Under a bright red tent behind concrete walls four stories high stands the start of the industrialized world's hopes for a futures of limitless energy.

Inside the tent, concrete is being poured, floors laid and cables installed to carry power to what is called the Tokomak Fusion Test Reactor.This is the machine that is expected to demonstrate that the fury of the hydrogen bomb can be tamed and tapped for its energy. It is halfway to completion.

Fusion involves the joining of light elements under such terrific force that they release massive amounts of heat. The main fuels for fusion are deuterium and tritum, heavy isotopes of hydrogen, which can be extracted in abundance from seawater.

Nuclear power plants now operates on the principle of fission, or splitting apart of atoms.

"We are on a schedule that will allow us to demonstrate the scientific feasibility of fusion in 1983," Dr. Melvin B. Gottlieb, director of Princeton University's Plasma Physics Laboratory, said in an interview, "We know it's going to work. We have every confidence that we will do it."

Demonstrating the feasibility of fusion means matching the temperature of the sun inside a machine. That temparature -- 100 million degrees -- must be sustained for at least one second, magic numbers scientists have talked about reaching for 30 years.

It also means a fusion reaction rate that pumps more energy out of the fusion machine than is being pumped in that loses less heat through the walls of the machine than is being trapped inside.

"What we're striving for is an energy breakeven point," Gottlieb said. "That's where we'll be able to say we've done it, we've achieved fusion."

By the time the Princeton machine is ready to operate in 1981, the Department of Energy will have spent $284 million to build it. That will be on top of more than $2 billion spent on fusion research since 1951. Reflecting growing confidence in the program, DOE is asking for $403.6 million for fusion research in fiscal 1981, $30 million of it for the Princeton Tokomak Fusion Test Reactor.

"The rapid technical developments in fusion research in the last two years suggest a new evaluation of the program is now appropriate," Dr. Nelson D. Pewitt, deputy director of the Office subcommittee recently. "That such a review is appropriate is testimony to the progress made in the fusion program."

If fusion is demonstrated in 1983, as Gottlieb is convinced it will, some scientists think that commercial electricity can be produced from fusion before the end of the century. An accelerated program to do so would cost at least $20 billion.

The way the fusion program is conceived now, the Department of Energy does not expect to operate the first commercial fusion reactor until 2005, and the second one 10 years after that. Sticking to this timetable would cost an estimated $14 billion.

Under this schedule, a network of fusion electric plants would be in place in the United States in 2025. If the United States decides to speed up fusion development and spend the extra dollars, fusion electricity might be commercial 10 to 15 years sooner.

The commercial development of fusion would bring a swift halt to at least that part of the energy crisis that involves burning oil, coal or uranium to generate electricity.

The breakthrough that inspired such confidence came on July 4, 1978, when a machine called the Princeton Larlge Torus, half the size of the Tokomak test reactor, reached reached a temperature of 60 million degrees and held it for one-twentieth of second.

Since then, scinetists at Princeton and at Tennessee's Oak Ridge National Laboratory, Massachusetts Institute of Technology and General Atomic Research have made steady progress in fusion research.

Oak Ridge scientists have demonstrated they can ignite more deuterium gas in a confined space than was theoretically thought possible. While this may seem like a small step, a doubling of the density of the gas means a quadrupling of the power output. Any time that is done, scientists move closer to what Gottlieb calls "energy break-even."

Oak Ridge scientists also have created larger and more powerful devices caled neutral beam machines, which are the heaters that raise the temperature of the deuterium fuel to more than 50 million degrees. At that temperature, the gas becomes a plasma who electrons have been stripped away.

Supplementing the more potent neutral beam machines will be made radio frequency heaters, which are being tested at the Princeton Large Torus. One advantage of radio frequency heating is its lower cost. Another advantage is that it heats the gas to a plasma in a way that results in less heat loss. Together with bigger neutral beam devices, the radio frequency heaters promise to raise the temperatures of the gas in the Tokomak reactor to the hoped-for 100 million degrees.

Nothing will produce higher fusion temperatures better than the Tokomak itself.

"To get to higher temperatures and longer confinement times, what you have to do is build a bigger device,"

Gottlieb said the Princeton Tokomak (Russian for doughnut-shaped machine) will start up in late 1981, reaching full power sometime in 1983. He said he sees no technical obstacle anywhere ahead that will stop the machine from extracting 100 million-degree temperatures from a fusion reaction for pulses of one second or longer.

"It's gone so well that everybody's telling us we built a machine that's too conservative," Gottlieb said. "Now they're all saying we should have taken a bigger risk and built a bigger machine."

Fusion wasn't always so promising. When Dr. John M. Deutch left MIT three years ago to become undersecretary of energy, his first assignment from then-Energy Secretary James R. Schlesinger was to cut $200 million from the fusion budget that year.

"We looked into it very carefully and I remember briefing Jim [Schlesinger] and saying, 'You can't do it,'" Deutch recalled in an interview. "I said. 'There is a real possibility this will become a serious candidate for electric power generation in the next century.'

"I'm convinced that feasibility will be proven," said Deutch, who has resigned to return to teaching chemistry at MIT. "I'm confident it will happen at Princeton in the next three years."