Test Site 300, near Tracy, Calif. — Scientists working in a new, $55 million facility hidden in the dry, brown hills between Livermore and Tracy hope to advance development of a defensive weapon any future adversary of the United States will respect - a particle beam device that could destroy incoming nuclear warheads.
The possibility of developing charged-particle beam (CPB) weapons has long been discussed, and research has been going on for some 25 years in the Soviet Union and the US. But when President Reagan, in a speech early this year, speculated on the use of such devices as defensive weapons, his remarks created quite a stir.
Development of this ''ray gun,'' to use comic-book terminology, is by no means a certainty. Among scientists who have expressed serious doubts about its feasibility are John Parmentola and Kosta Tsipis, physicists at the Massachusetts Institute of Technology (MIT). They and other critics have cited problems of accuracy, cost, and the ability of an adversary to devise countermeasures.
Recently, a group of reporters and photographers was admitted to Site 300 - a tightly guarded and usually off-limits test facility of the Lawrence Livermore National Laboratory - for a guided tour of the new Advanced Test Accelerator (ATA).
Scientists who conducted the tour clearly hope the ATA will enable them to take a concept that first appeared decades ago in science fiction closer to scientific fact.
Later this summer, the ATA team headed by Richard J. Briggs will begin ''beam physics experiments'' at the new facility. Basically, that means generating and controlling electronic pulses that eventually will be directed at a target at the end of a 540-foot-long, three-section tunnel.
In the words of the scientists themselves: ''The ATA is designed to produce short pulses of electrons, each about 21 meters long, in several modes. Typical modes are 1 pulse per second, 5 pulses per second, and a burst of 10 pulses separated by one-thousandth of a second, repeated every two seconds.
''The current of the electron beam rises from zero to 10,000 amps in about 20 billionths of a second, and is maintained at that level for at least 50 billionths of a second before decreasing to zero in another 20 billionths of a second.''
To generate this powerful beam, ATA takes 18,000 volts of electricity from a Pacific Gas and Electric Company substation, uses transformers to boost it to 250,000 volts, then stores it in a series of 235 capacitors called Blumlein sections. These sections store the electricity for 20 millionths of a second and release it in 70 billionths of a second. When this pulsed voltage is applied to a cold cathode ''disc,'' a plasma of electrons is created. These particles are injected into a strong magnetic field, which accelerates them to an energy level of 2.5 million electron volts (MeV).
When the electrons come out of the 64-foot-long injector chamber, their velocity is 0.985 times the speed of light. That increases to 0.999 times the speed of light by the time the pulse leaves the 256-foot accelerator chamber - and the electron beam's energy increases, at the same time, from 2.5 MeV to 50 MeV.
Next the electrons are magnetically guided through a 140-foot-long transport section, which separates the accelerator section from the experimental section. It also provides a chamber for evaluating the beam's characteristics before beginning the actual experiment. A nearly complete vacuum is maintained in the accelerator, but the experimental tank is at atmospheric pressure.
As it passes through the different chambers, the diameter of the beam is reduced from 2 inches to 0.8 inches. It is either guided into an 80-meter-long experimental tank or into the open air for testing.
Testing in the open will not take place, said Dr. Briggs, until the scientists can control the high-energy radioactive pulses. Because of the extreme electrical charge, the beam creates its own magnetic field, which keeps it from flying apart. But it can also buck like an unheld water hose through which water is suddenly sent at high pressure. Keeping the beam ''on track'' is one technique that has to be mastered if it is to have practical value.
The theory is that the electron beams will, in effect, bore a low-pressure ''tunnel'' through the air that will permit following pulses to hit objects with such intense and concentrated energy that the targets will be destroyed.
Reporters at the recent briefing were shown pictures of a valve that had been mistakenly left open during a preliminary test of the new accelerator. An aluminum disk covering the valve opening was pierced, the interior of the valve was melted, and chunks of metal were knocked off the rear of the valve.
Briggs and other scientists who talked to reporters about the project routinely referred to the charged-particle beam pulses as ''bullets.'' The image is apt: Just like a bullet's, the CPB's destructive capacity is in its impact on the target, Briggs explained, not in its radioactivity or any explosive quality.
Although expressing confidence in the final result of their program, none of the scientists or military officers at the press showing would make specific predictions about when a particle-beam weapon might be developed (by either the US or the Soviet Union), what its size might be, whether it could be used in space, or its possible range. The Navy has special interest in particle beam devices because of its need for a defensive weapon to protect giant aircraft carriers against nuclear-armed cruise missiles. The kind of device envisioned would have to be not much larger than present big naval guns.
In a paper titled ''A Perspective on Charged-Particle Beam Weapons,'' W.A. Barletta of the Lawrence Livermore Laboratory wrote in February 1982:
''The deciding criterion for or against a CPB weapon may be, 'Can this weapon perform a task that no other can?' A unique capability requirement could be stated in very practical terms:
* Does a CPB weapon provide a 'real' ballistic missile defense?
* Can CPB weapons protect the surface fleet?
''The potential high payoff of CPB weapons can be illustrated by considering the example of a shipboard point defense weapon.''
Mr. Barletta concludes that because of its ''zero time of flight,'' and because it ''can deliver lethal energy to one target and be switched to another in milliseconds'' - and therefore is ''not sensitive to saturation attacks'' - and because it could destroy a nuclear missile far enough away to keep the resulting blast from damaging the target being defended, the particle beam, if otherwise feasible, can be the solution to defending surface ships against cruise missile attacks.
MIT's Parmentola and Tsipis examined, in an article in the April 1979 issue of Scientific American, the possibility of using particle beam weapons in space to intercept ICBMs, on ships to destroy low-flying cruise missiles, or on land to defend missile silos. Their conclusion: ''It is . . . highly questionable that such a system could function at all, let alone be operationally effective, in any of the three missions we have examined here.''