Throughout history, the advantage in warfare has always been on the side that could achieve the greatest concentration of firepower at the right place and time. At the battle of Crecy in 1346, for example, the powerful and rapid-firing English longbow won out over the crossbow. After routing the crossbowmen in the front ranks, the longbowmen made mincemeat of wave after wave of heavily armored French knights on horseback. Though outnumbered more than three to one, the English lost only 100 men of all ranks, while the French lost 10,000! With the introduction of gunpowder and steady improvements in the design and engineering of guns, the bow and arrow disappeared.

By 1590, the bow and arrow was replaced with gunpowder, a "higher technology" improvement, providing the capability of greater concentration of firepower without loss of mobility. Gunpowder employed a more efficient physical principle: chemical combustion.1 As we move into the next decade, we are again on the verge of fielding additional "higher technology" weapons.

The next generation of tactical battlefield weapons will include directed energy or laser weanons against men, electro-optical sensors, and other light-sensitive targets. Speaking in late 1981, Defense Advanced Research Projects Agency director Robert Cooper said that $2 billion was "an enormous amount of money...for what still remains an exploratory development program. Yet he added that "it's the collective judgement of high officials in the Pentagon that laser weapons present a high potential for payoff. There is a good chance that we will put a high-energy lase weapon system on the battlefleld."

The word laser is an acronym for Light Amplification by Stimulated Emission of Radiation. Here is a 1975 description: The laser is really a simple device composed of an energy absorbing medium such as a ruby rod; an excitation source such as a flash lamp; a container of cavity to hold the laser medium and mirrors and lenses to direct and focus the laser beam. Operation of the laser begins when the power source is turned on and the excitation source pumps energy into the medium.

The atoms which had been at rest in the medium, are raised into an excited state from which they soon drop back to their normal level. While dropping back to their normal state, the atoms give off light energy called a photon. The photon strikes another photon and so on until a chain reaction is caused. This chain reaction of photons grows until the laser light is finally emitted from the medium. The output can then be focused and directed as a pencil thin beam of laser light.

TYPES OF LASERS Since this early definition, tremendous advances in research and technology have taken place. In the ruby laser mentioned above, synthetic ruby crystals are utilized as the energy absorbing material. This medium is not efficient, as only about 1% of the light that goes into the rod emerges in the form of laser light. Most of it ends up in the form of heat, which must be removed or its effects on the laser rod may break up the beam or damage the rod itself.

The heat removal required in ruby and other crystalline or glass lasers poses a serious problem. Although the external light source efficiently deposits energy throughout the transparent rod, the excess heat is much slower in leaving the solid. This fact, combined with some complications due to energy levels in the material, limits operation of the ruby laser to no more than a few pulses per second except at very low power levels, and also sets upper limits on the practical output power. Crystalline or glass lasers are easier to cool, and can produce much higher peak power, but they are not practical from a military tactical standpoint. They can only produce about one shot per day, due to heat dissipation problems.

For continuous operation at high power output, a laser material that can quickly dissipate residual heat is necessary. Much research has been done with gas lasers; however, military uses for gasdynamic lasers are limited. The atmosphere does not transmit the beam well, and there is very little tolerance for error in design or manufacture of critical components. The size and complexity of gasdynamic lasers have little tactical application on the battlefield. Most demonstrations of laser weapons in the works involve the chemical laser. As its name implies, it derives its energy from a chemical reaction, the combination of hydrogen and fluorlne to produce molecules of hydrogen fluoride-in a vibrationally excited state.

The term "chemical laser' usually refers to a hydrogen fluoride laser. The reaction in a chemical laser can be triggered by an electrical discharge. The starting point is a fuel containing hydrogen and an oxidizer containing fluorine. Because of problems with fuel stability, however, substitute fuels are frequently used. The laser beam is produced through a chemical sort of chain reaction. There are some drawbacks.

The cavity into which the laser gas flows must be kept at a very low pressure, typically 1% or less of atmospheric pressure. There must be a way to get rid of the gas after it is passed by the laser mirrors. For a space based laser system, it could simply be vented outside. On the ground, however, because of atmospheric pressure, a vent must let the air in. This is necessary since hydrogen fluoride is toxic in concentrations of as little as three parts per million. The Army is currently working on methods to pack the waste hydrogen fluoride into canisters to prevent inadvertent venting into the air.

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