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Laser Defense Outlook Brightens

March 2008
By Robert K. Ackerman
E-mail About the Author

 
Boeing’s  Airborne Laser system uses adaptive optics to focus a megawatt-class beam on a ballistic missile target, destroying it. The chemical-pumped laser soon will be able to destroy a ballistic missile in its boost phase.
Longtime research, new technologies bring reality closer.

The dream of zapping incoming missiles traveling at supersonic speeds into nonexistence is becoming closer to reality as laser science transitions from the laboratory to the field. Research into several different laser technologies is bearing fruit, and soon warfighters and civilians may be protected from threats as simple as mortar rounds or as complex as nuclear-armed intercontinental ballistic missiles.

At the top of the laser roster is the Airborne Laser system. Last summer, that system demonstrated the ability to track, target and illuminate an airborne target with a low-power laser. By August 2009, the complete system should be ready to shoot down a ballistic missile using a megawatt-class laser.

“The airborne laser is the first airborne directed-energy laser weapon produced by anyone,” declares Col. Laurence A. Dobrot, USAF, acting program director for the Airborne Laser program. “That’s going to revolutionize the way we conduct those types of operations. Battle at the speed of light is something that we have not dealt with before, and the airborne laser is a pathfinder to that future.”

Col. Dobrot shares that the program has made significant progress in the past two years. In last summer’s test, the system locked onto an uncooperative target vehicle—a KC-135 aircraft with a missile painted on its side—using its infrared tracking system and its tracking laser. The system’s beacon laser then illuminated the target to determine the degree of atmospheric distortion. That data in turn allowed the system’s main laser to compensate for the distortion and to strike the target. The main laser was represented by a low-power beam to prevent destruction of the target aircraft.

The system’s main high-energy beam is a chemical-pumped laser that features adaptive optics. When cued by the beacon laser, the system deforms a series of mirrors to pre-distort the killing beam before it leaves the aircraft. In effect, the system compensates for atmospheric distortion by treating the atmosphere as an optical lens. It then emits a pre-adjusted beam that will strike its target with maximum efficiency and concentration after undergoing that lens effect.

The main laser is powered entirely by its chemical reaction, and the turbopumps that fuel that reaction are powered by a peroxide-powered gas generator. The system’s lesser lasers draw power from the aircraft’s own engine generation. No extra power units are needed, the colonel explains.

The toughest hurdles that had to be overcome over the past few years involved technology integration, Col. Dobrot offers. Many of the system’s technologies came directly out of the laboratory. The high-energy laser alone comprises six laser modules generating megawatt-class laser power. No one ever had fired those six modules in sequence before this program, the colonel notes. The testbed for that construct was an old Boeing 747 aircraft cabin and fuselage in a hangar at Edwards Air Force Base, California. Engineers were able to fire the laser almost 70 times with consistent performance after early tuning, he recounts.

The adaptive optics were tested by ground-based astronomers who were able to remove starlight distortion—twinkling, to the naked eye—to produce sharp images that rivaled those of the Hubble Space Telescope. Engineers applied that work to eliminate atmospheric beam distortion.

Information processing technology actually exceeded expectations, the colonel says. Over the length of the program, engineers were able to transition signal processing from multiple boxes to single cards.

The high-energy laser currently is being installed aboard its aircraft. Ground testing should begin this summer with activation tests. Other airborne tests will evaluate performance against faster targets than a piloted subsonic aircraft. Col. Dobrot notes that one test already evaluated tracking and targeting capabilities against a supersonic F-16 in a zoom climb. The difference in speed between a slow-moving object and a supersonic one does not affect the system’s ability to track a target, as the laser’s aiming turret slews to keep the target in frame, the colonel offers.

 
A technician at Northrop Grumman’s Space Technology sector checks diagnostic instruments used to monitor infrared laser beams fired by the company’s laboratory demonstrator for the U.S. military’s Joint High Power Solid-State Laser (JHPSSL) program. Successful tests have paved the way for scaling up the solid-state laser from 27 kilowatts to an eventual goal of 100 kilowatts. 
If successful, the August 2009 test would be followed by efforts to expand the system’s envelope. Engineers would try to find limits to the capability of an aircraft carrying a megawatt-class laser. This effort will include tests against different types of targets, both airborne and ballistic. A second laser-equipped aircraft also would be built with enough design improvements to make it operational.

The biggest potential pitfall may not be technical but budgetary. Next year will bring a new administration and Congress, and no one can predict what their budget priorities will be.

Some land-based directed energy systems show promise. These largely are infrared lasers that destroy incoming rockets by focusing heat on their warheads causing them to explode, instead of destroying projectiles with a burst of energy. Many of these exploit solid-state technology, which generates laser energy without the large logistics footprint of chemical lasers.

One system is the Laser Area Defense System, or LADS. Under development by Raytheon Company, this laser builds on the Phalanx close-in air defense system that protects U.S. Navy ships.

Michael Booen, vice president for advanced missile defense and directed energy weapons at Raytheon in Tucson, Arizona, explains that work on LADS began about two years ago. The original aim was to protect U.S. forces in the Baghdad, green zone from mortar attacks. To build a laser defense system quickly, the company sought off-the-shelf technologies that could produce a prototype within six months.

Engineers selected a 20-kilowatt solid-state fiber laser from the Air Force Research Laboratory, which then partnered with the company on LADS’ development. The laser emits a 1-micron infrared beam that heats incoming warheads until they detonate. Within their six-month deadline, engineers were able to blow up two mortar shells in static tests at a range of 500 meters (1,650 feet).

Tests with the Phalanx gun system showed that it could destroy incoming mortar shells and rockets, so some of these land-based Phalanx systems were deployed to Iraq and Afghanistan. To deploy LADS, the company plans to swap out the conventional 20-millimeter Phalanx gun with the 20-kilowatt laser. The laser would operate using the existing Phalanx radar and command and control system. Tests scheduled for later this year will evaluate the full LADS system against incoming mortar rounds. If these tests are successful, the system will be ready for deployment, Booen says.

Booen offers that the 20-kilowatt laser has a range similar to that of the kinetic-kill Phalanx system. But, advances in solid-state lasers are increasing beam power, which will extend the range of the laser defense system. This will make it more effective than the Phalanx gun. And, the laser will have an “unlimited magazine,” Booen says, as it will need only a steady flow of electricity to keep pumping photons onto incoming targets.

“We’re entering an era where we actually have systems that can be deployed and that are ready today,” Booen emphasizes.

While stronger lasers can be incorporated incrementally as they become available, the existing 20-kilowatt unit is capable of shooting down Katyusha rockets in addition to mortar rounds. Even unmanned aerial vehicles, particularly their imaging sensors, would be viable targets.

Those stronger solid-state lasers may emerge from two separate efforts funded by the U.S. Army. Part of the Joint High Power Solid-State Laser (JHPSSL) program, both aim to develop a 100-kilowatt solid-state laser that can be used to shoot down incoming artillery and small rockets.

One effort, led by Northrop Grumman Corporation, Redondo Beach, California, completed its first milestone early last year. This entailed developing a gain module operating at 4 kilowatts. These gain modules will be combined into a laser chain that in turn will be combined with others to produce the laser, explains Jay Marmo, JHPSSL Phase III program manager at Northrop Grumman.

This system effectively combines several individual laser beams coherently into a single beam in phase. The beam is an infrared laser operating at 1.06 microns.

The laser chains are nominally 15 kilowatts, Marmo continues. Combining these chains will lead to whatever power designers want to attain. In Phase II, two-chain technology generated about 27 kilowatts. This demonstrated both the ability of multiple chain modules to generate a laser and the necessary wavefront correction for adequate beam quality, he says.

With the laser chain phasing being demonstrated successfully, company engineers feel confident that they can increase the power of the system. The engineers are striving to make this technology more compact and better performing, Marmo says. Achieving these goals would generate 30 kilowatts of laser output from a two-chain system with better beam quality. This also will allow the addition of more laser chains to reach the 100-kilowatt goal. That should be attained this year, he notes.

Marmo allows that most of the technological hurdles have been overcome. What remains are largely engineering issues. As these are overcome, it may be possible to scale up the laser beyond the 100-kilowatt goal. Ultimately, engineers will reach a ceiling in scalable power, Marmo admits, but that limit has not been determined yet.

First uses likely would be in mobile ground-based laser systems, he allows. Ships also could install this type of laser for close-in defense, and ultimately the system might be placed on aircraft.

Another effort in the JHPSSL program is run by Textron Defense Systems, Wilmington, Massachusetts. This work is built around a proprietary company technology it calls ThinZag, reports Dr. Dan Trainor, director for laser systems at Textron Defense Systems Corporation.

In this technology, the slab is fabricated in a thin design so that heat can be extracted easily. Its zigzag pattern helps average out non-uniformities. This geometry allows the use of multiple large slabs in a single module. A single module acts as a base, and then multiple modules are arrayed in series to act as a power oscillator.

The slabs also are made of ceramic neodymium:yttrium-aluminum-garnet (YAG), which enables the fabrication of larger slabs than those made with crystalline material. These slab constructs can be multiple centimeters high and tens of centimeters long.

John Boness, vice president for applied technology at Textron Systems, states that this concept allows the construction of large laser devices with a simple configuration having fewer parts. “It’s simpler, and you can build very large lasers based on this fundamental concept without getting into thousands of parts,” he emphasizes. It also generates only one beam, so the system does not have to deal with the challenge of combining multiple beams.

The design already has generated a 20-kilowatt laser beam using one module. Trainor explains that the company has configured two modules to generate power, and the next step will be to add a third module in late spring. To reach the 100-kilowatt target, the laser will require six modules. “Once we’ve done one [module], all the rest are exactly the same,” he says. “If you put two together and operate them as a power oscillator successfully, it follows that putting six together entails no new engineering or physics.”

Boness warns that the company cannot expect “no surprises” along the way as it scales up to six modules. Yet, the firm does expect its progress to be fairly repetitious. Trainor notes that there is a limit to the number of modules that can be configured, particularly as engineers accumulate more gain than they can handle. That might be overcome by building larger and more powerful modules that can generate a stronger beam despite being fewer in number. Both physics and manufacturability pose potential limitations.

One of the long poles in the tent involves manufacturability, Boness notes. The laser features precise optical components and exotic materials with limited sources. Those mandate long lead and fabricability schedules, so design changes can be a major problem down the line. Other constraints may involve the availability of these exotic materials, particularly those that must come from sources overseas.

Controlling the beam phase has been the most significant challenge faced in development, Trainor allows. The inefficiency of solid-state lasers and necessary thermal management generates phase errors that required control. He states that the company has been successful in that realm.

Research continues at Lawrence Livermore National Laboratories in a solid-state laser that has been in development for some time (SIGNAL Magazine, April 2005). This laser has achieved an output of 67 kilowatts by using five ceramic YAG diode blocks. This represents a change both in power—up from 30 kilowatts—and in material. The laboratory had been using four diode slabs made of neodymium:gatalinium-gallium-garnet (GGG) crystal, but it switched to ceramic YAG a couple of years ago.

Robert Yamamoto, program manager/chief engineer for the Solid-State Heat-Capacity Laser program, explains that the laboratory switched materials for several reasons. YAG blocks are easier to fabricate and scale up in size, which also translates to greater laser power. Neodymium:GGG can be made only in sizes up to six inches. And, the ceramic material resists cracking better than the crystalline neodymium:GGG.

The transparent ceramic slabs also allow greater versatility in the pumping architecture. Instead of pumping off of the face of the neodymium:GGG slab, YAG slabs enables pumping off of the four edges of the slab. The laboratory’s slabs are framed with samarium-doped YAG, and this provides better uniformity of temperature, which improves laser beam quality. The laboratory currently is buying its YAG blocks from Konoshima Chemical Company in Japan, but it also has its own internal research and development effort in transparent ceramics for laser-gain media, Yamamoto notes.

Yamamoto shares that the laboratory has not received funding for the final push to achieve a 100-kilowatt beam with a solid-state laser. It continues to strive to improve beam quality with its existing construct. This will increase the distance a beam can travel through the atmosphere. The laboratory hopes to further validate its edge-pumping architecture as a key to significant beam quality improvement, which in turn may lead to renewed funding.

Web Resources
Airborne Laser: www.airforce-technology.com/projects/abl
Raytheon LADS: www.raytheon.com/technology/rtn07_sslad
Northrop Grumman JHPSSL: www.st.northropgrumman.com/capabilities/directed_energy_syst/laser_systems/solid_state_lasers/jhpssl.html
Textron Defense Systems Lasers: www.textrondefense.com/products/laser_systems.htm