An extraordinary plan to beam more power to orbiting
communications satellites than they can get from the sun may make use of a free-electron
laser under development at Berkeley Lab's Accelerator and Fusion Research Division. "If
you want to do better than the sun, you need a powerful source," says Alexander
Zholents of the Center for Beam Physics, who has helped design a laser called IFRA
(Ignition Feedback Regenerative Amplifier). At 200 kilowatts peak power, IFRA would be the
most powerful free-electron laser in the world. Remarkably, much of it can be built
virtually "off-the-shelf" using key components designed at Berkeley Lab for the
B-Factory at the Stanford Linear Accelerator Center (SLAC).
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THE 476-MEGAHERTZ RADIO-FREQUENCY, HIGH-ACCELERATING-
GRADIENT CAVITIES OF THE IFRA LASER WILL BE SIMILAR TO THESE, DESIGNED AT BERKELEY LAB AND
BUILT AT LIVERMORE FOR THE B-FACTORY AT SLAC. SOPHISTICATED COOLING CHANNELS ARE EMBEDDED
IN THEIR COPPER SHELLS. THE CAVITIES ABSORB BEAM-INDUCED ELECTROMAGNETIC FIELDS FOR A
CLEAN INTERACTION OF THE BEAM WITH THE ACCELERATING FIELD.
Photo courtesy of Lawrence Livermore National Laboratory.
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In the early 1990s NASA pursued an Earth-to-space power initiative known as SELENE (for
Space Laser Energy), but the project ended before any operational tests were performed.
Fortunately, SELENE found a champion in businessman and physicist Harold Bennett,
president of Bennett Optical Research, Inc. of Ridgecrest, Calif., who secured a grant
from the California Highway to Space program to continue research and development.
Free-electron laser light, if focused directly on the photovoltaic material of a solar
panel and tuned to its most sensitive frequency, can generate ten times as much electrical
power as sunlight. Bennett reasoned that SELENE could help satellite companies meet the
mounting power demands of communications satellites with lighter, cheaper-to-launch
spacecraft. As a result, fewer satellites would be needed, relieving an increasingly
crowded geosynchronous orbit -- and the companies could save billions.
SELENE's investors should also turn a profit. "We can buy power at five cents a
kilowatt-hour and sell it to the satellite owners at $350 a kilowatt-hour," Bennett
says. "The return on initial investment should be 30 to 35 percent per year."
While the technical challenges to the SELENE dream are stiff, they are not daunting.
"There are two main components," Bennett says. "One is the free-electron
laser now being developed at Berkeley Lab. The other is the adaptive optics, and we're
doing that here."
SELENE's home will be near Ridgecrest in the Mojave Desert, where for 360 days a year,
"in the words of our favorite song," as Bennett puts it, "the skies are not
cloudy all day."
Even in clear skies, however, atmospheric distortion is a challenge. Bennett's solution
is a 12-meter compound mirror with adaptive optics -- the same technology used by
astronomical telescopes such as the Keck 10-meter designed at Berkeley Lab to get rid of
starry twinkles, but working in reverse to focus a laser beam on an orbiting satellite.
Since the mirror is so large, airplanes can fly right through the laser beam without
damage, although the beam will be turned off if any airplanes do wander into the region --
an unlikely event, as the SELENE site is under restricted air space: the former Navy test
range at China Lake.
Except for the mirror and power lines (possibly from a nearby geothermal plant), SELENE
will be entirely underground. Meanwhile, with major in-kind support from Bennett's
company, the Navy is preparing a comprehensive land use management plan for the entire
high desert region, as required by the Desert Conservation Act.
Bennett's search for a suitable laser to power his dream took him far afield. When John
Madey, inventor of the free-electron laser, told him that a machine with a peak power of
100 kilowatts was under construction at the Budker Institute of Nuclear Physics in
Novosibirsk, Russia, Bennett traveled to Siberia in July 1993 and inspected it for
himself.
"I needed someone who could evaluate it thoroughly," Bennett says.
"That's when I approached Kwang-Je Kim at Berkeley Lab." Kim concluded that the
Budker machine could indeed be adapted to SELENE's purposes, but that a laser with twice
the peak power could be built in California.
"At that point Kwang-Je Kim, Max Zolotorev, and myself were working on SELENE, and
we had our own ideas," says Zholents. "We came up with an idea for a simple,
reliable facility that could run 365 days a year, 24 hours a day."
Zholents says that one of his incentives was the fact that Robert Rimmer and his
colleagues in the Beam Electrodynamics Group had already designed powerful radio-frequency
cavities for the B-Factory. Twenty-six of these unique rf cavities have been constructed
at Lawrence Livermore National Laboratory for use at SLAC.
Although IFRA's 476-megahertz rf cavities will be virtually the same design, the IFRA
layout is quite different from that of the B-Factory. Electrons are accelerated in the
front stretch of a racetrack longer than a football field, and the electron bunches pass
through long undulator magnets in the backstretch. The powerful coherent synchrotron
radiation is sent to the giant mirror to be beamed into space.
"One of the challenges of the project is to produce a high quality electron beam,
with an average beam power of about ten megawatts," Zholents says. "Typically
only a small fraction of this power is converted to light in the FEL, so another challenge
is to slow the returning electrons before sending them to the dump," where otherwise
they could produce radioactive isotopes. Most of the energy invested in accelerating the
electrons is recouped in the cavities as the returning beam decelerates.
Powerful free-electron lasers like IFRA bring the dream of beaming power across space
-- one long held only by science-fiction writers and a few visionaries like Nikola Tesla
-- closer to practical reality.
The possible uses of SELENE do not end with communications satellites. Ground-based
lasers could power orbital space tugs with photovoltaic wings and ion-thruster engines --
or high-flying electric airplanes, for that matter. Indeed, the NASA scientists who first
suggested laser-beaming power from Earth thought it would be a nifty way to keep a lunar
base running -- which is perhaps the true origin of the acronym SELENE, the classical
Greek name for the goddess of the moon.
IN THE IFRA DESIGN, ELECTRONS ARE ACCELERATED IN THE FRONT STRETCH OF A 100-METER LONG
RACETRACK. AS THE ELECTRON BUNCHES ENTER THE UNDULATOR MAGNETS IN THE BACKSTRETCH, THEIR
DENSITY IS MODULATED TO THE DESIRED WAVELENGTH, TUNING POWERFUL COHERENT SYNCHROTRON
RADIATION WHICH IS SENT TO A GIANT MIRROR AND BEAMED INTO SPACE. A SMALL FRACTION OF THE
LASER LIGHT IS REFLECTED BACK AND SERVES TO "SEED" THE DENSITY MODULATION OF THE
NEXT ELECTRON BUNCH. ANOTHER FRACTION, SHIFTED TO THE ULTRAVIOLET, IS USED IN AN ELECTRON
GUN TO CREATE NEW BUNCHES OF ELECTRONS.
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