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Here's something from aviation week.
Solid-State Laser Programs Advance to High-Power Tests

Jan 11, 2009

By Graham Warwick

Directed-energy weapons are set for key tests this year, including the attempted shoot-down of a boosting ballistic missile, but they may not dispel lingering doubts over the military utility of high-power lasers.

Instead, the real future of high-energy laser weapons could be taking shape in laboratories, where later this year competing solid-state lasers are expected to run at power levels exceeding 100 kw. Different designs for 150-kw. electric lasers will also be tested in the lab this year as a step toward ground, and later airborne, demonstration of a fieldable laser weapon early next decade.

The military wants speed-of-light weapons with pinpoint accuracy, unlimited magazines and variable effects, but while the megawatt-class Airborne Laser and kilowatt-class Advanced Tactical Laser provide high power levels, their size and logistic issues with the hazardous chemical fuels limit their potential. "Warfighters want an electric laser," says Don Seeley, deputy director of the U.S. Defense Dept.'s High-Energy Laser Joint Technology Office.

Solid-state lasers promise to be much smaller and lighter, easier to integrate on to mobile and airborne platforms, and powered by electricity generated on board. Compared with fuel-hungry chemical lasers, electric weapons offer longer run times and unlimited shots.

The Pentagon's flagship effort - the Joint High-Power Solid-State Laser (JHPSSL) program - is nearing completion with the laboratory demonstration of lethal power levels. But the High-Energy Liquid Laser Area Defense System (Hellads), a U.S. Defense Advanced Research Projects Agency program, is poised to take the lead in demonstrating a deployable weapon.

Northrop Grumman and Textron Systems are developing competing 100-kw. solid-state lasers under JHPSSL. Textron is also building a more powerful derivative of its JHPSSL laser for Hellads, while General Atomics is developing the unique "liquid laser" that gave the Darpa program its name.

Full-power firings of the JHPSSL devices were planned for the end of 2008, but are now expected in February-March for Northrop Grumman and summer 2009 for Textron. Both companies have completed 30-kw. firings as a step toward full power levels. The 100-kw. demos will complete the program, but the solid-state lasers are candidates for the U.S. Army's High-Energy Laser Technology Demonstrator program to test a truck-mounted system in 2013-15 that can counter rocket, artillery and mortar projectiles.

"JHPSSL is stoking the fire of military interest in high-energy lasers," says Dan Wildt, Northrop Grumman vice president of directed-energy systems. In addition to counter-rocket/artillery, potential uses include precision strike and aircraft self-defense for the U.S. Air Force, and anti-terrorist force protection and ship anti-missile defense for the Navy.

JHPSSL is demonstrating two different approaches to scaling solid-state lasers to high power. Northrop Grumman uses a "master oscillator power amplifier" configuration where the beams from eight lasers are combined optically to get to 100 kw. Textron uses a power oscillator configuration where a single beam goes through a chain of gain modules to produce a 100-kw. laser.

Northrop Grumman's design is based on 15-kw. building blocks, or "benches," says Wildt. Eight of these benches stacked in two columns will produce "well over 100 kw.," he says. Inside each bench are four gain modules, comprising neodymium-doped yttrium aluminium (Nd:YAG) crystals, or "slabs," pumped by light from diodes to amplify the laser beam. The beams from the benches are then tiled - laid side-by-side - and their phases controlled so they combine optically into a single beam.

Beam quality is a key parameter for solid-state lasers, both to maximize the energy on the target and to enable beam combining for scaling to higher powers. The JHPSSL goal is better than two times the theoretical ideal "diffraction limited" beam divergence. Electrical-to-optical efficiency is another key parameter, as it drives the size of the power supply and thermal management systems needed to operate a high-energy laser on a platform. JHPSSL's goal is 15-20%.

"Tiled apertures lose efficiency in the far field due to the fill factor in the beam, so you need to get them as close together as possible," says Seeley. But Wildt says it offers manufacturing advantages, as production can be set up around standard laser chains packaged into 1 X 1-meter benches. The company recently launched the first weaponized laser product, the Firestrike module, based on JHPSSL technology.

In Textron's laser, the gain modules are in series and one beam goes through all the chains - six 15-kw. amplifiers producing a 100-kw. laser. The design is based on the company's ThinZag ceramic Nd:YAG slab laser. "The beam zig-zags through each gain module slab, which takes out non-uniformities from the pump light," says Seeley.

Textron says ThinZag's advantage "lies in its inherent simplicity, fewer parts count and ability to scale to extremely high powers without recourse to beam combining techniques." JHPSSL allowed the company to make a late entry into the Hellads program and still meet Darpa's schedule for a laboratory "shoot-off" between competing lasers later this year.

Hellads differs from JHPSSL in being the first program to impose size and weight requirements on a complete laser weapon system. The goal is to produce a 150-kw. weapon that fits within 3 cu. meters and weighs no more than 5 kg./kw. - more than 10 times smaller and lighter than any other high-power laser.

"JHPSSL is about scaling to 100 kw. in a laboratory. Hellads has higher power and aggressive targets for weight, size and run time, all within a form factor that fits on a tactical platform," says program manager Don Woodbury. Hellads is small enough to fit inside a bomber, transport or tanker and still allow the aircraft to perform other missions.

The original Hellads was conceived by Michael Perry, president of General Atomics' Photonics division, as a "radically different approach" to making a deployable laser weapon system. Perry says earlier work on ground-based high-energy lasers showed battlefield smoke and dust would degrade the beam. "We had to get the laser off the ground, but the issue was its size, weight and performance." The problem is not the laser head itself, which is "pretty small," he says, but the electrical supply and thermal management systems required to power and cool the weapon.

Instead of optically combining the beams from chains of lasers, Hellads is a single large laser. "In a typical solid-state laser, a slab of Nd:YAG is pumped with diode light and cooled at the sides, so there is a thermal gradient - the slab is hot in the middle and cold at the edges," says Perry. "A slab laser is a ceramic or crystal, and at high energy density it shatters from thermal shock, like a hot glass in cold water.

"The liquid laser design is completely different. It eliminates the thermal gradient and allows it to work a very high power," says Perry. The design is classified, but essentially the beam passes through a series of thin-disk laser amplifiers and the coolant in which they are immersed. The system comprises two 75-kw. modules, but they plug together to produce a single 150-kw. laser resonator, and there is no beam combining.

General Atomics has been working on Hellads since 2003. Textron entered the program only recently, having convinced Darpa it could scale up its ThinZag technology to meet the power and weight requirements. The design has three 50-kw. laser modules - called unit cells - similar to the JHPSSL power oscillator configuration "but with several significant design differences based on lessons learned," a company official says.

"JHPSSL was a great starting point and made it possible for Textron to be a competitor," says Woodbury. "They came in late and had to start from scratch, but they have made great progress in the laboratory and have gone well beyond JHPSSL in power, beam quality, run time and footprint." Darpa plans a shoot-off in the summer, with the winner going on to build the full laser. "All the science is in a unit cell; we simply replicate it to get to 150 kw.," he says.

The winning design will be integrated into a laser weapon system being developed by Lockheed Martin under contract from the Air Force Research Laboratory. It includes beam and fire control, power supply and thermal management.

Ground tests are planned for 2011 and will include a demonstration of the system's ability to shoot down two SA-10-class surface-to-air missiles in flight simultaneously. "We want as realistic a tactical environment as possible," says Woodbury. "The next step is to line up support for an airborne demonstration. The system will be ready in 2012, and we could see a demo in 2012-13."

To save time and money, the ground demonstration will use the beam director from the Airborne Laser Laboratory. This has representative functionality, but an airborne demo would require a smaller, simpler beam control system. This, and tapping into the aircraft's power and cooling, are necessary to achieve Darpa's size and weight goals. "The technology exists to build a system at the size required, but we have to demonstrate Hellads works first."

After completing the ground demonstration, Darpa plans to transition Hellads to the services. The agency is already talking to the Army about a mobile ground defense platform, but Woodbury believes an airborne demonstration would be a "game changer." Hellads will be compact enough that the platform will not have to be dedicated to carrying the laser. "A B-1 could have two bays of kinetic weapons and one bay with the laser. The AC-130 could still carry the big gun," he says.

http://aviationweek.com/aw/generic/story.jsp?id=news/aw011209p1.xml&headline=Solid-State%20Laser%20Programs%20Advance%20to%20High-Power%20Tests&channel=awst



Of course earlier from wired


http://blog.wired.com/defense/2007/07/lockheeds-death.html


and here's the LM patent

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=1&f=G&l=50&co1=AND&d=PTXT&s1=%22directed+energy%22&OS=%22directed+energy%22&RS=%22directed+energy%22
 

Antonio

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Thanks avatar, that's extremely interesting. One again reality met SF :)
 
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