Heather Andrews

Effect of reflections and losses in Smith?Purcell free-electron lasers

We have included the effects of losses in the grating surface and reflections at the ends of the grating in the theory of Smith-Purcell free-electron lasers. Computations show that losses typically increase the start current by about 10%. The complex reflection coefficient for the evanescent wave is computed using numerical simulations, and is found to have a magnitude on the order of 30%. This typically increases or decreases the start current by about 10%, depending on the phase of the round-trip reflection.

Three-dimensional theory of the Smith–Purcell free-electron laser with side walls

We present an analytic theory for the exponential-gain (growth) regime of a Smith-Purcell free-electron laser amplifier (oscillator), which includes the effects of transverse diffraction in the optical beam. The optical mode is guided by the electron beam, having a mode width that depends upon the gain length. For the case of a wide electron beam, the dispersion relation converges with that of the 2-D theory. When the electron beam is narrow, the conventional cubic-dispersion relation is replaced by a five-halves dispersion. The dispersive properties of the grating divide device operation into four distinct regions, two amplifier and two oscillator. The number and location of physically allowed roots changes depending on operating region. Additionally, in the narrow-beam case, new challenges arise in satisfying the boundary conditions required for operation as an oscillator

NEEDLE CATHODES FOR HIGH-BRIGHTNESS BEAMS

At the tips of sharp needles, the surface electric field is enhanced by many orders of magnitude. This intensifies thermionic emission and photoemission of electrons through the Schottky effect, and reduces the effect of space charge. The increased current density improves the brightness of electron sources by orders of magnitude. In addition, at very high fields (>109V/m), field emission and photo-field emission produce very high current density. Arrays of needles can be used to achieve high total current.

Compact long wavelength free-electron lasers

The idea of using the Smith-Purcell effect to build a compact (table-top) long wavelength (0.1 -1 mm) free-electron laser is quite old. However, it is only recently that a complete theory for the operation of such devices has been proposed. The current state of the theoretical and experimental efforts to understand these devices will be summarized.