Livermore laser targets battlefield environment

An engineering team at Lawrence Livermore National Laboratory (LLNL; Livermore, CA) led by Bob Yamamoto has taken steps toward meeting a "$150 million challenge" from the U.S. Army Space and Missile Defense Command...

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An engineering team at Lawrence Livermore National Laboratory (LLNL; Livermore, CA) led by Bob Yamamoto has taken steps toward meeting a "$150 million challenge" from the U.S. Army Space and Missile Defense Command to deliver a mobile field demonstration of their Solid-State Heat Capacity Laser (SSHCL) for test firing from a hybrid-electric high-mobility multipurpose wheeled vehicle (HMMWV) in about 18 months. Detailed budgets were being submitted to the Army at that time in hopes of obtaining an authorization to proceed that would start the 18-month clock ticking.

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Bob Yamomoto, deputy program manager of the solid-state heat-capacity laser (SSHCL) program at Lawrence Livermore National Laboratory, adjusts components for the diode-pumped SSHCL. Cooling hoses and electrical power wires supplying the three modules can be seen behind him.
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In two years, the SSHCL has progressed from a 10-ft-long 10-kW flash-lamp pumped neodymium glass system to an approximately 1.5-m diode-pumped neodymium-doped gadolinium gallium garnet (Nd:GGG) tabletop laser (see figure). Hoses pump cooling water and wire-bundles deliver battery power (both from an adjacent room) to diode-pumping assemblies in each of three laser modules arranged serially in a linear configuration. Yamomoto's team intends to double the number of modules in the coming year to produce the 40 kW of output power required for the field demonstration. Three additional modules, for a total of nine, are expected to ultimately provide the 100 kW of power required for the actual battlefield laser weapon.

Each module consists of four 720-bar diode arrays pumping an 8-cm2 × 2 cm-thick Nd:GGG crystal at 808-nm wavelength, with approximately 500-µs-wide pulses at a 200-Hz pulse repetition rate. The diode arrays are provided by Decade Optical Systems (DOS; Albuquerque, NM). Alternating vertical and horizontal pumping configurations from diode arrays situated above, below, and at the sides of each module facilitate compactness. Each of the 720 diode bars puts out about 100 W of optical power for 72 kW of peak power per array, according to Mark Rotter, principal investigator for the SSHCL.

The GGG crystals produced by Northrop-Grumman Synoptics (Charlotte, NC) were chosen for the gain media instead of YAG because refractive-index homogeneities limit the usable YAG boule diameters, Rotter said. The surface area of the GGG slabs must be increased from 8 to 10 cm2 for the 40-kW field demonstration, and then up to 13 cm2 for the actual 100-kW weapon. The number of diode bars per array must also increase from 720 to 1250 for the jump to 100 kW.

Single-aperture design

The laser uses a single-aperture design to avoid the phase-locking complexities of multiple-beam designs, and delivers a beam quality between three and four times the diffraction limit from an unstable resonator configuration. An intracavity deformable mirror, intended to adaptively correct for thermally induced aberrations and to provide beam quality on the order of two times the diffraction limit, has been placed on the original flash-lamp pumped system, which is slated for field testing at the Army's High Energy Laser Systems Test Facility (HELSTF) in the spring of 2004 at White Sands Missile Range in New Mexico.

The overall SSHCL design addresses thermal aberration risk primarily through a pumping configuration distributed uniformly through the gain medium. The GGG slabs can operate for 10 seconds of cumulative run time before reaching their temperature maximum of 100°C. The crystals are then cooled back to 20°C in 1 minute by insertion into a heat sink consisting of copper plates separated from the gain medium by only 2 mils. For actual field operation, the Livermore engineers expect that multiple sets of interchangeable gain media will allow the battlefield weapons system to operate continually for more than 10 seconds out of every minute or so. Gain-media exchange systems are scheduled for inclusion on the diode-pumped SSHCL system by this Christmas.

No major scientific hurdles appear to impede project development, but downsizing of components, in some cases by factors of three or four, and hardening the construction to move from tabletops into military vehicles is proceeding at an aggressive pace. Private-industry production of components should facilitate the process, Yamomoto said. "We're closer to having a real thing on the battlefield, just because we've been working with industry for the last several years," he said.

Each of three diode-cooling units that can cumulatively chill and recirculate about 150 gallons per minute of room-temperature water is currently about the same size as the diode-pumped SSHCL. The cooling system must still go through two generations of downsizing to provide all of the needed cooling from just one unit that would be about the size of the refrigerator in a wet bar. General Atomics (San Diego, CA) has already produced a prototype of the desired system, and the intervening generation is close to production, Yamomoto said.

Similarly, the system of pulse-forming network cards (PEI; Huntsville, AL), which regulates the pulse shape and timing sequence of the approximately 110 A of current at about 150 V going into each of the nine diode stacks in each diode array, needs to either shrink by about a factor of four or increase its capacity to run four times as many diodes. The lithium-ion battery (SAFT America; Cockeysville, MD) power source is nominally the same type that would be available on a HMMWV. "We've been characterizing and using these batteries, again to get us closer to having the whole system ready for battlefield deployment," Yamomoto said. "That's the whole premise."

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