FREE-ELECTRON LASERS: Upgrade supports space-laser energy initiative

In October, a House-Senate conference committee approved a $10 billion defense budget to upgrade the Free Electron Laser (FEL) at the US Department of Energy Thomas Jefferson National Accelerator Facility (Jefferson Lab; Newport News, VA).

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In October, a House-Senate conference committee approved a $10 billion defense budget to upgrade the Free Electron Laser (FEL) at the US Department of Energy Thomas Jefferson National Accelerator Facility (Jefferson Lab; Newport News, VA). Among researchers interested in using an FEL to beam power from the ground to orbiting satellites, the budget recommendation was no doubt good news.

A Jefferson Lab announcement last summer that they had experimentally demonstrated 1.7 kW of average power output from their 1-kW FEL served as a hopeful sign for Alexander Zholents, a physicist at Lawrence Berkeley National Laboratory (LBNL; Berkeley, CA). He said the Jefferson Lab experiment showed that FEL power can be increased predictably and controllably by upgrading design specifications.

If the compromise defense budget is ultimately approved as is by Congress and the President, the $10 million FEL item will provide two-thirds of the funds needed to upgrade the Jefferson Lab FEL to a 10-kW output. A lot more than 10 kW will be needed to power satellites in space, but going from 1 kW to 10 kW might be thought of as a giant step in the right direction.

Last spring, researchers at LBNL completed a feasibility study for an FEL system that, in combination with other existing technologies, could power communications satellites in geosynchronous orbit more efficiently than such satellites are now powered by solar radiation. The design is expected to boost the 5-kW power levels of standard communications satellites by almost an order of magnitude. Actually assembling all of this technology into a working system, however, will require strong commercial interest and broad industry support for the space laser energy (SELENE) initiative-whose acronym shares its name with the Greek goddess of the moon.

The researchers have designed an ignition feedback regenerative amplifier (IFRA) that they say could provide the world's most powerful FEL at 200 kW of average power. The Berkeley IFRA design seeks to double the peak-power performance of a proposed 100-kW FEL at the Budker Institute of Nuclear Physics (Novosibirsk, Russia) using 476-MHz RF cavities designed at LBNL for the Stanford Linear Accelerator Center (SLAC; Palo Alto, CA), according to Zholents.

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PHOTO. The IFRA will send a 200-kW signal at 0.84 µm to a communications satellite through a projection adaptive optics telescope. A small fraction of the laser light is fed back to the IFRA to seed density modulation of the next electron bunch and to create new bunches of electrons.

In the IFRA layout, an 80-MeV linear accelerator will accelerate electrons in the front stretch of a 100-m-long racetrack, while special high-order-mode loads of the RF cavities absorb beam-induced electromagnetic fields to provide a clean interaction between the electron beam and the accelerating field (see figure). Then undulator magnets in the backstretch will modulate electron density in the accelerated beam, thereby tuning the coherent synchrotron radiation to the desired wavelength of 0.84 µm. The RF cavities will also recoup most of the original accelerating energy as the returning electron beam decelerates.

Of course the next step is to actually transmit the beam to a satellite. Harold Bennett, president of Bennett Optical Research (Ridgecrest, CA), hopes to accomplish this using a 12-m compound mirror with adaptive optics, also based on existing technology used in astronomical telescopes. In fact the LBNL feasibility study that led to the IFRA design originally grew out of a request from Bennett to evaluate the Budker laser as a power source for the project, Zholents said.

Bennett argues that a single 40-kW laser-powered satellite could take the place of six to eight conventional 5-kW solar-powered satellites and save "over half a billion dollars." He also estimates the total project cost of $400 million as roughly equivalent to "the cost of one large geosynchronous satellite."

Such arguments will have to find their mark in terms of significant and specific commercial goals and benefits before the work done so far at Berkeley moves from the feasibility-study phase to the work-in-progress phase, Zholents said. He estimates that even in the best of scenarios-abundant financial backing ($100 to $150 million for the IFRA portion alone) and several laboratories working in parallel on different aspects of the IFRA—the project would take at least a year to organize and another four to actually complete. Simultaneous efforts and additional cost would also be required for the optical-transmission system as well as the design and launch of satellites that would be compatible with a laser-power transmission system, he said.

Hassaun Jones-Bey

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