1. Improved  FEL performance with novel resonator 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. 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

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.  FEL schematic Specimen Chamber Detector SEM Electron Beam Grating Polyethylene Window TPX Lenses THz Radiation

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

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

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

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

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

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

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

12. Grating Horn Ruled surfaces Electron Beam Opening angle

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

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

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