APPENDIX 20

ARTICLE FROM THE DECEMBER 1981 SCIENTIFIC AMERICAN ENTITLED "LASER WEAPONS" 1

COULD ORBITING LASERS DEFEND A NATION AGAINST MISSILE ATTACK? THE TECHNOLOGICAL OBSTACLES ARE INSURMOUNTABLE; FURTHERMORE, SUCH WEAPONS WOULD BE VULNERABLE TO SIMPLE COUNTERMEASURES

(By Kosta Tsipis)

A high-energy laser can readily burn a hole through a metal plate of considerable thickness; indeed, the laser serves as a cutting tool in industry. This capability has naturally led to speculation that the laser might serve as a weapon of war. What is envisioned by some military planners is a weapon rather like the ray gun of science fiction. The laser beam would be pointed at an attacking_missile or aircraft or at some other target and would almost instantly destroy it. Because the beam propagates with the speed of light there would be no possibility of outrunning or evading it. In preliminary tests under controlled conditions lasers have destroyed small, remotely guided aircraft.

Recently a small group of people in the U.S. Congress, the Department of Defense and the aerospace industry have contended that high-energy lasers have the potential for destroying intercontinental ballistic missiles in flight. Maintaining that the U.S.S.R. has already mounted a large effort to develop lasers as antimissile weapons and that the U.S. therefore confronts a "laser gap,' these people are urging the Reagan Administration to greatly expand the U.S. laser-weapons program, which is now receiving about $300 million per year. The main objective would be to deploy a network of very large laser weapons in earth orbit within about 10 years. The orbiting weapons would have the mission of destroying Russian intercontinental missiles soon after they were launched. Another objective would be to develop laser weapons that would be fired from the ground to attack enemy satellites or to defend valuable targets against aircraft and tactical missiles.

The effects of such hypothetical weapons on the world military balance and on the prospects for nuclear-arms limitation might someday merit detailed appraisal. For now, however, a more fundamental question must be addressed: Is it technically feasible to build an effective laser-weapon system? I shall argue here that the objectives set forth for laser-weapons development could not possibly be achieved in 10 years. Indeed, unless a number of fundamental impediments to the use of lasers as weapons are overcome, the objectives could never be achieved. Several of the difficulties arise from the physics of the propagation of a laser beam over long distances. Other difficulties are technological and economic.

The potential of lasers as weapons has been assessed in a series of workshops organized by the Program in Science and Technology for International Security of the physics department of the Massachusetts Institute of Technology. Participants in the workshops have included some of my colleagues and me from M.I.T. and investigators from other universities, from industry and from the national weapons laboratories. We have concluded that lasers have little or no chance of succeeding as practical, cost-effective defensive weapons.

A laser generates an intense stream of electromagnetic waves, all of which have exactly the same frequency, phase and direction of motion; the waves are said to be coherent. The property of coherence is essential to weapons applications of the laser because in order to cause damage a beam of laser light must be intense and well collimated and the waves that make up the beam must be in phase. In principle the light intensity of a laser is unlimited; in practice it depends on the size of the laser and the properties of the material in which the coherent light is generated.

The working medium of a laser can be a solid, a liquid or a gas, but high-energy lasers generally employ a molecular gas. To initiate laser action external energy must be supplied to the molecules of the gas. A fraction of the energy increases the kinetic energy of the molecules and therefore simply heats the gas, but some of the energy is absorbed into the internal vibrational and rotational motions of the molecules. A molecule excited in this way leaves its lowest vibrational or rotational energy state (the growing state) and occupies a higher one. As a result the low energy states are and a significant number of molecules enter an excited state. The condition is called a population inversion.

*

1 Copyright 1981 by Scientific American, Inc. Reprinted with permission. All rights reserved.

A molecule in an excited state can return to a state of lower energy by emitting a photon, or quantum of electromagnetic radiation. The frequency of the radiation is determined entirely by the difference in energy between the two states. The operation of a laser depends on a peculiarity of the interaction of photons with matter. When a photon emitted by one excited molecule impinges on another modecule in the same excited state, the photon can stimulate the second molecule to emit an additional photon of the same phase and frequency as the stimulating photon. Both photons can then stimulate similar emissions from other molecules, so that the number of identical photons moving through the collection of molecules grows exponentially. The nature of this process is suggested by the word laser, which was coined as an acronym for “light amplification by stimulated emission of radiation." In a laser a collection of molecules subject to a population inversion is enclosed in an optical "cavity" with parallel mirrored surfaces at the ends. Photons emitted by the excited molecules travel back and forth through the laser medium, reflected by the two mirrors, and stimulate additional molecules to emit photons with the same frequency and phase. Because only the photons that follow a path exactly perpendicular to the mirrors remain in the cavity long enough to be amplified, the light is formed into a well-collimated beam. A portion of the beam can be allowed to leave the cavity by making one of the mirrors partially transparent.

A photon wich a frequency in the visiable region of the electromagnetic spectrum has little energy: less than 1019 joule, or watt-second. Nevertheless, the energy output of a laser can be many thousands of joules, emitted in an exceedingly brief period, sometimes as short as a few millionths of a second. The reason is that enormous numbers of atoms (perhaps 1023) can be stimulated to radiate many times during this short interval.

Three kinds of high-energy laser have been considered as potential weapons. They are classified according to the mechanism that creates the population inversion in the working medium. In the gas-dynamic laser a gas such as carbon dioxide is generated by combustion. The gas is formed at high temperature, with the result that most of the molecules are in excited states. Then the gas is cooled suddenly by expansion through a series of nozzles; the cooling is so rapid that the molecules occupying excited states do not have time to return to the ground state. A population inversion is thus created, and laser radiation is emitted immediately after the expansion.

An electron-discharge laser achieves a population inversion by means of an electron beam directed through the gaseous working medium. The electrons give up part of their energy through collisions with molecules of the gas, causing transitions to higher vibrational or rotational energy states. By this mechanism the population inversion can be sustained continuously.

In a chemical laser two elements or chemical compounds are combined to form molecules of a new compound; for example, the gases hydrogen and fluorine can be combined to form hydrogen fluoride. The molecules are created in an excited state, and by controlling their environment it is possible to achieve the stimulated emission of radiation before they return to the ground state by dissipating their energy as heat.

These three methods can generate a population inversion in a large collection of molecules with good efficiency. There are practical limits, however, to the size of an efficient optical cavity for a laser and to the amount of power it can handle. Much of the research and development now under way in the field of high-energy lasers has the aim of pushing back those limits.

A laser weapon would differ in three important ways from all weapons that have been deployed up to now. First, it would transport destructive energy to the target in the form of an intense beam of electromagnetic waves rather than in the form of an explosive charge carried in a missile or a shell. Second, the energy would move with the speed of light, roughly 300 million meters per second; a supersonic missile, in comparison, has a speed of between 1,000 and 2,000 meters per second. Third, the laser beam must actually strike the target in order to damage it, whereas an explosive warhead can be effective at a considerable distance. Hence for a laser weapon to destroy its target the position of the target must be known to within a distance equal to the shortest dimension of the target, and the laser must be pointed with the same precision.

Given these characteristics of a laser weapon, one can thing of three kinds of mission it might perform. Mounted on a satellite orbiting the earth it could attack intercontinental ballistic missiles during their boost phase, which lasts for about eight minutes, or it could attack enemy satellites in their orbits. Mounted on the ground it could attack aircraft or enemy satellites as they passed overhead, or

mounted on a naval ship it could defend against missiles homing on the vessel. A laser weapon mounted on an aircraft could attack enemy aircraft or missiles.

In any of these missions the laser-weapon system would have to complete successfully an entire sequence of operations. The system would have to detect the target and distinguish it from possible decoys or other objects in the background. The system would have to point the laser beam at the target, follow its motion and fire the beam through the intervening medium. After each firing of the laser it would be necessary to determine whether or not the target was hit. In the case of a miss the system would have to determine by how much and in what direction the beam was misdirected, then correct the aim and fire again. After a hit the weapon system would have to determine whether or not the target had been destroyed; if it had not been, the aiming and firing would have to be repeated. Ultimately the system would have to communicate the results to a central command post and engage a new target if necessary.

To do these things the system would need several devices in addition to the laser. One device is a large mirror that could be moved under precise control to point the beam at the target. Another is a complement of sensors capable of detecting, identifying and determining the position of the target with the requisite precision and stability. Control devices would be needed to couple the output of the sensors to the aiming mirror. An additional, specialized set of sensors would be needed to assess the damage to the target or the distance by which the beam missed the target. Of course the system would also require a means of generating and storing energy and a mechanism for supplying the energy to the laser in intense pulses at the appropriate times.

A laser-weapon system operating in space and attacking targets that are also above the earth's atmosphere could send its beam over long distances, because light propagates without impediment in a vacuum. The beam would, however, diverge somewhat owing to diffraction, which is a consequence of the wave nature of light. Assuming that the mirror has a perfect surface and shape, the angle at which the beam diverges is inversely proportional to both the frequency of the laser light and the diameter of the mirror. Because one would want to keep the spreading of the beam to a mimimum, a laser intended to serve as a weapon would work best if it generated light with a high frequency and if it were equipped with a large aiming mirror.

A laser beam traveling through the atmosphere is attenuated and dispersed by a number of processes. The molecules of the air and the particulate matter in it (dust, water droplets and smoke particles) both scatter and absorb light. An infrared beam from a carbon dioxide laser would lose half of its intensity after traveling four kilometers in cool, dry air or 1.5 kilometers in hot, humid air. Clouds, smoke, dust, fog or thick haze would absorb a beam almost completely. In short, the efficacy of a laser weapon operating in the atmosphere would depend on the weather. Such dependence is a serious drawback in any weapon, but it is particularly troublesome in a defensive weapon, which would have to respond to an attack launched at a time (and therefore in weather conditions) of the enemy's choice. Even in clear weather a laser beam can be deflected, dispersed or completely interrupted by atmospheric phenomena. Turbulence causes rapid local changes in the density of the air, which can deflect a beam of light or make it diverge. The twinkling of stars and distant lights is a manifestation of this effect.

A considerable fraction of the energy in a laser beam is absorbed by the atmosphere. As a result the air in the path of the beam is heated; the heated air expands, creating a channel of low-density air. Light waves bend away from the hotter, less dense regions of a medium and so the beam diverges. The phenomenon is called thermal blooming; it is a common reason for the defocusing and divergence of a laser beam in air.

A final difficulty in propagating a laser beam through the atmosphere is the risk of creating a plasma. Since light waves are a form of electromagnetic radiation, an intense light beam is accompanied by a strong electric field. At an intensity of about 10 million watts per square centimeter (the exact value depends on the frequency of the radiation) the field is so strong that it removes electrons from atoms in the air, thus ionizing the air and creating a plasma. The plasma absorbs the beam and interrupts its transmission. The effect sets an upper limit on the intensity of a beam of laser light that can propagate throught the atmosphere.

A laser weapon would damage a target by overheating it, that is, by concentrating on it more thermal energy than it could withstand without malfunctioning. Damage arises only from the fraction of the energy that is absorbed by the surface of the target. For example, a target of shiny aluminum would absorb only 4 percent of the

radiation from an infrared laser that reached the target. The rest would be reflected and would cause no damage.

The proportion of the laser energy that would be absorbed by a target depends on the frequency of the radiation, the material of the target and the condition of the target's surface. Visible and infrared radiation are mostly reflected from a polished metal surface, so that in general much less than 10 percent of the energy carried by the laser beam would be absorbed and cause damage. The absorption of ultraviolet radiation by a metallic surface is much higher; more than half of the ultraviolet energy reaching a target would cause damage.

Overheating might destroy or incapacitate a large target such as a missile by any of several mechanisms. The amount of energy per unit area that would have to be delivered to the target in order to damage it would depend on the mechanism chosen and the vulnerability of the target to that mechanism. For example, the electronic circuits of an unprotected satellite would probably malfunction if the craft were illuminated continuously for several minutes by a laser beam with an intensity of about one watt per square centimeter. This roughly 10 times the intensity of sunlight at the top of the atmosphere. The absorption of 1,000 watts per square centimeter for one second (a total absorbed energy of 1,000 joules per square centimeter) would melt a metal surface a few millimeters thick. To deposit that much energy, however, an infrared laser would have to provide about 20,000 joules per square centimeter, since most of the energy would be reflected by the target.

A laser that sends out its energy in brief but powerful pulses might reach an instantaneous intensity of a million watts per square centimeter, even though the average power would be much lower. The surface of a target struck by such pulses would rapidly lose its shininess, and the fraction of the beam's energy absorbed would increase with each pulse. It is therefore possible in principle to burn a hole in a target with a pulsed laser beam.

When the target is in the atmosphere, a beam intensity of roughtly 10 million watts per square centimeter would cause the air immediately in front of the target to ionize, creating a layer of plasma where the beam strikes the surface. The plasma would absorp the energy of the beam and grow incandescently hot (to about 6,000 degrees Celsius). The plasma would rid itself of this energy in two ways: by emitting ultraviolet radiation and by expanding explosively. These two mechanisms could increase the proportion of the beam energy coupled to the target to about 30 percent and thereby reduce the amount of energy the laser would have to generate.

A pulsed beam of extreme intensity could evaporate the metal at the surface of the target. The evaporated metal would fly away from the surface at a high velocity, and its momentum would be balanced by an equal and opposite momentum impinging on the target. The impulse generated in this way could tear or crack a metallic target.

From the physics of these effects it is possible to arrive at a good estimate of the capabilities a laser weapon would need in order to carry out a particular mission. The mission I should like to consider in some detail is that of an orbiting laser weapon intended to destroy enemy intercontinental ballistic missiles during their boost stage. Although this mission is the most remote application of laser weapons in terms of development time and practicality, it is conceptually the most interesting mission. It is also the one most often mentioned in public discussions of laser

weapons.

The missile-defense lasers would be deployed on satellites in orbits some 1,000 kilometers above the earth. At this altitude a satellite would be within striking distance of launching sites in the U.S.S.R. for only a short period during each orbit. To ensure that at least one satellite would be within range at all times the total force would have to include about 50 satellites. A single satellite would have to be capable of destroying an entire force of perhaps 1,000 missiles during the boost stage, which lasts for about eight minutes. Therefore the satellite could devote about half a second to each missile.

My colleagues and I have found that an efficient damage mechanism for a ballistic-missile interceptor would be to crack the surface of the missile by impulsive loading. Cracking would result from the absorption of about 1,000 joules per square centimeter during each of several brief pulses. The energy would be delivered by a beam with an intensity at the target of about a million watts per square centimeter and a pulse duration of a few hundred-millionths of a second. Laboratory experiments indicate that about 10 pulses would be needed to punch a hole in the missile. How much energy must such a laser develop? Assume that the weapon is a pulsed hydrogen fluoride laser and its beam-directing mirror is optically perfect and measures one meter in diameter. Since only about 10 percent of the light that strikes the target is absorbed and contributes to the damage, the laser must deliver 10,000

joules per square centimeter per pulse at the target. The area covered by the beam 1,000 kilometers away would be about the same as the area of the mirror: almost 8,000 square centimeters. To achieve an energy flux of 10,000 joules per square centimeter over this area the total energy of the beam would have to be almost 80 million joules per pulse. If the pulses were to last for roughly 100 microseconds, the power of the laser would be almost a million megawatts, which is quite unattainable. (A large commercial power station has a generating capacity of a little more than 1,000 megawatts.)

A lesser amount of absorbed energy from a continuous laser beam might damage a distant target by melting a hole in its skin instead of fracturing the skin. For example, an aluminum skin two millimeters thick would melt when it had absorbed about 400 joules per square centimeter. If the reflectivity of aluminum is assumed to be 90 percent, a 100-megawatt carbon dioxide laser would need about 100 seconds to inflict such damage on a target 1,000 kilometers away. This rate of damage is clearly inadequate, since the laser weapon has only half a second at best in which to deal with each rising missile.

One way of alleviating these difficulties might be to enlarge the pointing mirror. With a mirror four meters in diameter a 100-megawatt hydrogen fluoride laser could damage the target by melting in about a second. Making such a mirror sufficiently rugged and of the necessary optical quality, however, is beyond the technical capabilities of the U.S. or any other nation. There are scant prospects for constructing an optically precise four-meter mirror.

The fuel requirements of a laser-weapon system represent another insurmountable obstacle. Even if the laser itself and its energy-staging system operated with perfect efficiency, such a continuous-wave hydrogen fluoride laser would consume some 660 kilograms of fuel for each missile destroyed. In order to shoot down 1,000 missiles, then, each satellite would have to be supplied with 660 metric tons of fuel, which represents about 20 loads for the U.S. space shuttle. The 50 satellites needed to ensure continuous coverage of Russian launching sites would require 1,000 shuttle flights for their energy stores alone. Four shuttle craft, each making two trips per year, would take 125 years to deliver the fuel.

The assumptions that underlie this discussion of a hypothetical missile-defense system are unrealistically optimistic. It should be pointed out first that a 100-megawatt hydrogen fluoride laser does not exist, and there is no indication that such a device could be developed in the foreseeable future. Furthermore, the efficiency of the laser and of the energy-staging system will never approach 100 percent. The efficiency of existing lasers is a few percent, and it might someday attain 30 or 40 percent. An energy-staging system can at best reach 30 percent efficiency. Hence the total energy store for each satellite would have to be increased by a factor of at least 10 and more likely 30.

It is conceivable that a laser weapon suitable for deployment in space could eventually be constructed. Even so, I doubt that it could be exploited successfully because it would be vulnerable to a number of relatively simple and inexpensive countermeasures. During the long time it would take to assemble each platform in space the system would be extremely vulnerable to attack by an antisatellite weapon exploded nearby. Even a completed network could be temporarily incapacitated at crucial times by blinding its sensors, by jamming its communications or by confusing its detection and tracking system.

The other conceivable use of a space-based laser weapon in as an antisatellite system. The practicality of the concept is highly questionable. In the first place satellites in orbit are already vulnerable to explosive weapons, which can be placed accurately in space or even made to home in on a warm object in orbit. A space-based antisatellite laser would itself be vulnerable to the same weapons. The laser system would also be complex and fragile and therefore expensive and difficult to maintain. It is highly unlikely that antisatellite lasers will ever become more cost-effective than mechanical satellite killers launched from the earth.

I turn now to the potential of high-energy lasers as weapons operating in the earth's atmosphere. Missions such as the protection of aircraft and ships from enemy missiles and the destruction of enemy aircraft might in principle be carried out by a laser weapon. Several weapon systems that can accomplish the same tasks already exist, however, including supersonic self-guided missiles and rapidly firing cannon. The question, then, is whether a laser would be a superior weapon. Could it provide such short-range protection more cheaply or more effectively?

The first thing to consider is the physics of the propagation of a laser beam in the atmosphere. I have already described blooming, absorption and atmospheric ionization. Blooming and absorption alone would reduce the intensity of an infrared laser beam by a factor of from 100 to 300 over a distance of five kilometers. If the size of