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ONCE
again, science fiction has predicted science fact. Remember those
movies where the hero (or villain) uses a beam from a compact laser
to blow a rocket out of the sky? Last December, that generic bit
of sci-fi drama took a step closer to reality. In a demonstration
at the White Sands Missile Range in New Mexico, the solid-state
heat-capacity laser (SSHCL) burned a 1-centimeter-diameter hole
straight through a 2-centimeter-thick stack of steel samples in
6 seconds. The electrical current to do so came from a wall outlet
and cost no more than 30 cents. While large chemical lasers have
successfully shot down tactical rockets, the SSHCL design supports
the weight and size requirements for a future mobile deployment.
The SSHCL, designed and developed
at Lawrence Livermore, is the prototype of a laser tactical weapon,
which shows promise as the first high-energy laser compact enough
in size and weight to be considered part of the Armys future
combat system (FCS) for short-range air defense. The FCS is a component
of the Armys vision of sensors, platforms, and weapons with
a networked command and control system. The more advanced version
of the laser weapon system, now under development, will be battery-powered
andat 2 meters long and less than a meter acrosssmall
enough to be mounted on a hybrid-electric high-mobility multipurpose
wheeled vehicle (Humvee). In this configuration, the Humvees
generator and batteries could power both the vehicle and the laser,
requiring only diesel fuel to support full operation.
The SSHCL offers speed-of-light
precision engagement and destruction of a variety of targets, including
short-range artillery, rockets, and mortars. There is a current
need for effective protection against these weapons on the battlefield.
The project is sponsored by the U.S. Army Space and Missile Defense
Command and has a number of commercial partners, including General
Atomics, Raytheon Co., PEI Electronics Inc., Northrop Grumman Corp.,
Goodrich Corp., Armstrong Laser Technology Inc., and Saft America.
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Lawrence
Livermore laser technician Balbir Bhachu monitors the performance
of the 13-kilowatt neodymium-doped glass version of the solid-state
heat-capacity laser during a low-power test. |
Meeting
the Challenges
The
SSHCL delivered to White Sands for testing last September has an
amplifier composed of nine disks of neodymium-doped glass (Nd:glass).
In this prototype, an electrical source powers flashlamps, which
in turn pump the disks, which then release the energy in pulses
of laser light. The average output power of the SSHCL is 10 kilowatts,
and it can deliver 500-joule pulses at 20 hertz in 10-second burstsessentially
vaporizing metal. The prototype requires 1 megawatt of input power
to produce a 13-kilowatt laser beam. Project manager Brent Dane,
of Livermores Laser Science and Technology program, notes
that the ultimate objective of the project is to build a next-generation
system with enough electrical efficiency to produce a 100-kilowatt
laser beam from the same 1 megawatt of input power. The final version
will be capable of firing 200 pulses per second.
The Livermore team is focusing
on the technological challenges that remain to building the 100-kilowatt
system. Dane enumerated the three areas of concentration: growing
large crystals of neodymium-doped gadoliniumgallium
garnet (Nd:GGG) for amplifier disks; developing the technology needed
to make diode arrays large, powerful, and cost-effective; and defining
the laser architecture and technology that will allow high-quality
beams to propagate precisely over long distances.
Although the prototype uses
Nd:glass for its laser amplifier disks, the final version will use
Nd:GGG. There are many reasons for choosing Nd:GGG,
explains Mark Rotter, an electrical engineer who is leading the
diode-pumped Nd:GGG effort. Compared with Nd:glass, Nd:GGG
boasts a higher mechanical strength and higher thermal conductivity,
which, in combination, will allow us to rapidly cool the disks between
runs and reduce the turnaround time between laser firings. The Nd:GGG
is also twice as efficient in converting pump energy to output beam
energy. The challengeto grow the crystals large enough
to manufacture the nine 13-square-centimeter slabs needed for the
100-kilowatt laseris well on its way to being met. Northrup/Grumman
Poly-Scientific, the commercial partner responsible for growing
the crystals, is now producing high-optical-quality Nd:GGG crystals
up to 15 centimeters in diameter. The ultimate goal is to grow crystals
approximately 20 centimeters in diameter.
To pump these Nd:GGG amplifier
disks, the SSHCL will use arrays of laser diodes instead of flashlamps
because diode arrays are more compact and efficient than flashlamps
and, more importantly, diode radiation generates less heat in the
Nd:GGG laser crystals. The challenge is to make the diode arrays
large, powerful, and cost-effective and to come up with a cooling
scheme that will work in the field.
Lawrence Livermores
Ray Beach, who leads the diode array portion of the project, explains,
Cooling high-average-power laser diode arrays is a unique
and challenging problem in the field of thermal engineering. Although
laser diodes are extremely efficient devices by ordinary laser standardsthey
typically convert 50 percent of their electric input power into
light outputthe remaining 50 percent of the input power shows
up as high-intensity heat from a very compact source. Because the
arrays operate near room temperature, there isnt much opportunity
to radiate away heat or use standard electronic cooling techniques
such as forced air.
Livermore
engineer Barry Freitas came up with a revolutionary packaging technology
that solves the problem of creating high-density diode arrays. In
this approach, small laser diodes are soldered to low-cost silicon
substrates that are etched with thousands of tiny (30-micrometer-wide)
microchannels. Cooling water flows through these microchannels,
which act as high-performance heat sinks. The team used this packaging
design to create the worlds highest average-power diode array41
kilowatts of peak power from a 5- by 18-centimeter package. Arrays
that produce 100 kilowatts of power are in production. Work is under
way with Armstrong Laser Technology to commercialize the silicon-based
diode laser package to support the production needs of the 100-kilowatt
laser development.
The team is also working
on an optical system that will make a beam of high enough qualitythat
is, sufficiently narrow, intense, and well-shapedto propagate
10 kilometers and still hit and disable its target. In the
final system, the laser pulse will travel through nine slabs of
crystal, and no matter how good the optics are, the beam will pick
up distortions along the way. Its those distortions in the
wavefront that we are addressing, because they decrease the power
that can be extracted in the laser beam and cause that beam to diverge
more on the way to the target, explains Dane.
A team led by Jim Brase
in the Physics and Advanced Technology Directorate is developing
an adaptive resonator system that will sense distortions in the
wavefront and correct them in the system. The resonatorwhich
is based on adaptive optics technology developed at Livermoreincludes
a deformable mirror, control electronics, and sensors to detect
the shape of the laser pulses wavefront. A deformable mirror
will be placed inside the laser resonator, and a wavefront sensor
will be used to measure the output beam during operation. The sensor
measures the difference between the actual shape and a perfect,
flat wavefront. Computer-controlled actuators on the mirror then
raise or lower small sections of the mirrors surface to correct
distortions in the incoming light so that a high-quality beam is
maintained from the laser resonator.
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A life-size model, developed by General Atomics and PEI Electronics,
of a mobile 100-kilowatt heat-capacity laser built on a prototype
of a hybrid-electric, high-mobility multipurpose wheeled vehicle
(Humvee) shows the potential compactness of a full-power weapon
system. |
Future
Looks Bright
The solutions to these challenges
are being incorporated into an SSHCL testbeda module made
up of a three-slab Nd:GGG amplifier pumped by laser diode arrays.
This testbed will be configured as a laser system to demonstrate
the pulse energy at a high repetition rate in 2003. The final version
of the SSHCL, which would have an output power of 100 kilowatts
under burst mode for several seconds, is expected to be ready to
demonstrate to the Army by 2007.
Meanwhile, at the White Sands
Missile Test Range, the Army, with Laboratory support, is putting
the prototype through its paces, testing it on aluminum and steel
to determine what types of power and pulse format will optimize
the final weapon system. The Army will also use the prototype to
address issues such as lethality, beam degradation due to atmospheric
effects, and precision optical pointing and tracking.
The future for the solid-state
laser looks promising, notes Dane. The system we delivered
to White Sands is just the starting point. The goal is to have a
laser weapon system that is small, cost-effective, and mobile, which
protects against tactical threats while meeting the sponsors
other military requirements. Were confident well meet
these goals.
—Ann Parker
Key Words: laser
diode array, neodymium-doped glass (Nd:glass), neodymium-doped gadoliniumgalliumgarnet
(Nd:GGG), solid-state heat-capacity laser (SSHCL), tactical laser
weapon, U.S. Army Space and Missile Defense Command (SMDC).
For further
information, contact C. Brent Dane (925) 424-5905 (dane1@llnl.gov).
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