Potential defense applications of free electron lasers.

Potential Defence Applications of Free Electron Lasers 

The history of science abounds with examples of discoveries that at the time appeared to have no practical relevance to progress. They were often regarded as mere toys or at best as theoretically "interesting" experiments until further studies revealed them to be major breakthroughs. Such an event took place in 1960 in the research laboratories of Hughes Aircraft Company when the first working laser was invented. Initially, the laser was regarded as an interesting curiosity which permitted scientists to generate light flashes at intensities 10 000 times stronger than hitherto possible. Nobody then could have predicted the impact of this invention on virtually every level of human existence. It serves now in manufacturing, surveying, communications, in medicine and last but not least in defence. Here, it proved to be a revolutionary method of improving the accuracy of tactical weapons in the form of laser rangefinders, target illuminators or missile guidance systems.

Barely 20 years after the invention of the laser a different method of lasing was rediscovered during research into secure communications with submerged submarines. At the time it was regarded as a useless offspring of laser technology. Listed under US Patent No. 3 822 410 of July 1974 it describes a lasing method which had been developed in 1963 by John Madey, then a physics student at the Californian Institute of Technology. The method was given the name Free Electron Laser (FEL). By 1977 Madey could prove with a rudimentary experimental device that the system was workable. But nobody saw any potential application until the early '80s when it was established as a scientific fact that FEL systems can project extremely high-powered beams at frequencies which could be varied over an unprecedented range. This type of advanced light amplification now enables one to push beyond the microwave spectrum of earlier radiation-generating methods into the realm of millimeter waves. Perhaps a brief explanation of conventional laser technology is in order at this stage.

In the traditional laser, a brief power input - commonly an electrical impulse - is introduced into a precisely shaped cavity which is closed at both ends by mirrors. One of these is totally reflective, the other is semitransparent. Inside this resonant cavity is some type of amplifying medium - a gas, a liquid or even solid matter. The power surge interacts with the medium's atoms and elevates their bound electrons to a higher energy state. The aim is then to recall them rapidly under the action of an excitation radiation to their original position because while doing this they shed the absorbed excess energy by emitting photons. This flash of light bounces back and forth between the cavity's mirrors. A minor portion is lost through the semi-transparent mirror, but the rest keeps oscillating and building up in intensity, until it emits an extremely concentrated beam of coherent light.

The FEL operates in a similar way, but the method of injecting the power is radically different. Instead of stimulating the nucleus-bound electrons of a traditional laser amplifying medium, a beam of fast moving free electrons - the reason for this laser type's name - is fired from an outside source (an electron gun or a powerful radio frequency generator) past an array of alternately polarized magnets built into a resonant optical cavity not unlike that of a conventional laser. The force of the magnetic fields bends the trajectory of the electron beam, causing the electrons to zigzag, or better to "wiggle" on a slalom-like course at almost the speed of light. The reason for the "wiggling" is to pry a high yield of photons from the electrons and to compress the resulting beam into a single narrow wavelength, this being determined by the distances between the magnets. The wiggling process is the key to the FEL. Each electron contains a charge which manifests itself in the form of an electric field. By wiggling the electrons back and forth, their electric field lines generate waves of electromagnetic radiation in the form of photons.

The electrons moving through the FEL's magnet array are compelled by its field forces to wiggle at the same frequency. This is important because each electron thereby creates a wave of electromagnetic photon radiation of identical frequency. Because the electrons are moving at almost the speed of light, Einstein's laws of relativity come into play. As a result, the actual spacing between the magnets appears to the electrons to be contracting along their direction of travel. Hence they start bunching at much shorter intervals, or wavelengths, than might be logically expected. The result is fascinating. What should normally appear as a centimeter wavelength becomes vastly shorter, and the shorter the wavelength, the higher obviously the frequency. The now attainable frequencies reach from the microwave end of the wavelength spectrum to the 1 000 000 GHz domain of visible light. This is not unusual as far as conventional lasers go, but these operate at the fixed resonance frequency of the lasing medium. An FEL device will be capable of equally efficient operation over this wide span of frequencies once a practical way of shortening or lengthening the space between the magnets is found. Such lasers can then be tuned like a wireless set, but most significantly, their power too can be varied to meet specific applications by adjusting the amount of power contained in the original electron beam prior to its injection into the system.

The FEL is so arranged that all the generated photon waves are in phase and the generated power is thereby vastly amplified. At this stage the bunching of the electrons becomes essential. If billions of electrons wiggle at the same frequency, the power released (depending on the input energy) can be tremendous, easily attaining the megawatt range. In order to be of practical value, this power, which appears in the form of photons, has to be harnessed. In small, low-power systems this is done with the help of a classic resonant cavity resembling a travelling wave tube in which the amplification takes place. After it has done its work the stimulating electron beam is channeled by magnetic force to exit at almost right angles into a damping device. In a megawatt FEL this becomes a problem because it, too, has to discard the excess energy remaining in the electron beam, which uses only a fraction of its power for the production of photons. Because it would require considerable magnetic energies to bend the still very live beam at right angles, most current designs only divert it slightly from the system's optical axis and the energy is dumped at its open front end.

In the USA FEL research is primarily conducted under the auspices of the Strategic Defense Initiative (SDI). It's goal is to develop extremely high-powered devices for anti-missile defense. For this purpose the FEL seems to beat every other competitor in the field. But it can be assumed that low-power, tunable FELs are also under development for EW purposes as short-distance directed-energy weapons. Current work in this field is Top Secret and consequently next to nothing has been made public. The FELs involved are small, look like ordinary laser systems and can therefore easily be hidden from the public eye. This is not possible with their SDI-related relatives. These are veritable monsters which, even in their experimental versions, are some hundreds of feet long. Today's operational systems can cover several square miles. The first of these is expected to be ready by 1994.

RF vs. Induction Linear Accelerators

The reason for the size is that a megawatt FEL requires a very energetic beam. For SDI purposes this beam must carry an energy of some hundreds of MeV (million electron Volts) to produce a laser beam of some tens of megawatts. Two ways of creating the electron beam are available. They are the Radio Frequency Linear Accelerator (RF-Linac) and the Induction Linear Accelerator (I-Linac). The I-Linac method seems to be the preferred approach for high-power applications, while the RF-Linac excels in provoking extremely short wavelength lasing. With the latter method the electrons are accelerated and energized by the pressure of microwave energy fed into a wave-guide assembly through which the electron beam is passing. The RF-Linac FEL produces packets of laser pulses, each of about 20 picoseconds or trillionths of a second. Each packet lasts about 100 microseconds and the power output of the FEL is determined by the density of pulses within a packet and the repetition rate at which the packets are generated. Though the device generates a pulsed laser output, because the pulsing is so short, its effects resemble those of a conventional continuous-wave laser.

The I-Linac builds up the electrical potential in the electron beam by letting it pass through a series of "transformers" where the beam itself acts as a secondary winding, the primary winding being supplied by the electrical energy from an outside source. In essence, the induction method resembles an amplification system, the electron beam continuously gaining energy before entering the "wiggler". The laser beam produced by an I-Linac device is a single pulse of extremely high intensity with an average life of only 50 nanosec (billionth of a second). Its intensity is such that after leaving the wiggler current practise is to reduce its pin-point density by diffraction in a vacuum chamber several miles along and so produce a broader beam before permitting it to enter the optical system, which would otherwise be punctured and melted at the first impact. In fact the use of FELs hinges on the availability of special, extremely heat-resistant and/or cooled optics with which the laser beam can be directed at the target.

One problem inherent in all laser systems is their inefficient input/output power ratio. Conventional lasers convert only a small percentage of the input power into coherent light. The conversion ratio of a FEL is much better: it currently reaches more than 10%. This has been achieved quite simply. As the electrons interact with the magnetic fields of the wigglers and emit the photons to produce the laser beam, their velocity gradually diminishes and they even begin to absorb energy from the magnets. This lowers the system's efficiency drastically. This is solved by tapering the cavity towards its exit to provide increasingly shorter wiggling paths for the electrons. Workable experimental systems have already been built, raising the efficiency considerably. It could be raised still higher by recycling the energy potential remaining in the electron beam - once it has done its duty in the cavity to produce photons. Scientists believe that an efficiency of 20% could be achieved.

Once teething problems are solved the FEL will offer a number of significant advantages over conventional lasers. The FEL-generated laser beam shows an exceptional wavelength purity which cannot be attained with any other type of laser, and, as already mentioned, the wavelength can be adjusted to meet specific requirements. For example, it can be tuned to wavelengths which are expected to facilitate penetration of the ambient air with minimum loss. This is an extremely important factor if the FEL is to be used as a directed-energy weapon. The passage of such high-energy beams through the air creates some very unusual, highly undesirable and not yet fully understood physical phenomena. These are very complex and some say can only be studied empirically, but computer simulation studies based on known physical facts are also underway. Lightning during a thunderstorm creates similar effects. Lightning usually zigzags, forks and leaves a luminous trail. While on its way down, it actually burns the air, dust and aerosols in its path, thereby losing energy and so being deflected and forking.

Blooming Effect

The laser beam projected by a directed-energy weapon tends to suffer the same fate. While on its way to the target, it collides with the air's molecules, losing an enormous amount of its power. At many wavelengths and under specific conditions of air pressure and humidity the intense heating of the air leads to a phenomenon known as "thermal blooming", which tends to diffuse the beam's energy. Thermal blooming is caused by a succession of plasmas, small clouds of ionized gas created by the beam's energy. These clouds are nearly impenetrable to energy and therefore deflect and split the beam. One suggested remedy is to pulse the beam to give the air adequate time to cool down before the next pulse arrives so that each pulse bores further forward - essentially digging a tunnel through the air - until the target is reached. This process takes place at the speed of light, which means that a pencil-thin vacuum can be created in the beam's trail, since the surrounding air, through which this "tunnel" has been drilled, does not have the time to flow back into it. This method is feasible but can be used only for relatively low-power applications, such as close-quarter defense weapons against anti-ship missiles. It is not suitable for the megawatt energies needed for SDI purposes.

Raman Scattering 

Another obstacle posed by the ambient air is a phenomenon known as stimulated Raman scattering. This transmutes the laser beam's original wavelength to another, or even several different ones, a repetitive process which causes it to lose its energy. Some maintain that such phenomena do not occur on all wavelengths, and this is where the accurately tunable FEL shows its advantages. With traditional lasers the beam loses about 30% of the originally generated energy per mile. If the proper wavelength could be discovered where atmospheric attenuation is low, the FEL could be tuned to that.

Should experiments planned for the '90s establish that such a wavelength does not exist, another approach is being studied to circumvent thermal blooming and the Raman effect. This consists in burning a tube-like tunnel through the air by giving the laser projection an optical ring shape. This tube would stretch from the projector to the target, the plasmas forming primarily on its outer fringe. The actual weapon beam would be projected nanoseconds later through the center of this tube "drilled through the air". As the air in the center of the tube is rarefied and plasma generation is greatly reduced, the weapon beam would hopefully suffer only a minimal loss of energy. This means that the weapon beam's energy potential is only marginally affected and that the beam reaches the target carrying a sizable number of megawatts. This approach obviously doubles the power requirements needed and considerably complicates the weapon system.

In essence, apart from the creation of suitable optics, a full understanding and mastery of the thermal blooming phenomena are the two major obstacles in creating high-power FELs. What has been achieved so far in SDI research may indeed be called spectacular. An experimental I-Linac FEL, utilizing a 4 meV beam as input power and a tapered wiggler has repeatedly produced peak powers in excess of 1-gigawatt on a wavelength of 9-millimeters. Tests are currently under way utilizing a 50-meV beam accelerator and a 25m-long wiggler. The results should be impressive.

The Reflectors

As for the optics it is claimed that substantial progress has been made. Parabolic optical surfaces have expanded the optical beam and reduced the power loading on the mirror surface by a factor of 50. Grazing incidence and deformable mirrors have been developed, and by the end of last year two liquid-cooled deformable mirrors were close to completion. Should all attempts fail to design and construct suitable optics, high-power FELs would not be usable, much to the advantage of Neutral Particle Beam (NPB) weapons. The latter do not involve lasing and the beam can consequently be steered and focussed by magneto-optic devices, i.e. by magnetic field forces alone. The problem here, however, is that the beam tends to be deflected by the earth's magnetic field.

Short-Range Applications

In short-range applications, however, FELs have become prime candidates for electronic warfare and short-range anti-missile defence. A naval vessel armed with frequency-agile FELs, for example, could by electronic means alone blunt a missile attack. The FELs would have to be tuned to the sensor wavelengths of the approaching missiles. To be effective such FEL jammers would depend on perfected and fully automatic. Electronic Support Measures (ESM). The moment the ESM system picked up a radar-guided missile's transmission frequency it would send a command to the FEL's fire-control which would tune the device to the detected missile's frequency. Even a low-power laser beam hitting the missile's sensor will either blind it or at least confuse the weapon's radar computer, causing the missile to run wild. Medium-powered lasers, which need not be overly large, might be used to destroy the missile itself. Over a short range the air attenuation of the beam is negligible, and the full power of a megawatt beam hitting the missile is bound to damage it severely. This was already proved some years ago under controlled conditions; however, at that time a chemical laser was used which required a power plant large enough to provide a small city with electric light.

Communications

Another potential application is communications. Battlefield COMSEC (Communications Security) of tactical networks could be greatly enhanced. A communication FEL beam can be easily modulated for carrying a message. An experimentally proven system which enables aircraft to communicate with submerged submarines works with a modulated FEL beam tuned to the blue/green part of the visible light spectrum. High-definition FEL radar systems are also quite feasible. In civil communications systems FELs can be used as a power source for microwave links in any desirable frequency. The advantages are low power consumption, extremely stable frequency and small size as compared to today's transmitter systems. But the potentially most exciting applications outside the military sphere are in the medical field. In fact the SDI project contains a major sub-program exclusively aimed at the medical sciences. However, this aspect of FEL technology is outside the scope of this article.

Detection and Security

Finally, RF-linac technology can be applied to the generation of a harmless neutron beam for non-destructive testing and detection. The US Federal Aviation Administration intends to install such devices at airports to inspect baggage, since they can even detect explosives. The same linac type is also being considered for a process that changes nuclear waste's atomic structure, shortening its half-life and resulting in a faster decay. Depending on its successful development, this method might make it possible to eliminate the need to bury radiating waste.

Conclusion

The Free Electron Laser is without doubt one of the major inventions of the century. Its military potential has unfortunately overshadowed its many possible civil applications. In the weapons field contemporary FELs promise to become the germ of the "ray gun" of the future, which hurls powerful bolts of energy at the enemy with the speed of light. No more gunlaying systems would then be needed, nor target tracking: the weapon would simply be aimed and fired, and a well-aimed beam would guarantee a hit since nothing can outrun the speed of light.

PHOTO : Various mirrors capable of withstanding the energy of FELs are being developed. This is a Lockhead multi-element model.

PHOTO : Unlike their physical applications, the operating principles of FELs are simple. This diagram depicts a ring accelerator type.

PHOTO : The first lasers were minute experimental devices. Above, a synthetic ruby is inserted into the resonant cavity.

PHOTO : A complete lower power FEL system as it would be used for ECM or medical purposes is a compact unit. Hughes has been working extensively on devices of this type.

PHOTO : The USSR is also thought to be quite active in the field of gigawatt FELs. This picture is an US Dept. of Defense rendering of a Soviet high-power weapon.

PHOTO : In Europe several free electron laser projects are under way. MBB has published this interesting concept for a high-energy battlefield laser weapon system.

PHOTO : It is interesting to compare this picture of a Hughes/Navy tracker/pointer laser weapon system with the US DoD's artist concept of a Soviet unit shown earlier.