Design Considerations for a High-Efficiency High-Gain Free-Electron Laser for Power Beaming

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Design Considerations for a High-Efficiency
High-Gain Free-Electron Laser for Power Beaming
C. Muller and G. TravishUCLA Department of
Physics Astronomy, Los Angeles CA. USA
The Concept
Comments
The Design
Abstract
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Compression Diffraction
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Prototype Design
Selected initial parameters for study

• Once saturation occurs, the energy is extracted
  linearly
• Diffraction becomes a problem
• Need to maximize extraction efficiency
• Need high peak current

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

• Wavelength
• Good atmospheric transmission
• Good photovoltaic conversion
• Existence of seed laser
• Pick 840 µm
• Undulator
• Want long period so that beam energy is high
• Dont want unwieldy undulator period
• Will need a long undulator
• Will need to taper
• Want high FEL coupling -gt high K
• But, want reasonable magnetic field and large gap
• Optimal focusing lattice
• Pick 6cm period and K3 (0.5 T)
• Beam
• Modest RF photoinjector quality
• High (magnetic) bunch compression

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Opinions on High Power FELs

• 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

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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.
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Conclusions
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GOAL Produce 1 kW electricity in space.
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.
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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.
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

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References
Simulation Optimization
FEL Power Beaming K.-J. Kim, et al., Proc. FEL
Conf. 1997. M. C. Lampel, et al., Rocketdyne
Internal (1993). Laser Space Power http//power
web.grc.nasa.gov/pvsee/publications/lasers/laser_I
ECEC.html G. A. Landis, IEEE Aerospace and
Electronics Systems, Vol. 6 No. 6, pp. 3-7, Nov.
1991. http//powerweb.grc.nasa.gov/pvsee/publica
tions/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/absor
b.html High Power FEL http//www.jlab.org/doug
las/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).

• Key is to maximize FEL efficiency
• But, we dont 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

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Efficiencies
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) lt 64
Solar Panel Conversion lt 50
Atmospheric Transmission 80 (1)
Ground Optics Transmission gt 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
efficiencies
Optimized Results
2.6

• 20m 5 overall taper starting at 12.5m
• 40m 15 overall taper starting at 12.5m

6.7
Efficiencies as high as 13 were achieved, but
with an unrealistically long (150 m) undulator.

• NOTES
• 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.
• The FEL efficiency is to be maximized by
  simulation. 10 was taken as a starting goal.
• We assume a 60 wall plug to RF efficiency and a
  10 RF to beam efficiency.

http//pbpl.physics.ucla.edu/
Work supported by DOE BES grant DE-FG03-98ER45693
Work supported by ONR grant N00014-02-1-0911