Princeton University scientists have made a major advance in the race to tame nuclear fusion - the hydrogen bomb process - which a top government energy expert said yesterday could lead to the production of the first practical working fusion reactors.

Using a small, donut-shaped test reactor, the scientists produced sun-like temperatures of more than 60 million degrees centigrade inside a kind of magnetic bottle.

"It is the first time we've produced the actual conditions of a fusion reactor in a scale-model device," said Dr. Stephen O. Dean, director of the Department of Energy's magnetic confinement systems division

"This is the biggest thing that has ever happened in fusion research," he said.

"The question of whether fusion is feasible from a scientific point of view has now been answered," Dean added. "The practical questions of what price it will cost and when it will be duplicated commercially are now engineering and economic questions."

The potential payoff is unlimited electrical energy, one of the prerequisites for the long-term survival of advanced civilization.

The attractions of taming the fusion process are great. Fusion, the joining of two atomic particles at high temperatures, is the same nuclear reaction that powers the sun.

The reaction is the most efficient one known, producing 180 times more usable energy than it consumes. Its fuel, "heavy" forms of hydrogen, is as inexhausible as sea water. One glass of water contains fusion fuel equal to the energy in a ton of gasoline. One pound of "heavy" hydrogen produces the same energy as 5,000 tons of coal.

Heavy hydrogen refers to isotopes of hydrogen known as deuterium and tritium. They possess much greater mass than ordinary hydrogen. They occur naturally and can be extracted from sea water.

The fuel has the added attraction of "burning out" without leaving radio active waste like that from modern atomic power plants.

Current nuclear reactors use fission, the same process used in the atomic bomb. They depend on a limited world supply of uranium and plutonium.

Scientists have known about the fusion process for more than 40 years. It was first made to work in the hydrogen bomb.

But the problem has been controlling it for peaceful uses. The basic problem has been to contain a searing plasma of hydrogen ions long enough and in thick enough concentration for them to smash together with sufficient energy to fuse, releasing energy.

Two conditions must be met for the ionized plasma to undergo a continous fusion: the reaction temperature must be at least 44 million degrees centrigrade, and each cubic centemeter of the plasma must contain at least 100 trillion ions for one second.

Two years ago both American and Soviet scientists achived 10 million degrees in experiments. The Princeton group, working under government contracts, reached 25 million degrees last December.

Using a donut-shaped device called Princeton Large Torus, a concept originally developed by the Soviets, the New Jersey scientists produced a temperature of 60 million degrees in recent weeks and they estimate that their scale model will reach up to 100 million.

They did so without damaging the confinement of the plasma in its strong magnetic field - a fear long expressed by scientists.

The experiments were conducted in a 3-foot-high and 10-foot-wide donut-shaped device that encloses two smaller donuts. The smaller plasma donut is about 30 inches in diamete. It is surrounded by a vacuum chamber about 3 times as large, which forms a "magnetic bottle."

An intense beam of hydrogen atoms is shot through the walls of the magnetic bottle to heat up the plasma and make the reaction occur.

The process was developed by Dr.Harold P. Eubank of Princeton's Plasma Physics Laboratory and Dr. H. H. Hazelton of Oak Ridge. Tenn.

It is to be announced in a scientific paper to be delivered Aug. 23 at an international conference on controlled fusion and plasma physics at innsbruck, Austria.

The Department of Energy is expected to announce the advance next week, although official press spokesmen yesterday denied that.

Dr. Harold Furth, one of the chief Princeton researchers, said DOE officials had asked the scientists to refrain from comment.

"All I can say is we've developed something really important," he said. "Nothing has happened of this significance in fusion research in the last 10 years."

Dean said the temperature experiment was one of two major break-throughs expected in the fusion reactor field. The second, expected next year, will be to sustain the temperature for longer periods.

"We now know how to set the size of the reactor and how to calculate and scale it up in size," he said.

The temperature and time elements will be brought together in a full-scale test reactor, currently under constructuon at Princeton. It is scheduled to be completed in 1982.

It will probably be 20 to 30 years before the commercial applicability of fusion reactors is demonstrated, Dean said.