The Carter administration believes the first electricity to be generated in the United States from thermonuclear fusion will be no sooner than the year 2,005 at an additional cost between now and then of $14 billion.

Despite the late timetable and heavy cost, the administration will seek $500 million in President Carter's next budget for fusion research. The Carter administration believes that while there have been no breakthroughs in fusion research there has been enough steady laboratory progress not to cut the program below its present funding levels.

"The learning curve is on the way up." John M. Deutch, director of energy research for the Department of Energy, said in an interview. "I think there's little doubt we will have a demonstration of the scientific feasibility of fusion by 1982.

That is when the first Tokomak fusion machine at Princeton University is expected to reach the point where it can confine and control the thermonuclear chain reaction that generates heat of almost 1 million degrees. The Tokomak (the Russian word for doughnut, the shape of the machine) at Princeton will begin to operate in 1981 but will not go to full power for another year.

Progress after 1982 is expected to be slow. Deutch said the energy Department now thinks it can operate the first commercial fusion reactor by 2,005 and the second one 10 years after that. He foresees at least three and possibly four or five commercial fusion plants in operation by 2025.

"It is quite possible," Deutch said, "that we will be in a position to produce serious fusion power by that time."

The cost to get to the first experimental fusion reactor is now estimated at $14 billion from now until 2005. The United States has already spent more than $2 billion on the research and development of fusion.

In addition to the Tokomak, the Energy Department has decided to strengthen its scientific support of an alternative method of controlling fusion called the "mirror" approach. This involves the use of magnetic mirrors to confine the hot gas produced in the fusion process.

The Energy Department will also continue support of laser fusion work carried out at the Los Alamos Scientific Laboratory in New Mexico and the Livermore Laboratory in California. The Laser fusion work at both laboratories involves secret research directly related to the nuclear weapons program.

"I think the way the program will step out is a little more work with the mirror approach and a little less with the others," Deutch said. "We want to broaden the program and develop alternatives to Tokomak. I don't think anybody can say which approach will work the best."

The Soviet Union and Japan have each proposed joint programs for fusion research with the United States in the last few weeks. The Common Market countries have chosen to stay together and build what they call the JET (Joint European Torus) machine in Great Britain to demonstrate the scientific feasibility of fusion.

The Japanese proposal to the United States has not yet been made public but the Soviets have suggested jointly building a Tokomak machine bigger than the Princeton device in some third country like Poland or Finland, where rubles and dollars can be spent easily on construction. The machine suggested by the Soviet Union would cost at least $1 billion.

"The Russians see this as the next way to share expenses for these large machines," Deutch said. "The Japanese are trying to make energy research initiatives to help our joint balance of payment problems."