To infinity ... and beyond!

Jean Hoffman
Senior Editor

It may sound far-fetched, but the day is coming when lasers will beam power to spacecraft, lunar/Martian rovers, and sherpalike robots that carry payloads up a thin tether into the upper atmosphere and beyond. Although researchers worldwide concur that power-beaming capabilities are decades away, a range of experimental efforts give promise that lasers may, in the not too distant future, provide cheap, safe, and reliable access to space.

LASERS AS CONDUCTORS
The notion of beaming power is not recent. It was the brainchild of Nikola Tesla, one of the most prolific inventors of the 20th century. Although little is known about the mechanism behind his so-called Death Ray, Tesla said the device could focus tremendous amounts of energy into a thin beam so concentrated it would not scatter, even over huge distances. It is uncertain if Tesla ever successfully built a full-scale version but recent advances in lasers, optics, and solar-panel technologies may soon turn his power-beaming vision into more than just a tool of destruction.

Engineers at NASA and the Space Transportation arm of EADS (European Aeronautic Defense and Space Co.), for example, were among the first to successfully power a miniature airplane and rover, respectively, with lasers.

NASA engineers from the Marshall Space Flight Center in Huntsville, Ala., and Dryden Flight Research Center, Edwards, Calif., beamed propulsive power to a propeller-driven model plane from the ground. Dryden engineers built the Mylar film-covered plane from balsa wood and carbon-fiber tubing.

The radio-controlled craft has a 5-ft wingspan and weighs about 10 oz. It is outfitted with a panel of photovoltaic cells optimized to the laser wavelength by a team of participants from the University of Alabama. The photovoltaic cells sit on the fuselage underbelly and convert the laser energy into electricity for the tiny 6-W motor that spins the propeller.

Likewise, engineers at the EADS Space Transportation facility in Bremen, Germany, collaborated with Swiss engineering firm FiveCo, to power a minirover via laser. The 20-cm (8-in.) long vehicle reached speeds of 1.6 cm/sec (0.63 ips) and maneuvered with ease. The laser — 250 m (820 ft) away — power beamed energy to photovoltaic cells at the center of a large silvery disc fixed atop the rover.

The experiment serves EADS's ultimate goal of harnessing the sun as a giant, inexhaustible electric powerplant. Under the Solar Power Initiative (SPI) they propose (within the next 50 yr) putting a generator in geostationary orbit to collect solar energy and direct it in concentrated form at the earth where fields of photovoltaic cells would pick it up.

According to EADS SPI team leader Frank Steinsiek, there are two obvious wireless-transmission options: Laser (top choice at EADS) or microwave (the preferred option of Japanese researchers). According to Steinsiek, EADS prefers laser technology because it requires much smaller orbital structures. Second, laser beams concentrate energy more effectively and better control lateral dispersion over long distances. There are also no negative effects on electronic communications or navigation systems in the vicinity.

One of the main obstacles to address with the minirover was in pointing the laser beam at its target. EADS spent two years tackling the problem in collaboration with the University of Kaiserslautern in Germany. The result is a technology that lets the laser beam transmit position recognition data in addition to energy — similar to carrier frequencies of radio waves. The receiver is correspondingly equipped with sensors that continually determine its position in relation to the transmitter. This helps ensure the photovoltaic cell panel is always positioned at right angles to the laser beam.

B.Y.O.B. —BRING YOUR OWN BEAM
Another option for power beaming is the so-called Space Elevator. Yuri Artsutanov, a Russian engineer, first proposed the Space Elevator in the 1960s. Since then a number of space elevator concepts have been presented. A common one uses a nanocarbon-reinforced composite tether (cable) that will originate from a fixed platform near the equator and end at a counterweight 35,786-km (22,241 miles) above geosynchronous orbit (GEO).

As the earth rotates, inertia from the counterbalance (possibly an asteroid) will work against centripetal force and keep the tether taut. Climbing machines will scale the tether using electricity generated by solar panels and a ground-based booster light beam. The climbing robots will travel at speeds of about 200 km/hr (120 mph), do not undergo accelerations and vibrations, can carry fragile payloads, and have no propellant stored on board. They will be able to release payload in GEO or send it into outer space.

Last October teams from around the world participated in the second NASA Beam Power Challenge. The contest is the first step in evaluating power-beaming propulsion for the climber-bots. The event, part of the NASA Centennial Challenges, seeks novel solutions to NASA mission challenges from nontraditional sources of innovation in academia, industry, and the public. Some of the teams did manage to climb the tether but their power density was too low to do so within time limits. So the $200,000 in prize money has been rolled forward to 2007.

The basic purpose of the Beam Power Challenge is to boost interest (and funding opportunities) for technologies that within decades would enable fabrication of a tether strong enough for climbing machines to carry 5-ton payloads from earth into space. Visionaries predict a space elevator will lessen dependence on conventional rockets and associated fuel and thus reduce the cost to access space a hundredfold.

One Beam Power Challenge team hailed from the University of British Columbia (UBC). Its efforts illustrate the difficulties of building a space elevator. Damir Hot, team captain for Team Snowstar, says the UBC robotic vehicle was outfitted with six large solar-cell modules.

The team's focus was to maximize available power output and drivetrain efficiency. Their effort produced the competition's largest climber. It also had the most solar-cell area and the lowest total mass, a combination that also made the climber the most fragile. The high efficiency and power output was apparent during qualification, says Hot, a senior year physicist at UBC, when the climber smoothly scaled the tether cable using only reflected sunlight from the concrete.

Unfortunately, says Hot, technical glitches along with the fragility and size of the structure kept the team from competing beyond qualifying rounds when they pushed mechanical systems to the limit. The 30-member university team also had difficulties building the large climber around the ribbon within the strict time limits of the contest for each attempted climb.

Contest rules let teams bring their own beam: microwave, spotlight, laser, or other. Team Snowstar had tried beaming power via laser in the lab, but they opted instead to use a 7-kW Xenon arc — lamps that are a commodity in the film industry. The lamps' light spectrum closely matches that needed by the solar cells attached to their climber.

Its wide beam also eased the task of tracking the robot at great distances. This was important; wind gusts during the event caused up to a 6-m (19.7-ft) oscillation in the tether. According to Hot, few if any of the teams were able to test climbers to the competition spec and had little experience with high winds. Anybody using laser-power beaming in next year's contest will likely need to develop a method for ensuring the laser hits the cells even during wind gusts.

Team Snowstar is already in high gear for next October's competition. It hopes to use a stacked diode laser in place of the Xeon arc lamp. "Diode lasers have advantages over CO₂, free electron, and other types of lasers," says George Kamps, Snowstar R&D team lead and a recent UBC physics graduate. First, they are "tunable," and can be made to emit at a desired frequency. Use of tunable lasers lets designers optimize wavelengths to the solar cell or other means of converting energy to electricity.

"The second important factor is laser efficiency. A standard helium-neon laser pointer is less than 1% efficient. CO₂ lasers are better at a few percent," says Kamps. "Diode lasers are currently up to 60% efficient at turning input electricity into laser light. And this translates into vast power savings on the ground."

And finally, says Kamps, there is logistics. "A multikilowatt CO₂ laser system has at least one large fragile glass tube, as well as gas and water lines, a large support structure, and many other cumbersome parts. A multikilowatt diode laser, in contrast, fits in the palm of your hand. Hook up a coolant-in and coolant-out hose, attach positive and negative leads, push a button, and it works."

The main disadvantage for diode lasers in the near term is optics. Often lasers are thought of as a cutting beam of light, explains Kamps. "Most of the time this is true. A bare diode bar, though, doesn't emit light in a beam. It emits light in a wide oval-shaped cone. Collimating this beam is challenging and requires some sophisticated optics." The good news, Kamps says experts in diode laser optics have come up with some solutions.

FUTURE LASER WORK
According to Kamps researchers will need to address three main issues in future powerbeaming efforts that diminish laser power at high altitudes:

Attenuation from the atmosphere: The atmosphere absorbs radiation at several bands in the electromagnetic spectrum. In some frequency ranges it is nearly perfectly transparent, but in others it is nearly opaque. The choice of laser frequency must allow for this effect. Fortunately there are a large number of transparent windows in the atmosphere. The problem becomes one of choosing a laser that works in the best atmospheric window.

Other atmospheric problems stem from too much power going into too narrow a beam. This causes an effect called blooming where the molecules of gas in the atmosphere break down and ionize along the beam path. The ions absorb the laser light and, thus, reduce the power transmitted to the target. Firing the laser in pulses, using a wider beam, or using multiple beams can all solve this problem.

Beam defocusing: Even "perfect" optics cause an effect called diffraction whenever light passes through an aperture such as a lens. This diffraction blurs the beam somewhat and scatters the light. In addition, the atmosphere acts almost like a huge piece of frosted glass. It blurs, bends, and scatters the laser beam as it passes through moving air pockets of different temperature and density. Fortunately, researchers have borrowed an idea from astronomy to counteract this effect.

Astronomers have come up with a system called adaptive optics that corrects for the effects of the atmosphere in real time. A telescope will simultaneously observe both the target star of interest and a nearby bright "guide star." Part of the light in the system goes to a computer that analyzes the guide-star signal and computes the effect of the atmosphere. This analysis is used to stabilize and clarify the image. The computer does this by controlling a series of deformable and movable mirrors. An analogous system could dynamically reshape the laser beam to compensate for atmospheric effects.

Target tracking: Even with a laser beam, it's tough to hit a 10 m² target that sits 60,000 km (37,282 mile) away. The atmosphere will make the climber appear to dance around because of a rippling effect similar to that observed at the surface of a pond. A rock on the bottom will appear to dance if there are ripples on the pond surface. An analogous effect makes stars twinkle in the night sky.

PHOTOVOLTAIC CELL OPTIONS
There are several types of photovoltaic cells under development which could prove useful for power-beaming applications. The 2006 Snowstar climber used monocrystalline silicon cells made by Dutch Solar BV in the Netherlands.

Monocrystalline will continue to be a strong contender for use in the space elevator. Silicon is readily available, and much is known about how to process it. Even if a better option eventually emerges, Kamps contends silicon cells will remain valuable tools in laser system R&D.

Gallium arsenide (GaAs) cells are also attractive both currently and in the future, Kamps continues. "The most efficient solar cells available are called triple-junction solar cells. One of those junctions is made of GaAs. The GaAs absorbs light from a fairly narrow part of the electromagnetic spectrum. It is layered with two other semiconductors, germanium and GaInP₂ (gallium indium phosphate). These other two layers absorb part of the spectrum that the GaAs doesn't absorb well. The triple-junction solar cells can, therefore, draw more of the useful energy out of incident radiation if it is spread over a broad range of frequencies like sunlight."

However, it is more desirable for a laser-based beaming system to have cells tuned to the same narrow band as the beaming laser. "Research shows that it is possible to get cells up to 52% efficient with a laser at 806 nm (the near-infrared part of the spectrum)," says Kamps.

Other interesting technologies use nanocrystals. "The idea is to make very small nanocrystalline semiconductors that serve as tiny solar cells," says Kamps. The technology is still fairly new, he adds, but one of its main advantages is that nanocrystalline cells are "sprayable," so they can be applied like spray paint.

The SnowStar team has also been working on active solar-cell bypassing, a software and electronics-centered solution to the problem of cell shading and the resulting string power loss. Cell shading refers to when some photovoltaic cells get less light than others. Because cells are wired in series, the output of the worst-performing cell, drags down the output of the whole string. "In our climbers so far," says Snowstars' Hot, "we have made extensive use of bypass diodes to protect from this effect. But in the future innovative new solutions such as active bypassing will improve array efficiency. This issue is not likely to be major one in space, but it definitely affects our small climber for the competition."

MAKE CONTACT
Advanced Motion Controls, (805) 389-1935, a-m-c.com
Dutch Solar BV, +31 570 613 329, www.dutchsolar.com
FiveCo, +41 21 693 86 71, fiveco.com
Lumina Power Inc., (978) 532-4666, www.luminapower.com
nLight Corp., (360) 566-4460, nlight.net
UBC Snowstar Space Elevator Team, (604) 763-1468 or [email protected], www.snowstar.ca