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Design Considerations for a High-Efficiency High-Gain Free-Electron Laser for Power Beaming
C. Muller and G. Travish
UCLA Department of Physics & Astronomy, Los Angeles CA. USA
The Concept
Comments
The Design
Abstract
of re searchers as a m eans of d elivering energy to orbit ing satellites and stations. This paper considers
the use of a seeded high-gain high-efficiency Free-Electron Laser (FEL) amplifi er based on a
conventional linac as the source fo r power beaming. W h ile the wall-plug eff iciency of a single pass
FEL is likely to be considerably lower t han a recirculating system, elect rical
Prototype Design
eff iciency is unlikely to be a serious consideration fo r first-generation
power-beaming syst ems. Moreover, t he simplicity of t he proposed scheme
scales well from existing and completed experiments. I n the end, creating
Wavelength
a high duty-cycle high-brightness electron beam is still a challenge, and
Good atmospheric transmission
the relative merits of a single pass versus Energy-Recovery Linac (ERL),
Good photovoltaic conversion
and a high-gain system versus oscillator, need to be judged technically and
experimentally. This study considers the case of delivering 1 kW (1 kJ) of
Existence of seed laser
opt ical power (energy) to low-eart h orbit from a tapered high-gain FEL.
Pick 840 µm
Various eff iciencies and design consideration are discussed, and an initially
opt imized design obtained through the use of FEL simulations is proposed.
Undulator
P ower beaming from ground-based systems to space-based platforms has been proposed by a numb er
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)

…
 Compression & Diffraction
Selected initial parameters for study
Parameter
Central Wavelength
Once saturation occurs, the energy is extracted linearly
Diffraction becomes a problem
Need to maximize extraction efficiency
Need high peak current
Value
840 nm
Beam Energy
226 MeV
Beam Current
500 A
Beam Emittance (norm. rms)
5 µm
Beam Energy Spread
0.15%
 Opinions on High Power FELs
Qu i c k T i m e ™ a n d a T I F F (U n c o m p re s s e d ) d e c o m p re s s o r a re n e e d e d to s e e th i s p i c t u re .
Beam
Modest RF photoinjector quality
High (magnetic) bunch compression
High rep-rate multi-bunch system
Normal RF — probably L-band
Pick 500 (3.5 nC) A, 5 µm, 1000 bunches, 100 Hz
 GOAL: Produce 1 kW electricity in space.
Seed Laser
Ambitious 1kW average power
Pulse format matches electron beam
 Power Beaming from Ground to Space Using:
High brightness multi-bunch photoinjector
High average power linac
High average power seed laser
Long FEL undulator
Ground based optics
Analysis begins by estimating efficiencies and ground optical power required.
Power beaming assumed efficiencies. The assumptions are based on simplistic
arguments, and are meant only to provide an order-of-magnitude estimate of the energy
requirements.
Parameter
Efficiency
Geometric (Diffraction)
< 64%
Solar Panel Conversion
< 50%
Atmospheric Transmission
Ground Optics Transmission
~ 80% (1)
> 50%
Beam to FEL Conversion
= 10 % (2)
Wall to Beam Conversion
6 % (3)
FEL output to Space Power
12.7 %
Wall plug to Space Power
0.076 %
Undulator Parameter
Focusing (betafunction)
6 cm
3.0
87 cm
 Acknowledgments
The authors thank Professor James Rosenzweig for supporting and encouraging this
work, and Sven Reiche for helping us with Genesis 1.3 as well as holding many fruitful
discussions.
 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 ≈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.
Measured output of a standard silicon solar cell as a
function of incident wavelength [7]. The dashed line
indicates the ideal (unity quantum efficiency) spectral
response.
 References
 Simulation & Optimization
 Efficiencies
Undulator Period
“Wall plug” efficiency is not always that important
Cost of photons vs. cost of electricity is more relevant
Simplicity of single pass accelerator should be considered
100KW class FEL is producible now using existing, tested technology
ERL, recirculation, etc. should be investigated for long term systems
FEL Power Beaming:
K.-J. Kim, et al., Proc. FEL Conf. 1997.
M. C. Lampel, et al., Rocketdyne Internal (1993).
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
Qu ickT ime™ an d a T IFF ( Unc omp resse d) d ecom pres sor a re n eede d to see t his p ictu re.
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).
Optimized Results
efficiencies
2.6%
20m: 5% overall taper starting at
12.5m
6.7%
40m: 15% overall taper starting at
12.5m
Efficiencies as high as 13% were achieved, but with an
unrealistically long (150 m) undulator.
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).
NOTES:
1) It is important to note that while the efficiencies listed are reasonable estimates, the strong effect of
atmospheric turbulence has not been taken into account. Here we assume that techniques such as
adaptive optics can be used to limit the effect of the atmosphere.
2) The FEL efficiency is to be maximized by simulation. 10% was taken as a starting goal.
3) We assume a 60% wall plug to RF efficiency and a 10% RF to beam efficiency.
http://pbpl.physics.ucla.edu/
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Work supported by ONR grant N00014-02-1-0911
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Work supported by DOE BES grant DE-FG03-98ER45693