AAWSAP DIRD, Laser Lightcraft Nanosatellites, November 2010

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UNCLASSIFIED/;'POR: OPPICIAE USE 014Lf
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K"" 1.5.t
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Electron Beam:
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Photocathode injector creates picosecond electron pulses at 7 MeV. *
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Superconducting accelerator increases electron energy to 100 MeV.
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Peak current: 1100 A.
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Average current: 0.5 A.
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Length: 0.1 mm.
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Radius: 0.1 mm.
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Electron beam recirculated for energy recovery.
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Optical resonator based on short Rayleigh length optical mode (see Figure 32):
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Cavity length (resonator mirror separation distance), 5: 16 m.
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Optical waist (natural mode width), wo: 0.1 mm.
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Mirror radius of curvature, w: 2.6 cm.
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Rayleigh length, zo: 2 cm.
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Optical (Laser Beam) Output:
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Power: 2 MW.
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Wavelength, i: 1 µm (tunable by controlling electron beam, undulator and
resonator properties).
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Short Rayleigh length§ (SRL) optical mode gives several advantages:
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Reduces optical intensity on resonator mirrors to avoid mirror damage.
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Single optical wavefront is amplified giving excellent beam quality.
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FEL interaction is altered with SRL mode.
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SRL intensifies interaction at mode focus.
• Rapidly changing optical amplitude and phase.
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SRL accelerates electron bunching and energy extraction
• Electrons interact with optical radiation field along undulator.
• Electrons "see" intense optical electric field at mode focus.
• Electrons "see" rapidly changing optical radiation phase at mode focus.
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Natural mode width/optical waist (for 5 ~meters & laser beam i ~microns) is
millimeters.
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High-power lasers typically run in multiple transverse modes; however:
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High-power FEL requires short Rayleigh length and small optical waist.
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High-power FEL amplifies single mode without damage to gain medium (a
vacuum).
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FEL efficiency increases as Rayleigh length decreases.
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For megawatt-class FEL system, high operating current plus active mirror
alignment system acts to stabilize optical mode against Naval warship vibrations,
t K = eBrmsAund/2rcmc<, where e is the electron charge, Brm, is the root-mean-square magnetic field strength, Aund
is the undulator period, m is the electron mass, and c is the speed of light.
' MeV = Mega-electron Volt.
§SRL = distance for the area of the beam waist to double.
UNCLASSIFIED'I i'FAA: 061i1Clali.k W&& 8PtLY
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