Power Beaming using FELs

Power beaming from ground-based systems to space-based platforms has been proposed by a number of researchers as a means of delivering energy to orbiting satellites and stations. Our work considers the use of a seeded high-gain high-efficiency Free-Electron Laser (FEL) amplifier based on a conventional linac as the source for power beaming. While the wall-plug efficiency of a single pass FEL is likely to be considerably lower than a recirculating system, electrical efficiency is unlikely to be a serious consideration for first-generation power-beaming systems. Moreover, the simplicity of the proposed scheme scales well from existing and completed experiments.

The concept of power beaming has been around since the 1960’s but technical and economic hurdles prevented implementation [ ]. The Free Electron Laser has been examined as a possible source for ground-based beaming already. This work takes advantage of recent progress in photoinjectors and high average-power lasers. We consider a system capable of delivering 1 KW of electrical power to a platform in geo-stationary orbit.

Table: Selected initial parameters for study

Parameter	Value
Central Wavelength	840 nm
Beam Energy	226 MeV
Beam Current	500 A
Beam Emittance (norm. rms)	5 µm
Beam Energy Spread	0.15%
Undulator Period	6 cm
Undulator Parameter	3.0
Focusing (betafunction)	87 cm

Study Considerations:

Wavelength

• Want good atmospheric transmission
• Want good photovoltaic conversion
• Need the existence of a seed laser
• Pick 840 µm

Figure: Measured output of a standard silicon solar cell as a function of incident wavelength . The dashed line indicates the ideal (unity quantum efficiency) spectral response.

Undulator

• Want long period so that beam energy is high
• Don’t want unwieldy undulator period
• Will need a long undulator
• Will need to taper
• Want high FEL coupling -> high K
• But, want reasonable magnetic field and large gap
• Optimal focusing lattice
• Pick 6cm period and K=3 (~0.5 T)

Beam

• Restrict design to modest RF photoinjector quality
• Assume high (magnetic) bunch compression
• Assume high rep-rate multi-bunch system
• Assume normal RF — probably L-band
• Pick 500 (3.5 nC) A, 5 µm, 1000 bunches, 100 Hz

Seed Laser

• Allow for ambitious 1kW average power
• Assume pulse format matches electron beam

Simulations:

We used Genesis 1.3 to simulate the FEL process in 3D including important effects such as diffraction and slippage.

• Key is to maximize FEL efficiency
• But, we don’t worry about ?wall plug? efficiency
• Assume perfect seed laser
• Assume optical (smooth) focusing
• Assume well compressed beam
• Use 3D FEL code Genesis 1.3
• Vary tapering gradient and taper start

Results

20m undulator

• 5% overall taper starting at 12.5m
• 2.6% efficiency

40m undulator

• 15% overall taper starting at 12.5m
• 6.7% efficiency

Efficiencies as high as 13% were achieved, but with an unrealistically long (150 m) undulator. One can assume a denser beam, with peak currents of 1 - 2kA, and achieve much higher efficiencies.

Conclusions:

Optimization of a high-gain FEL yielded a system capable of producing 1 KW of electric power in space using a 40 m undulator and a &Mac197;100 KW electron beam. This design relies on improvements to photoinjectors and lasers that may allow for high repetition-rate, high-brightness beam production and for high-power seeding of the FEL.

A paper was published on this study and is available for download:

C. Muller and G. Travish, Design Considerations for a High-Efficiency High-Gain Free-Electron Laser for Power Beaming, Proc. FEL 2003 (to be published).

A poster presented at the same conference (FEL2003) is also available as a PowerPoint slide.

Selected References:

FEL Power Beaming:

• K.-J. Kim, et al., Proc. FEL Conf. 1997.
• M. C. Lampel, et al., Rocketdyne Internal (1993).

Laser Space Power:

• http://powerweb.grc.nasa.gov/pvsee/publications/lasers/laser_IECEC.html
• G. A. Landis, IEEE Aerospace and Electronics Systems, Vol. 6 No. 6, pp. 3-7, Nov. 1991.
• http://powerweb.grc.nasa.gov/pvsee/publications/lasers/IAF90_053.html
• G. A. Landis, Acta Astronautica , Vol. 25 No. 4, pp. 229-233 (1991)

Microwave Beaming:

• http://home.earthlink.net/~jbenford/BenfordDickinson_Pwr_Beam.pdf
• J. Benford and R. Dickinson, Intense Microwave Pulses III, H. Brandt, Ed.,SPIE 2557, 179 (1995).
• P. Glaser, Science, 162 3856, pp 857-861 (1968).

Atmospheric Absorption:

• http://orbit-net.nesdis.noaa.gov/arad/fpdt/tutorial/absorb.html

High Power FEL:

• http://www.jlab.org/~douglas/FELupgrade/talks/TH204/
• D. Douglas, Proc. LINAC 2000

Tapering:

• http://linac.ikp.physik.tu-darmstadt.de/fel/tapering.html

Genesis 1.3:

• S. Reiche, NIM A429, 243 (1999).