About a dozen people, mainly military brass, were crowded into a control bunker three stories beneath the New Mexico desert here. Peering anxiously at a bank of monitors and computer screens, they watched as a laser beam the diameter of a Hula-Hoop flashed a half-mile across the desert floor, glanced off a focusing mirror, and lit on a section of a Titan missile.
Seconds later, the rocket stage suddenly blew up, scattering shards of metal hundreds of feet amid the mesquite and pion.
``I've been in this business for 12 years,'' says Capt. Arthur Schroeder, head of the Navy's work here, who watched the demonstration in September. ``It was the most dramatic damage and vulnerability test I've ever seen.''
Impressive as it was, it does not prove that lasers can be used to defend the United States against nuclear annihilation.
The test was simply one more small step in a long and arduous quest to see if directed-energy, or beam, weapons may ever be suitable for knocking down Soviet missiles.
Beam weapons are gaining prominence. Once confined to Buck Rogers fantasy, these ``death rays'' consist mainly of particle beams, which hurl streams of atoms or atomic particles, and lasers. These technologies have been elevated to new visibility under President Reagan's Strategic Defense Initiative (SDI), popularly known as the ``star wars'' program.
Indeed, they are one of the reasons that the United States has revived the idea of building defenses against intercontinental ballistic missiles (ICBMs) after scotching it in 1970s.
Earlier it was thought that there was no way to deal with tens of thousands of warheads and decoys that might be launched against the US in a full-scale nuclear assault. There still may not be.
But a defender's job would be easier if a system could knock out as many missiles as possible within the first few minutes of launching, before they had a chance to release their many decoys and warheads. Beam weapons flashing through space at or near the speed of light are prime candidates for the job.
Conceptually, they make captivating weapons: beams of pinpoint precision able to zap mankind's most destructive armament. But translating that vision into reality will be difficult.
Physicists have been toiling for more than a quarter of a century to fashion directed-energy weapons, as they are called. The Pentagon launched its first particle-beam research program, the Seasaw project, in 1958 at Lawrence Livermore National Laboratory. The aim: to build a particle-beam accelerator and study its potential for thwarting missiles.
Interest in laser weapons surfaced shortly after that. In the years since, enthusiasm for these exotic weapons has vacillated. Hopes raised by advances in technology were often dashed when people began to look at the cost and other problems tied to building a practical weapons system.
The military is still keen on beam weapons for everything from air-defense to zapping enemy satellites. The SDI program, however, focuses attention on the far more difficult task of destroying enemy missiles and warheads, for which $1 billion is being sought next year alone (about one-fourth the SDI budget).
Given the hurdles that remain, particularly the defensive tricks the Soviets may try (such as spinning a booster so a laser cannot dwell on one spot), even SDI officials do not see a practical and affordable beam-weapon system this century. Divining what the Soviets might do is like a chess game, says Louis Marquet, head of SDI's directed-energy programs. ``Unfortunately, the Soviets are very good at chess.''
Light from a normal lamp is a disorderly jumble of frequencies. Lasers generate concentrated beams of light that are almost perfectly parallel, identical in frequency, and the light waves move in phase with each other. This gives lasers their punch. In theory, they could be focused over thousands of miles of space to burn a hole in the skin of a missile or, in the case of lasers that emit pulses, thump the target like a sledgehammer.
The most powerful lasers now in existence are chemical. They draw their energy from the combustion of gases. Because they do not require huge power plants, chemical lasers are mainly being considered for parking in space, where they would be free from the distorting effects of the earth's atmosphere.
These lasers pack a punch. Ones far less powerful than that tested here at White Sands -- a 2.2-megawatt device that is the ``brightest'' in the West -- have already knocked down planes. But space-weapons lasers will have to be brighter (probably 10 times or more).
Such infrared chemical lasers also have a long wavelength. Because their beams spread out over great distances, they would need to linger on the same spot on a fast-moving missile for several seconds. They also would require exquisitely fabricated mirrors of up to 50 feet in diameter to keep them focused. This has caused them to fall from grace with some in the SDI community.
Any orbiting constellation of chemical-laser battle stations will have to meet several criteria: be reliable, be cheap enough to hoist into orbit and maintain, and be able to survive a direct attack -- for instance, from exploding satellites (space mines) the Soviets may park next to the weapons platforms.
``The difference between putting something up in space that can fire once or twice and something that will keep missiles from landing on top of you is a big one,'' says Jeff Hecht, author of the widely respected book ``Beam Weapons.''
The alternative is to use shorter-wavelength lasers, such as the free-electron and excimer lasers. These are now the fair-haired beams among SDI researchers. A free-electron laser uses a huge particle accelerator to generate the electrons that, when passed through a series of wiggling magnets, are the source of the device's ultraviolet light.
These lasers have been developing the quickest. ``They've come along in not many years from a scientific curiosity to reality,'' says Gerald Yonas, SDI's chief scientist.
In theory, a free-electron laser can be tuned to different wavelengths to allow its beam to slip through Earth's atmosphere. They also can be scaled to large powers and operated at high efficiencies. But for now, they exist only in early-stage laboratory models. Because the free-electron laser's accelerator requires a jumbo power source, it is a better bet for basing on the ground.
Prodigious electrical requirements are likely to keep the excimer earth-bound as well. The excimer does not require a particle accelerator, but it does use a lot of power in producing an ultraviolet beam from rare gases.
Ground-basing is not necessarily a woe. It makes the complex devices simpler to tinker with, easier to defend, and, as Dr. Marquet likes to point out, ``You could plug them into Hoover Dam, turn off the lights when the war starts, and deliver all the electricity into the devices.'' Which you may have to do: By one estimate, powering enough of these lasers to hit 2,000 targets may gobble up as much energy in a few minutes as New York City uses in several hours.
One scheme calls for placing the lasers on mountaintops and firing them high into space, where their beams bounce off huge relay mirrors and then off smaller aiming mirrors in lower orbits. Or the beams might simply be bounced off of ``catch and transmit'' mirrors in low-earth orbit. Either way, these devices will need mirrors of gem-like quality larger than any built to date.
To meet this requirement, scientists are considering using mirrors made up of many small segments, like a mosaic, all computer controlled. The same general principle (adaptive optics) is aiding scientists in overcoming another problem with ground-based lasers: atmospheric distortion. So far, however, experiments have only been carried out with low-power beams.
The other snag with short-wavelength lasers is that they can be self-destructive. An excimer laser may be able to disable a booster in two seconds, which would negate the effect of spinning it to counteract the beam. But the excimer could also buckle its own mirrors.
New mirror coatings are being developed, but this is considered one of the more intractable SDI technologies. At a conference this spring, James Stanford of the Naval Weapons Center in California noted that only 2 percent of the coatings now available meet even currently known requirements.
Of course, defenders could alleviate many of the problems with ground- or space-based systems by simply popping lasers into orbit at the first hint of a Soviet strike. This is where the nuclear-pumped X-ray laser comes in. This weapon appears to be advancing technically but losing ground politically.
The idea sounds simple: Explode a nuclear bomb in a small chamber ringed with rods and pointed at a target. When the explosion's radiant energy hits the rods, it produces a pulse of highly lethal X-rays, spraying them out in the instant before the device vaporizes.
Snags exist, however. Even though work on the secret devices at Lawrence Livemore has been moving quickly, scientists still have to invent more efficient ``third generation'' nuclear devices that will convert more of their energy into X-rays instead of explosions. Researchers will also have to control and aim the pulses to hit quick-moving targets.
X-ray lasers, too, have put the Reagan administration in the uncomfortable position of pursuing a weapon driven by a nuclear bomb (albeit theoretically a small one) to help make nuclear weapons ``obsolete.'' In theory, hundreds of such lasers could be orbited. But SDI officials now go to great pains to say that will not be done.
The pop-up scheme involves putting X-ray lasers atop missiles safely stored beneath the sea on submarines or on land-based launchers and lofting them into space at the first sign of a Soviet strike -- the pet idea of Dr. Edward Teller, inventor of the hydrogen bomb and an inveterate SDI booster.
To get the weapons into space quickly enough, however, they would require extremely fast launchers and perhaps the submarines would have to be parked vulnerably close to Soviet shores.
``The practicality of a global scheme involving pop-up X-ray lasers of this type is doubtful,'' said a recent Congressional Office of Technology Assessment study.
X-rays also do not penetrate Earth's atmosphere well. Thus if the Soviets were to use ``fast-burn'' boosters -- which would complete their flight within 100 seconds, while still in the atmosphere -- the weapon may not be effective for knocking out ICBMs in the all-critical boost phase, when warheads and decoys are in one package and the missile is easy to detect. Currently, the boost phase lasts from 3 to 5 minutes.
Livermore scientists are not ready to concede lasers cannot be made bright enough to eat part way into the atmosphere. ``It doesn't violate any laws of physics to do so,'' says George Miller, Livermore's deputy associate director for nuclear design.
But X-ray lasers are considered more likely for post-boost duty, when the missile is just beginning to cast off its warheads and is still somewhat easy to find. In addition, the X-ray lasers could be used during the midcourse phase, when the warheads and swarms of decoys are floating through space. However, because the X-ray laser is basically a one-shot device, some critics think it will be able to wipe out only a limited number of decoys and warheads.
The chief concern, however, seems to be that detonating a series of nuclear bombs in space might damage America's own battle stations and satellites. This point bothers even many in the SDI community.
``I don't find it to be a credible weapons system, even if it does work,'' says Stephen Rockwood, head of SDI work at the Los Alamos National Laboratory in New Mexico.
X-ray-laser proponents say they believe battle stations could be hardened against the effects of nuclear explosions. They also say the device holds such potential, either as a defensive weapon or one to take out Soviet satellites, that the US can't afford to give up studying it.
The particle beam -- a stream of atomic particles or atoms -- is the Arnold Schwarzenegger of directed-energy weapons: It comes in a large package and packs a potent punch. The beam penetrates a missile's skin and sizzle the insides, unlike most lasers, which deposit their energy on the surface.
This means particle beams could disable a target quickly. It also means they would be tough for Soviet scientists to foil, either by shielding the missile or spinning it. The particle beam's penetrating character, however, has its drawbacks: Because the beam immobilizes the internal electronics, it might take some time to verify that a target had been destroyed or disabled. Thus a particle-beam weapon may continue to fire at a target long after it had actually been ``killed.'' In the meantime, other war heads zip past.
The most likely candidate for a missile-zapper would be a neutral-particle beam, which, because it can't penetrate the atmosphere, would have to be parked in space. The particle beam's bulk is not endearing. Scientists figure a neutral-beam battle station might be 80 feet long and weigh 50 to 100 tons (the shuttle carries 33 tons). Up to 100 may be required. ``The problem for particle beams is one of packaging and engineering,'' says Dr. Rockwood. ``They will have to be compact, lightweight, and fully re mote controlled.''
Blunted by Earth's atmosphere, neutral particle beams would be of little use for boost-phase kills. But they look more suitable for post-boost and midcourse phases.
One type of charged-particle beam -- the electron beam -- can operate in the atmosphere. Indeed, it has to: Its interaction with the surrounding atmosphere helps hold it together. If shot in space, the beam would almost immediately disperse as its electrons repelled each other. Even if the electrons remained in a narrow stream, it would be bent uncontrollably by Earth's magnetic field (neutral beams are immune to such mischief). Thus, the electron beam is being looked at for use on the ground to zap war heads dropping from space. The idea would be to use them to defend ships or US missile silos and command posts.
The perfect weapon? Not quite. As yet, researchers have only been able to control the beams over very short distances in the atmosphere. One possible solution: Use a laser to ``tunnel'' a path for the particle beam through the air. Scientists at Sandia National Laboratory have tested this technique in a special gas-filled chamber. For now, however, the trick looks more like a coup for science than anything to make the Soviets nervous: The gas used in the tests doesn't exist in Earth's atmosphere.
At Livermore, meanwhile, researchers are enthusiastic about work they are doing with the Advanced Test Accelerator, a device nearly the length of a football field bunkered in the flaxen hills east of San Francisco. With something greater than the sound of cracking helmets, it propels pulses of electrons up to 50 million electron-volts of energy -- in effect creating synthetic lightning.
When technicians fire the beam into the air for the first time within the next several months, they're hoping to keep it controlled for some 75 feet -- something that would be a leap forward but would still fall shy of the several miles that will be needed for a weapon. ``You're talking about a long row to hoe,'' says physicist William Barletta, head of the beam research program at Livermore. ``We're still working on the basic physics.''
If and when scientists work out the physics, they'll also have to be mindful of the cost. ``For terminal defense, if we can't keep the costs down to $100 [million] to $200 million a copy, it won't be worth looking at,'' says Dr. Barletta.
Beyond this, star-wars officials are exploring even more exotic concepts to thwart missiles, though most of these ideas are not much more than theories now. Two examples: gamma-ray lasers and ``plasmoids.''
Like the X-ray laser, gamma-ray lasers would be pumped by a nuclear bomb. Because gamma rays are more lethal than X-rays, one SDI booster says such a device would be the ``ultimate directed-energy weapon.'' Plasmoids are clouds of energized atomic nuclei and electrons that scientists would like to hurl at warheads. But first they will have to find a way to make the cloud stick together in space.
Given the work to be done, it's perhaps not surprising that beam weapons in general are not envisioned as part of a first-generation defense. Their first role would probably be a supporting one -- doing such things as helping discriminate decoys from warheads.
Even if space weapons can be built, they will have to be knit together in a reliable system. Most experts agree that developing technologies to run the battle will be far harder than developing the weapons.