came in much better than we anticipated."

The most encouraging result of the Princeton achievement is that the 60-million-degree mark was reached without the hot hydrogen-deuterium gas mixture "clumping" together in swirls that would tend to cool the gas and spoil the fusion reaction.

"The fear was that the hot gas would clump up and leave cold spots that would slow down the fusion reaction," Gottlieb said."To our great joy, we found out that this is not go."

He said this means that the larger machine now being built can be equipped with a much thicker magnetic field to insulate the hot gas and raise its temperature to the desired 100-million-degree mark.

"The invisible layers of magnetic field that confine the gas can be increased in scale in the bigger machine," he said. "The insulation that the magnetic field provides can be steadily improved upon in the next machine."

Calling the attainment of the 60-million-degree mark "extremely gratifying," Gottlieb declined to describe it as a scientifi breakthrough.

"Breakthrough is not a scientific term," he said. "I have always avoided the word and I will continue to avoid it."

The way he described it, the Princeton Large Torus machine that achieved the 60 million degrees did so two weeks ago. He gave credit for the achievement to the use of a technique called "neutral beam injection" to help heat the gas confined inside the machine.

The 60-million-degree temperature is more than twice the temperature previously reached by the Princeton machine, a mark it made in December. The new record temperature is four times hotter than the interior of the sun.

The opposite of nuclear fission - the basis for today's nuclear power plans - where heavy atoms like uranium are broken apart to release energy, fusion is the combining of light atoms like hydrogen with an accompanying discharge of energy.

Fusions is clean producing no radioactive wastes to be disposed of. It is limitless, drawing its fuel from seawater from which the light elements like hydrogen and deuterium (a heavy kotope of hydrogen) can be extracted.

John M. Deutch, director of energy research for the Department of Energy, cautioned yesterday that the Princeton achievement does not change the national timetable for the commercial production of electricity from fusion. He reiterated that the first fusion plant would not be built before 2005 and that fusion would not be commercial before 2025.

"The first fusion reactor won't be seen before the first decade of the next century and it could be another 20 years before fusion goes commercial," Deutch said. "We're talking about a developmental budget of $10 billion or more in which we have no assurances that the engineering difficulties involved in generating electricity from fusion will be overcome."