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

1. 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  Compression & Diffraction  … 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 • Wavelength • Good atmospheric transmission • Good photovoltaic conversion • Existence of seed laser • Pick 840 µm • 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 • 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 • Seed Laser • Ambitious 1kW average power • Pulse format matches electron beam  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  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  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.  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   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://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). • 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  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. 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