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Improved m FEL performance with novel resonator

Improved m FEL performance with novel resonator. J.H. Brownell, A. Bakhtyari, H.L. Andrews, I.J. Owens Department of Physics and Astronomy, Dartmouth College, Hanover, NH USA M.F. Kimmitt Physics Centre, University of Essex, Colchester CO4 3SQ, UK.

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Improved m FEL performance with novel resonator

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  1. Improved mFEL performance with novel resonator J.H. Brownell, A. Bakhtyari, H.L. Andrews, I.J. Owens Department of Physics and Astronomy, Dartmouth College, Hanover, NH USA M.F. Kimmitt Physics Centre, University of Essex, Colchester CO4 3SQ, UK The miniature free electron laser under development at Dartmouth College is a benchtop device designed to produce coherent, tunable radiation over the entire terahertz spectral range. We will report on a novel resonator design which significantly enhances the output intensity without limiting the tuning range of the device.

  2. The THz Gap

  3. Desirable characteristics • Broad, continuous tuning, • Stability, • Sufficient power, • CW (narrowband) and pulsed, • Simple and economical operation, • Small footprint, portable.

  4. mFEL schematic SEM Electron Beam Polyethylene Window Detector THz Radiation Grating TPX Lenses Specimen Chamber

  5. Smith-Purcell effect “Open” resonator Wide tuning range

  6. Demonstrated tuningMeasured vs. Calculated wavelengths Measured wavelengths (microns) Calculated wavelengths (microns)

  7. x e-Beam Grating Coupling constraint Evanescent field profile Must optimize net gain.

  8. Gain Newton’s eq. Feedback Maxwell’s eqs. Loss & SP signal • “Closed” resonator: • Increases gain by constraining • Reduces loss • BUT limits tuning! Try partial closure.

  9. Typical power from a planar grating Beam: 29 kV, 40 micron waist Threshold Beam Detected power (a.u.) Beating Beam current (mA)

  10. Planar Horn Electron Beam Mirror surfaces Planar grating base Opening angle

  11. Planar Horn powerfor 20, 40, 90, 180 degree opening angles Beam: 29 kV, 50 micron waist Opening angle = Detected power (a.u.) Beam current (mA) Conforms to theory.

  12. Grating Horn Electron Beam Ruled surfaces Opening angle

  13. Grating Horn power vs. planar grating Beam: 29 kV, 58 micron waist Detected power (a.u.) Beam current (mA)

  14. Other Grating Horn configurations(Distinct boundary conditions) Electron beam (a) (b) Grating tooth depth (c) (d) (e) (f)

  15. Conclusion • Intensity is magnified by Planar Horn, and even more by Grating Horn, • Gain is increased by Grating Horn, • High spontaneous signal suggests SP-FEL operates in a fundamentally different way with the Grating Horn, • Many configurations to test for optimum performance. Support: Army Research Office National Science Foundation

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