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.

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.

Thermophotovoltaics (TPV)
(NASA)

The conversion of electromagnetic radiation from thermal (non-solar) sources to electricity is known as thermophotovoltaic (TPV) power generation. It is a concept that has been investigated for three decades, and has been shown to work. The overall thermal-to-electric conversion (TEC) efficiency of TPV systems has typically been lower than predicted, primarily because the broadband output of the thermal source does not match the spectral response of those solar cells that were available.

Recent developments in materials and techniques for shaping the output spectrum of a thermal source, and the ability to fabricate solar cells with tailored spectral responses have changed the situation dramatically. Several rare earth oxides, for example, have been shown to have altered spectral distributions in their emission spectra. The radiation changes from its normal broad band to a narrow line superimposed on a low itensity background. Rare earth oxides used this way are called selective emitters.

The thermal sources can be chemical (ie, a flame) or nuclear (reactors or radioisotopes). There are commercial applications for all three sources, but interest is mainly in the use of combustion sources. All applications, however, will use the same basic selective emitter/filter/solar cell technology, the choice of which will depend only on the temperature range available from the source.

This dual use technology has major implications for terrestrial energy and consumer applications. In the automotive industry, for example, it can eliminate the need for battery storage for electric vehicles, and incorporate the use of external combustion to reduce pollution. TPV can be used as a topping cycle in central utilities for increased fossil fuel plant efficiency, and as a portable source of power for all sorts of consumer applications. The possibility also exists for stand-alone power systems operating on natural gas that individual homeowners could use instead of central utility power.

The major issues at this time are lifetime of the selective emitters, monochromatic solar cell design and fabrication, and system engineering to optimize performance. LeRC holds a patent in the area of thin film selective emitters which needs to be reduced to practice. An alternative approach to using selective emitters is to incorporate multi-element filters to shape the spectrum from the thermal source rather than rare earth exide selective emitters. All the above issues still apply.

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