The idea of shooting down incoming missiles with a beam of charged atomic particles is still "in the realm of science fiction," according to four Massachusetts Institute of Technology physicists who released a 71-page report on the subject yesterday.

Bernard T. Feld, one of the authors, said "practical particle beam weapons do not appear to be appreciably more imminent than they were when first fictionally placed in the hands of Buck Rogers."

The MIT report comes at the time when dire warnings are being sounded about Soviet progress on these so-called particle beam weapons.

Retired Maj. Gen. George Keegan, former Air Force chief of intelligence, in discussing beam weapons on the Dec. 17 CBS television program, "Sixty Minutes," said, for example, that, "We are entering an era of space war technology which will completely invalidate the strategy of defense of this nation of the last 20 years."

He said the proton beam the Soviets are working on "would simulate a bolt of lightning" which would destroy U.S. warheads in a nuclear war. He has pedicted that the Soviets will have a beam weapon as early as 1983.

Aviation Week magazine has been claiming in a series of articles that the Soviet Union is making significant strides in beam technology.

Feld said in an interview that the Keegan and Aviation Week reports are "tremendously exaggerated."

The MIT report challenging the alarm about beam weapons supports the Pentagon's go-slow approach.

The Pentagon has been spending $16 million to $20 million a year on exploring beam technology, less than the cost of a single modern fighter plane.

Kostos Tsipis, one of the report's authors, said the Pentagon's particle beam research is "interesting physics" but is so far from being applicable to a weapon that even that relatively small amount of research money should be transferred to the Energy Department.

Scientists agree that an atom-smashing machine called an accelerator can produce a stream of charged particles that start out focused like a flashlight beam. But turning that beam into a death ray is something else again.

The MIT physicists said a beam of charged particles falls apart shortly after leaving the accelerator and hitting the air, robbing it of destructive power needed to stop a missile warhead.

Although an electron beam might be produced to bore a hole in the atmosphere for a death ray to race through, the physicists said, an enemy could foil it with such relatively simple measures as fog, chaff or decoy warheads.

In space, a charged beam of electrons, protons or ions would not hold together and also would be deflected from its target by the geomagnetic field, they said.

If an uncharged, neutral atomic beam were produced for space warfare, MIT said, it could be easily countered.

The study group -- comprised of MIT physicists George Bekefi and John Parmentola along with Feld and Tsipis -- said atomic beams would be impractical for these specific roles, citing the flaws:

Missile defense. Radars that would have to be used for antiballistic-missile (ABM) beam weapons based on land could be destroyed by the attacker, making the beam weapons useless.

Also, a charged particle beam would have to make a direct hit on a small incoming warhead. It would be bent by the earth's atmosphere on the way to the target.A charged particle beam ABM "suffers from the same basic vulnerabilities as earlier ABM schemes...."

Ship defense. It would be difficult for the beam to hit an incoming cruise missile, especially if the intruder took evasive action. The beam also could be foiled by explosives that rippled the air, by smoke and by decoy missiles.

Space warfare. Beam weapons deployed in space would be easy to destroy or disable by exploding devices near them or cutting their vital links to ground-control stations.

Since there are cheaper and better weapons available for those jobs, the report said, "it appears unwise and wasteful to expend sums for the development of particle beam weapons."

The report, entitled "Particle Beam Weapons" and distributed by MIT's Program in Science and Technology for International Security, concluded:

"There is serious doubt whether the necessary fire control and beam control systems needed for a weapon are technically feasible." Add to that the ease with which an enemy could foil a beam weapon, "one is forced to the conclusion that in all probability such a system is not a realistic prospect and that, with the current technological horizon that extends to more than a dozen years into the future, such a system could not advance beyond a possible prototype accelerator model."