Improved m fel performance with novel resonator
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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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Improved m fel performance with novel resonator

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.


The thz gap

The THz Gap


Desirable characteristics

Desirable characteristics

  • Broad, continuous tuning,

  • Stability,

  • Sufficient power,

  • CW (narrowband) and pulsed,

  • Simple and economical operation,

  • Small footprint, portable.


M fel schematic

mFEL schematic

SEM

Electron Beam

Polyethylene Window

Detector

THz Radiation

Grating

TPX Lenses

Specimen Chamber


Smith purcell effect

Smith-Purcell effect

“Open” resonator Wide tuning range


Demonstrated tuning measured vs calculated wavelengths

Demonstrated tuningMeasured vs. Calculated wavelengths

Measured wavelengths (microns)

Calculated wavelengths (microns)


Coupling constraint

x

e-Beam

Grating

Coupling constraint

Evanescent field profile

Must optimize net gain.


Improved m fel performance with novel resonator

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.


Typical power from a planar grating

Typical power from a planar grating

Beam: 29 kV, 40 micron waist

Threshold

Beam

Detected power (a.u.)

Beating

Beam current (mA)


Planar horn

Planar Horn

Electron Beam

Mirror surfaces

Planar grating base

Opening angle


Planar horn power for 20 40 90 180 degree opening angles

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.


Grating horn

Grating Horn

Electron Beam

Ruled

surfaces

Opening angle


Grating horn power vs planar grating

Grating Horn power vs. planar grating

Beam: 29 kV, 58 micron waist

Detected power (a.u.)

Beam current (mA)


Other grating horn configurations distinct boundary conditions

Other Grating Horn configurations(Distinct boundary conditions)

Electron beam

(a)

(b)

Grating tooth depth

(c)

(d)

(e)

(f)


Conclusion

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 OfficeNational Science Foundation


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