HELEX - High Energy Laser ExperimentalHELEX (High Energy Laser Experimental) a multi-megawatt gas dynamic CO2 laser mounted on a tracked armored vehicle (Leopard 2). Theoretical research was performed by by Diehl, Gmb in Nuremberg and MBB in Munich and commissioned by the Federal Ministry of Defense in Germany. The laser is powered by burning a liquid carried in storage containers : A common hydrocarbon fuel such as benzene (C6H6) is combined with an nitrogen compound oxidizer (N2O). After combustion, the high temperature gases are rapidly cooled by supersonic adiabatic expansion through a comb of very fine nozzles. This leads to a non-equilibrium distribution among many quantum levels of the CO2 molecule, some transitions are inverted. Energy is extracted from this population inversion by an optical resonator transverse to the flow. The 10,600 nm wavelength beam is directed by large mirrors on an adjustable scaffolding. The heat is carried away by the non-toxic waste gas via a diffuser.
The following description of HELEX is from Anderberg (1992)
GERMANY'S AIR DEFENSE LASER (HELEX)One of the most interesting HEL weapon projects is the German air defense system called HELEX, which is an industrial joint project between Diehl, Gmb., in Nuremberg and MBB in Munich. HELEX stands for High Energy Laser Experimental. The project is still in its early stages, although the initial work started in the late 1970s. MBB together with Diehl have been commissioned by the Federal Ministry of Defense in Germany to implement and study this experimental system as a continuation of the work done previously In the following discussion, the term HELEX refers to the industrial conception of the final weapon to be delivered to combat units if the experiments are successful. The project is interesting, not only because a comparatively large amount of information has been made public so far, but also because it tries to meet a precise military requirement. Since this is not only a research program but also a very extensive development program aimed at producing a well-defined laser weapon for a future battlefield, it will be described in detail. The idealized conceptualization. is given in Fig. 5.1.
Germany has a long common border with Poland and Czechoslovakia, which were Warsaw Pact (WP) countries, and the distances from important targets inside Germany to WP air bases and missile sites were very short. The time between an airborne attack launched from the WP air bases across the border could be extremely short, lasting only minutes. Thus, Germany was very vulnerable to low-level air attacks by combat aircraft missiles and standby weapons with the capability of engaging targets automatically. However, the distances are still comparatively short, and, even though the warning time is slightly longer, this limited distance will still be a problem for Germany's air defense. The present-day German air defense is heavily dependent on ground to-air missiles, fighter planes, and sophisticated chains of radar stations which feed the command and control system with information. In spite of all the money spent so far on this very complicated air defense system, it may be insufficient to counter future threatening situations in which the other side will use an increasing amount of more and more sophisticated electronic countermeasures. Air defense laser weapons could be one way to achieve the extremely short warning and engagement times that Germany will eventually require.
The main component of the HELEX is a gas dynamic carbon dioxide laser which emits an average beam power of several megawatts over the specified mission time. To carry the laser and all of its accessories, the basic chassis from a German tank, Leopard 2, has been suggested. The supply tanks for gas, water, etc. are used for the laser fuel, while the laser itself and its coolant water are carried in the chassis. As laser weapons have a direct-line-of-sight action, it is important to position the laser beam above the tops of surrounding trees and buildings. This problem is solved by using an elevator platform to carry a focusing mirror of more than one yard in diameter along with the passive surveillance and target acquisition system. The area of coverage of the HELEX will also be greatly increased by the elevated platform, since the time between the identification of a target and the laser hit is very short, and it may be possible to engage very low flying targets that quickly appear and disappear out of the immediate field of view.
A relatively simple technical principle has been used for the HELEX. The high-energy gas dynamic laser employed does not need a heavy and complicated gas pump or flow system nor does it require sophisticated cooling. The fuel is a common hydrocarbon burned together with a nitrogen compound oxidizer, both of which can be easily carried in the liquid storage containers. The hot gas flows at supersonic velocity through a comb of very fine nozzles, expands, and is transformed into the population inversion state required to amplify the laser energy. The gas then flows at supersonic speeds through an optical resonator (mirrored cavity), where stimulated emission occurs, and the laser beam is finally created. The beam leaves transverse to the gas flow direction. The used, nontoxic gas is vented into the atmosphere through a diffuser. At the same time, the exhaust gas carries off most of the waste heat. Overall, the function of the laser is similar to that of a rocket engine.
The emitted power of the high-energy gas dynamic laser is proportional to the amount of fuel used. The research to date indicates that the dimensions of even very high energy laser equipment will remain within acceptable limits from a technical point of view. The fuel consumption per laser shot corresponds roughly to the weight of a guided missile, but the fuel consumption of future-generation systems should be lower. If these estimates are correct, an HEL weapon like the HELEX should be able to fire something like 50 laser shots with the amount of fuel (5-10 tons) carried in the tank.
The wavelength of the HELEX system may be either 9,350 or 10,600 nanometers. Most reports on the system indicate a wavelength of 10,600 nanometers. However, the shorter wavelength may be a more appropriate choice, since the larger the focusing mirror is relative to the wavelength, the smaller the focal spot and the higher the energy density will be. Obviously, the desired effect requires as high an energy density as possible.
The optics of the HELEX Must cope with the difficult task of focusing enough laser energy on the target to destroy it in the air or cause it to crash. This has to be done on the battlefield even when the atmospheric conditions are unfavorable and at a combat range of at least five to ten kilometers if the HELEX is to be cost effective within the air defense concept.
Only mirrors suitable for use at the wavelength and high power levels of this system can be used to direct and focus the beam. The use of transmission optics such as lenses is not very feasible due to their high cost and fragility, and, in any case, the HELEX Will probably damage any lenses to some extent. The reflector at the top of the elevated platform is a concave mirror with a diameter of more than one meter. To achieve a sufficient effect at the target range, compensations for atmospheric turbulence, blooming, and other disturbances to the laser beam inside and outside the system are planned with an adaptive mirror. The mirror surface can assume the required shape and the correct axial angles with the aid of numerous piezoelectric (small electronic) actuators exerting mechanical forces on the mirror back. To enable the mirrors to withstand the HEL beam, a cooling liquid flows through fine channels on the rear of the mirror. Compensation by adaptive optics may double the range possible with a rigid mirror system.
The information necessary to control the mirror surface shape is furnished by the laser beam reflected by the target; thus, the beam itself becomes a sensor element in the closed control loop by which the target is tracked. It is a difficult problem to achieve a really high precision laser beam, and it is necessary to keep a focused beam on a single location of an extensive target for a considerable time. If a target moves at the speed of sound (Mach 1) and the beam must be coupled with it for, at least, a half a second, during this time the target will move nearly 105 yards. Keeping the laser on the same spot may be done either by using the variable reflection characteristics of the target or by a procedure where the deviation of the beam center from reference marks on the target is used as the control signal. Diehl has demonstrated this procedure by means of a rotating aircraft model.
It is not only necessary to keep the beam directed to the same spot on the target, but it is also a prerequisite for the HEL system that the beam can be focused correctly. Basically, the mirror at the top of the elevated platform functions just like a burning glass which concentrates the energy of the sun to such an extent that combustible material catches fire. The advantage of the coherent laser energy is that it can be focused sufficiently over distances of many kilometers to produce thermal effects at the final site. Adaptive optics can be used to focus the beam continuously, even as the target changes its position.
The HELEX will have some type of a passive surveillance and target acquisition system, such as satellite monitoring, which will probably cover the entire hemispherical air space of the protected zone and permit tracking of numerous targets simultaneously This is also the prerequisite for sequential engagement of targets by the laser weapon without any delay. The passive target acquisition makes radar surveillance and tracking unnecessary, and, as a passive surveillance system is used, it may be very difficult for an airborne attacker to find and counter the system beforehand by any electronic countermeasure activity. The HELEX will make it possible to carry out identification, threat analysis, and target selection and finally to hold the beam on the target on automatic, or, if desired, part of the sequence can involve a human operator to select target priority. However, the choice of the best or, at least, a suitable spot to hit on the target has to be done automatically to cope with the time constraints.
If the research and development of the HELEX air defense laser weapon is successful, battlefield commanders will have a powerful tool to cope with highly threatening situations. One air defense HELEX could effectively control an area against multiple low level, high-speed attackers with comparatively low operating costs. The effective range will be dependent on atmospheric conditions. Under very favorable conditions, the range against aircraft, helicopters, and missiles would be up to 6 miles; this would be reduced to 3 to 4 miles in the normally heavily polluted atmosphere over a battlefield. Due to the extremely short time for target detection, tracking, slaving, and firing, it would be possible to engage many targets in rapid succession. If one HEL weapon is defending a facility that is attacked by a squadron-sized enemy force, the laser weapon may very well shoot down all aircraft during their first attack. Reloading is simple; there is no minimum range, and different types of targets do not require the use of different types of ammunition. The main limitation of such a weapon as the HELEX is the reduction in range of the system under very poor weather conditions or when the pollution on the battlefield is extremely heavy. It is difficult to quantify these limitations, but it is obvious that the HELEX will not replace conventional gun and missile systems, not even at distances well within its range. Such HEL weapon systems will only be able to complement existing air defense systems. However, the survivability on the battlefield of a HELEX type system compared to a system dependent on radar technology will be very high, since the passive localizer will not reveal itself. Also, the mobility of a 20-40-ton tracked HELEX system will be high, and it will be possible after terminating one firing action sequence to change the location of the weapon quickly.
Many problems still must be solved before it is even possible to decide if the HELEX concept is a valid one. To date, tests have only been done in the laboratory The scaled-down experimental weapon paid for by the German Ministry of Defense will not be available until 1993 or 1994. If this weapon is a success, and if it is possible to solve all of the very difficult problems, the development of a final air defense high-energy laser weapon based on the HELEX concept may start in the mid-nineties and should be completed about ten years later. This means that theoretically such a weapon could be produced and handed over to the combat units at the beginning of 2005. Due to the technological difficulties involved in this concept, even such a distant delivery date may be overly optimistic.
Other countries have begun developmental work on possible laser weapons along similar lines. In France, several companies together with the French National Aerospace Research Agency (ONERA) are working on a HELEX-like experimental HEL weapon. There have also been some reports on a possible collaboration between France and Germany. In the United States, a similar idea is currently under investigation in the JAGUAR project.
The military specifications for the HELEX weapon are really very ambitious, and this, along with the technological difficulties, is the main reason for the high costs and the very long time necessary for research and development. It is debatable whether or not it would be more cost-effective to limit the requirements to simply damaging some very sensitive parts of the target such as sensors, canopies, and radomes and leave the actual destruction of the platform itself to conventional antiaircraft guns and missiles. This would mean that, up to 6 miles, much less energy would be required, and the sensitivity to the atmospheric conditions should be less. Such a weapon could possibly be fielded much earlier and at a significantly lower cost. Of course, there are even some limitations to this less demanding military requirement. Some of the targets are not all that dependent on their sensors, and, even if they are, it may be possible in the future to make the most crucial sensors insensitive to the effects of laser energy- Whatever the future holds for the HELEX, the fact remains that a high-energy air defense laser weapon capable of outright destruction will be expensive to develop and manufacture, and it will take many years before such a weapon can be successfully fielded. It is very possible that the whole idea will be abandoned because it simply proves technically impossible or just too expensive to implement.