a 6-second shot of laser light. End up with a 1-centimeter hole
in a 2-centimeter-thick slice of steel. In the past, this sort of
fire power was only available from large gas or chemical laser systems.
But thanks to a new technology developed at Livermore, this feat
was accomplished by a refrigerator-size laser system using about
30 cents worth of electrical current from a wall socket. Dubbed
the solid-state heat-capacity laser (SSHCL), this system garnered
one of 100 awards presented annually by R&D Magazine to honor
the most technically innovative products for the year. The SSHCL
can produce up to 13,000 watts in a single, high-quality beam with
output-pulse energies of more than 600 joules, making it the most
powerful solid-state laser in the world.
systems solid-state heat-capacity design paves the way for
a laser, now under development, that will produce 100,000 watts
in a single beam and opens up a range of applications for industrial
materials processing and military defense. (See S&TR,
for Tactical Laser Weapons.)
|Members of the solid-state
heat-capacity laser development team are, from left, Balbir
Bhachu, William Manning, Scott Fochs, Bruce Roy, James Wintemute,
Steve Sutton, Georg Albrecht, Brent Dane, and Mark Rotter.
New Operation Mode Means More Power
According to Brent Dane, who led the SSHCL team
in Livermores Laser Science and Technology program, the breakthrough
that made this laser possible involves a revolutionary, yet seemingly
straightforward solution to dealing with the high temperature gradients
that occur while operating a solid-state laser system.
Before the SSHCL, solid-state
lasers operating with high-energy pulses have been limited to power
outputs of less than 1,000 watts. The stumbling block to increasing
the power involved the heat generated during laser operations. Any
laser creates waste heat in the system, explains Dane. For
a solid-state laser, this heat is deposited inside the opticsthe
glass or crystalthat provide the gain for lasing. If not removed,
the heat can damage the optics.
Most solid-state laser systems
are continuously cooled while operating to avoid such damage. Waste
heat is conducted from inside the glass to the surface where it
can be carried away by coolant, such as water. This cooling process
occurs at the same time as the lasing, creating a large difference
in temperature between the optical materials relatively cool
surface and its heated interior. These large temperature gradients
lead to mechanical stress, physical deformation, optical distortion,
and ultimately, to the fracture of the optic.
Dane and his team have demonstrated a different operating mode, in which
the lasers cooling cycle is completely separate from its high
power bursts. During a burst, the waste heat accumulates evenly
throughout the glass or crystal material of the optic. At the end
of the burstwhich typically lasts 10 to 20 secondsthe
laser is shut off, and the optical material is aggressively cooled
over a period of from 30 seconds to several minutes. Operating the
system in this pulsed mannerseparating lasing cycles from
cooling cyclesmeans that there are no significant thermal
stresses on the optical material during lasing. The average power
emitted by the laser is now limited only by the power capacity of
the laser pump source, which for the SSHCL is flashlamps or another
Livermore-developed technology, the high-average-power diode array
(see the article on diode arrays, SiMM Is Anything
But Simple). This pulsed operation means that the average
power cap is removed for solid-state lasers, allowing us to scale
up the output to hundreds of thousands of watts, says Dane.
|Laser technician Balbir Bhachu
monitors the performance of the 13,000-watt solid-state heat-capacity
laser during a low-power test. The prototype uses an electrical
source to power flashlamps, which in turn pump nine neodymium-doped
glass laser disks that release energy in pulses of laser light.disk.
Advantages to SSHCL
This unique pulsed operating
mode makes the SSHCL the most powerful solid-state laser around.
When compared with other pulsed-format solid-state lasers, the average
power of the SSHCL during a burst is better than that of the competition
by more than tenfold. And when compared with the most powerful nonpulsed
solid-state lasers, the SSHCL exceeds their average power by up
to two times.
But what about other lasers
that are not solid-state based? According to Dane, the realm of
truly high-average-power laser systems has been historically dominated
by chemical and gas lasers. Because these lasers, by their very
nature, can flush out the waste heat from their systems along with
the expended (and often toxic and corrosive) combustion products,
it has been possible to scale their output powers to the highest
values ever obtained for any laser system. However, says Dane, the
SSHCL is now prepared to scale up to and challenge the highest powers
obtained from these lasers as well. SSHCL has other advantages over
these giants of the laser power world, including the ability to
operate at a shorter wavelength of laser light. A shorter wavelength
allows a laser beam to propagate longer distances through the atmosphere
with less beam spread. (The greater the beam spread, the lower the
power densitythat is, watts per unit areadelivered to
the target.) For example, given the same size beam, the smallest
theoretically achievable beam area spread for the powerful deuterium
fluoride chemical laser is 12 times greater than the beam spread
for the SSHCL, and for the carbon dioxide laser, it is 100 times
The SSCHL also has the advantage
when it comes to operational logistics. It can be installed and
operated on a mobile vehicle the size of a jeep and powered by electrical
generating equipment that consumes conventional fuels such as diesel
or gasoline. In practice, says Dane, advanced versions of the SSHCL
will also be able to use high-storage-capacity, rechargeable batteries
that are part of a vehicle. A subscale prototype SSHCL amplifier,
capable of 15,000 watts of output power using advanced lithium-ion
batteries, is being constructed and will be demonstrated at Livermore
military to industry, organizations and companies foresee a bright
future for the worlds most powerful solid-state laser system.
One interesting possibility,
notes Dane, is using the large pulse energies of the SSHCL to clear
orbital space debris from the paths of satellites and manned shuttle
flights. Precisely targeted laser pulses could cause the orbits
of space trash to decay, allowing the trash to harmlessly burn up
during reentry into the atmosphere. Other more down-to-earth applications
for the future include heat treatment of metals and thick-section
metal cutting and drilling.
Because the project is sponsored
by the U.S. Army Space and Missile Defense Command, the first application
is a military one: to defend against rockets, artillery, mortars,
and other tactical threats at close range (1 to 10 kilometers).
There is, Dane notes, currently no effective protection against
these weapons in the battlefield. Michael W. Booen, vice president
of Directed Energy Systems for Raytheon Electronic Systems, said,
Tests of this laser to melt metals and damage other materials
are convincing many audiences that the era of tactical, solid-state
weapons may be fairly close at hand.
The Livermore team and its
industrial partners (including Raytheon, General Atomics, PEI Electronics,
Northrop Grumman, Goodrich Corporation, Armstrong Laser Technology,
and SAFT America) are already working on a version of the SSHCL
capable of being transported and powered on the modern version of
the Humvee military jeep. This final laboratory demonstration version
of the SSHCL, which will have an output power of 100,000 watts under
burst mode for up to 10 seconds, could be ready to demonstrate to
the Army by 2007.
Key Words: R&D
100 Award, solid-state heat-capacity laser (SSHCL), tactical laser
weapon, U.S. Army Space and Missile Defense Command.
For further information contact C. Brent Dane (925) 424-5905 (firstname.lastname@example.org).