A Few Good Beams:
A Light-Speed Defense Against Mach-Speed Missiles
In 1991, during the Persian Gulf War, then-US Defense Secretary Richard Cheney observed at a news briefing that, "As Iraq has shown, modern technology can make a third-rate power a first-class military threat."
To some, this may have seemed like an ironic statement in the midst of almost-daily televised displays of just how technologically advanced Cheney's department had become in comparison with its opponents in Iraq. Photonics-based technologies, including laser guarded armature, infrared night vision and high-resolution satellite nimages, had enhanced the allied forces' ability to quickly dominate Iraqi troops on the fields and in the air.
Ballistic Missiles Proliferate
But the threat Cheney was addressing was Iraqi President Saddam Hussein's arsenal of Scud missiles, with which he simultaneously imperiled Israel, Kuwait, Saudi Arabia and the allied troops with the delivery of chemical and biological weapons. Cheney predicted at the briefing that by 2000 more than two dozen developing nations would have short-range ballistic missiles similar to the Scud, with a coincident trend extending the target range beyond the Scud's 300-mile reach.
Some analysists put the number today at 30 nations, including North Korea, which in August 1998 achieved a 1500-km flight with its Taepo Dong One missile. Pakistan and Iran have also revealed marked advances in missile technology in the past year.
This helps explain why high-energy alsers and their adjunct technologies are on the US military's list of critical technologies. Although six types of lasers are under this category, the list highlights chemical (i.e., hydrogen fluoride and deuterium fluoride) lasers are drawing "particular interest." The document notes technological parity in similar programs in Russia, the UK, France and Germany.
The rationale for military laser research leans heavily on its speed-of-light destruction of projectiles traveling at Mach velocities, a characteristic that also enable rapid retargeting in the event of multiple attacks. Another benefit, after a laser system is fully installed, is the low cost per shot. TRW Inc. of Redondo Beach, Calif., reported that the typical per shot of a chemical laser is a few thousand dollars as compared with missile-based defense systems, which run about $1 million per round.
Former President Reagan is credited with the suggestion of using laser- and other photonics-based defenses to protect US civilians against intercontinental ballistic missiles - long-range cousins of the Scud. But long before Reagan coined the term "nuclear umbrella" in 1983, the Department of Defense recognized the potential of laser energy as a defense against missiles.
In 1968, the US Navy began researching the ability of CO2 lasers to penetrate metals. Its focus then switched to deuterium fluoride lasers at 3.8 μm, which propagate better than CO2 wavelengths through the maritime atmosphere, and began scaling up the power. Within the first 10 years of the program, the Navy shot down the first missile - one of its own Tow missiles - using a 100 kW deuterium fluoride laser integrated with a pointer/tracker system.
"Because of this success, we began working to develop a megawatt-class laser," said Joung Cook, a research physicist at the Naval Research Laboratory in Washington. He said that, by 1982, the Navy produced the Miracl (mid-infrared advanced chemical laser) and Sea Lite beam director and integrated them at White Sands Missile Range in New Mexico. The laser, developed by TRW, could conceivable be packaged small enough to fit into a ship's 5-inch gun mount.
The Miracl borrows from rocket engine design. A gaseous oxidizer is ignited to produce fluorine in an upstream combustion chamber. As the fluorine feeds into individual modules, deuterium is injected into the flow to chemically combine with the fluorine atoms and produce the required population of excited deuterium fluoride molecules. The design of the modules includes several nozzle blades to produce an optically uniform downstream flow field that acts as the lasing medium. Although the range of a shipboard laser is limited by the horizon - never more than 25 km - the US Army recently proved the laser's ability to destroy a failing satellite in low Earth orbit.
Cook said the researchers used the Miracl to do scientific experiments, engineering testing and a variety of lethality evaluations needed for laser weapon development. In 1989, the system was used to shoot down a Vandal missile simulating the flight of a cruise missile; that demonstrated its technological maturity and effectiveness for fleet defense. The same year, he said, the Cold War ended and the warfare environment changed.
The change didn't happen overnight, but with the reduced threat of meeting the Soviet fleet on the open ocean, the Navy began discussing littoral warfare, which required ships to operate more often in shallow water, and often alone. This placed a higher priority on ship self-defense and created new challenges for shipboard missile countermeasures.
Among those challenges were the shorter time to respond to a missile launch and the angle of fire. During the Cold War era, ships traveling in a battle group put protection of the high-value units (e.g., aircraft carriers) first and their own defense second. This also meant that missiles traveling past defending ships at the perimeter often exposed their length, thus providing a larger target and requiring the laser beam to track rapidly across a column of air.
However, in littoral warfare, lone vessels are morelikely to view approaching missiles head-on. This means that a defensive laser beam remains more or less stationary and heats the same column of air. One a windless day, this could result in thermal blooming of defocusing of the beam.
"It is interesting to note that while the shift from open-ocean warfare to littoral warfare has made the laser weapon problem more difficult, it also increased its need," said Lt. Douglas Small, a researcher at the Naval Surface Warfare Center in Dahigren, VA. "That is because, as we get closer to shore, it is harder to detect incoming missiles. This leads to a reduction in the so-called reaction time - once detected, you have less time to actually stop the missile. The directed energy weapon with its speed-of-light travel time would counter this."
Gone, but not forgotten
Along with changes in naval tactics during the 1990s, military spending was changing, too. By 1990, the US Army Space and Missile Command assumed stewardship of the Miracl and Sea Lite equipment and, with it, much of the military's efforts to develop laser countermeasures against missiles.
To date, lasers are not an active part of ship defense. Cook said that, although the technological capability is there, scientists have to determine what size makes sense and what applications present the msot effective use of the ship's real estate and return for the investment.
All of these questions fall under the rubric of utility assessment which, in the absence of formal development of naval laser weapons, continues to propel research into alternative laser weapons, power sources, and size and cost issues.
The Navy footed nearly half of the $25 million price tag for development of a high-energy free-electron laser at the Thomas Jefferson National Accelerator Facility, a US Department of Energy laboratory in Newport News, Va. The lab is soliciting continued funding from the Office of Naval Research after making headlines last June for achieving a record 155 W continuous wave at 4.9 um. As of this writing, the laser had achieved steady output at 500 W.
The Navy's interest in free-electron lasers might be explained by these devices' advantages over chemical lasers for shipboard applications. Besides their tunability, they eliminate the reliance on potentially corrosive chemicals used to energize chemical lasers. Additionally, free-electron lasers' electrical pump source offers more options for extended operations at sea.
Another advantage - and disadvantage - of free-electron devices is that the lasing medium," according to Bill Colson, a physics professor at the Naval Post Graduate School in Monterey, Calif., and one of the technology's earliest theorists. "The vacuum allows us to reach higher peak powers, but we pay the price for that when we get to the mirrors, where the higher energy is focused."
In a free-electron laser onshore, this problem is solved by increasing the Rayleigh range between the mirrors to allow diffraction to distribute beam energy over a wider mirror area. But this is not an option aboard ship, where space is limited. Small, who received his postgraduate degree under Colson, wrote his thesis on a technique that could shorten the distance between mirrors without reducing the beam's ability to tear through a missile.
Small envisioned a free-electron laser with larger mirrors that have a smaller radius of curvature. This, he theorized, would reduce the beam's waist at the free-electron laser's magnetic undulator, while spreading the beam out at he mirrors' surface, thus enabling the mirrors to withstand the laser energy. "Our approach was to share the pain throughout the system," he said.
"Lt. Small's research is one of the more interesting theses to come out of Bill Colson's group," said Fred Dylla, manager of the Jefferson Lab's free-electron laser research. "(Small) indicated how this technology could evolve to become shipboard-compatible, and he showed there were no fundamental physics issues involved, but rather a matter of good, solid engineering."
But Colson pointed out that if the Navy ever deploys shipboard free-electron lasers, that day is distant. "The real challenge is developing a small-footprint, high-power source for the laser," he said. "Ship generators have three or four generators running on diesel, but you can also store energy in capacitor banks. Another method being explored is using flywheels."
Small's work has not been pursued by either the Navy or the Jefferson Lab. "Shrinking [the laser] in size was not an important goal," Dylla said of the Jefferson project, which also received substantial funding from industry sponsors with fewer concerns about the laser's footprint. "Size per unit doesn't matter if the cost per unit of light is low."
The Katyusha rocket, a sort of poor man's Scud, has a range of 12 to 20 km and can be launched in salvos from a truck or individually from a pipe using just a car battery. On the global horizon, the rockets pose little threat to world peace, but to residents living near Israel's border with Lebanon, they are a daily threat.
In February 1996, Miracl must have seemed an especially apt name to the Israelis who observed the technology in action, under the Nautilus program conducted at White Sands. In a collaborative experiment performed by the US and Israel, a Katyusha rocket was fired from a mobile ground launcher. Range radar quickly spotted the rocket and passed its trajectory to a US Army Sea Lite optical course tracking system. The tracker located the rocket in flight and directed the deuterium fluoride beam onto its warhead, detonating it seconds after launch.
Even before this event, laser-based countermeasures had reawakened the Army's interest, as evidence by a 100 percent budget increase for the Nautilus program from the previous year. But with the detonation of the rocket in February, interest caught fire.
To test prototype
By late April, the US and Israel documented their intent to begin joint development of a tactical high-energy laser defense system to defend Israel's northern frontier. In the White Sands demonstration, the megawatt output of the Miracl had been scaled down to represent a smaller, more transportable system. The new-generation laser retains Miracl's deuterium fluoride technology but includes a refined version of the Sea Lite tracking system.
Tom Romesser, vice president and general manager of TRW's Space and Laser Programs, said the laser and tracking system of the early prototype is in final integration. Live fire testing is scheduled to end by September of this year. Results will determine how many additional units will be built.
In the interest or more rapid deployment, Romesser reported that the first generation stationed along Israel's northern border will be nonmobile. "[The system] was originated as a high-priority rapid-development program to be deployed as soon as possible," he said. "The simplest thing was to put it in containers that could be screwed down to a concrete pad."
Development of a mobile system is contingent on what funds, if any, the program receives in the fiscal 2000 budget.
Current systems for missile defense, including the vaunted Patriot missile system, cannot compare in terms of rapid transport with the launch mechanisms that they encounter. During Operation Desert Storm, the swift advance of the US Army's 7th Corps far outpaced the range of support that Patriot systems could offer. The Army is developing Patriot transport methods to remedy this vulnerability, which also must be addressed by laser-based defenses.
In terms of maneuverability, diodepumped solid-state lasers make an attractive solution, a fact notlost on the Army Space and Missile Command. Researchers at Lawrence Livermore National Laboratory in Livermore, Calif., are aiming for development of diode-pumped lasers capable of 100 kW. According to George Albrecht, a physicist for the lab's laser division, the thrust of the technology hinges on a technique called heat-capacity lasing.
Solid-state media's heat capacity is very high. "So we burst the laser very rapidly to exploit the material's heat capacity and to let it heat up. During this burst, you get weapons-grade energies-plenty enough to down missiles," Albrecht explained. "One big advantage is you have separated the lasing function from the cooling function, so you don't have to cool the laser at he same time you lase."
This treats the laser's energy burst very much like munitions in a gun. "When you're out of munitions, you reload. When you're our of photon burst heat capacity, you cool," Albrecht said.
This technique poses several advantages, including the comparative cost-per-shot expenditures over the targets destroyed, which means more affordable training. "You can't fire 50 Patriot missiles to learn what the thing does and what it looks like," Albrecht pointed out. "But it's a few dollars apiece for a diode-pumped solid-state laser." Now, he added, thousands of shots can be fired for training.
Further cost benefits are reaped by circumventing logistical issues associated with missile defense systems-extra transport, fuels, special storage-which are a tremendous expense in wartime operations. Albrecht reported that the smallest effective system is envisioned as being mounted on a HumVee, but he emphasized that the application ultimately determines the size of the weapon.
Livermore's research is focusing on diode-pumped solid-state lasers that emit in typical neodymium state transition of 1.05 μm, but Albrecht said future developments are not limited to that. Funding from the Army Space and Missile Command, he said, has been steady.
Lasers take wing
Since Nautilus' detonation of the Katyusha rocket, the highest-profile and highest-confidence project pitting lasers against missiles has been the Airborne Laser Project. The scope of the project entailed cooperative efforts among several companies. Boeing in Seattle is developing the battle management system and modifying the 747-400 aircraft. Lockheed Martin Missiles & Space in Sunnyvale, Calif., is working on the target acquisition and beam control systems. And TRW, again, is responsible for developing the megawatt-class laser, in this case a chemical oxygen iodine device emitting at 1.3 μm. The project is under the auspices of the US Air Force airborne laser systems program at Kirtland Air Force Base in New Mexico.
The aim is to develop a seven-plane fleet of 747 freighter aircraft capable of destroying theater ballistic missiles in their early boost phase-an impossible task given current operational capabilities. If proved effective, the laser could provide more than just a tactical advantage. It would also act as a strong deterrent, since destruction during the boost phase essentially sends the missile-and its chosen weapons of mass destruction-raining down on the heads of its launcher.
Time lines for the project indicate that the first field test could come as soon as 2003, with the airborne destruction of a launched theater ballistic missile. If the work continues on schedule, a fleet could be in the air as soon as 2007. The airborne laser could be online by 2003 for emergency use.
The laser operates by reacting basic hydrogen peroxide with chlorine gas to produce highly excited oxygen molecules, which in turn mix these excited iodine atoms return to ground state, their energy is released as photons of 1.3 um. The light is amplified in the resonator and emitted at the wavelength, which is optimal for propagating in the 40,000 foot altitudes at which the aircraft operates.
According to Bob Smith, spokesman for Boeing, the airborne laser should be capable of up to 20 shots per magazine, but atmospheric conditions, of the shot could vary that number. As the aircraft rises, the attenuated atmospheric conditions require fewer chemicals per shot. Altitude and other factors also vary the time span of each shot, but it would average five seconds. Cost per shot is around $2000.
The operational capability of the airborne laser system calls for two aircraft to be in the air and operating as a team, so available shots actually double the 20 shots at any given time. When one aircraft system is running low on fuel, it trades places with a fully fueled backup, Smith said.
The laser's range is more than 200 nautical miles, said Richard Garcia, the Air Force's director of public affairs for the project.
TRW cannot reveal its lasers exact energy output and range, which are much more relevant than these factors are in lasers operating low to the Earth's surface. However, the company reported last August that a single module had achieved 110 percent of its required output specification.
Given TRW's near-30-year development of weapons-grade lasers, it is no surprise that the laser's output is the least of the challenges confronting the collaborators. "The laser is not the big issue. It's appearing to work as promised," observed Jon Grossman, a senior researcher at Rand Corp. in Santa Monica, Calif. "Fitting a laser into a 747 is not trivial. Tracking missiles is a challenge, and flying at altitudes to eliminate atmospheric distortion changes season to season. You also have to consider if the missile has been hardened against laser energy. Add all these factors together and you don't know if it will work in a real case scenario until you build it and put it in the field."