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 The Design Comments Work supported by DOE BES grant DE-FG03-98ER45693 ParameterEfficiency Geometric (Diffraction)< 64% Solar Panel Conversion< 50% Atmospheric Transmission~ 80% (1) Ground Optics Transmission> 50% Beam to FEL Conversion= 10 % (2) Wall to Beam Conversion6 % (3) FEL output to Space Power12.7 % Wall plug to Space Power0.076 % Abstract 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. 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. 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 Efficiencies   Analysis begins by estimating efficiencies and ground optical power required. GOAL: Produce 1 kW electricity in space.   Prototype Design ParameterValue Central Wavelength840 nm Beam Energy226 MeV Beam Current500 A Beam Emittance (norm. rms)5 µm Beam Energy Spread0.15% Undulator Period6 cm Undulator Parameter3.0 Focusing (betafunction)87 cm Selected initial parameters for study 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 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  Simulation & Optimization 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 FEL Power Beaming: K.-J. Kim, et al., Proc. FEL Conf M. C. Lampel, et al., Rocketdyne Internal (1993). Laser Space Power: G. A. Landis, IEEE Aerospace and Electronics Systems, Vol. 6 No. 6, pp. 3-7, Nov G. A. Landis, Acta Astronautica, Vol. 25 No. 4, pp (1991) Microwave Beaming: J. Benford and R. Dickinson, Intense Microwave Pulses III, H. Brandt, Ed.,SPIE 2557, 179 (1995). P. Glaser, Science, , pp (1968). Atmospheric Absorption: High Power FEL: D. Douglas, Proc. LINAC 2000 Tapering: Genesis 1.3: S. Reiche, NIM A429, 243 (1999).  Optimized Results  20m: 5% overall taper starting at 12.5m  40m: 15% overall taper starting at 12.5m Efficiencies as high as 13% were achieved, but with an unrealistically long (150 m) undulator. 2.6% 6.7% efficiencies 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. Conclusions  Compression & Diffraction Once saturation occurs, the energy is extracted linearly Diffraction becomes a problem Need to maximize extraction efficiency Need high peak current 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. Work supported by ONR grant N ……   