Distributed feedback surface-wave Smith-Purcell FEL

Smith-Purcell radiation is the emission of electromagnetic radiation by an electron beam passing next to an optical grating. Recent measurement of relatively intense power of such radiation was observed in the THz-regime [1]. To explain the high intensity and the super-linear dependence on current beyond the threshold it was suggested that the radiating device operated in the high gain regime, amplifying spontaneous emission (ASE) [1, 2]. We contest this interpretation and suggest an alternative mechanism. According to our interpretation the device operates as a distributed feedback (DFB) laser oscillator, in which a forward going surface wave, excited by the beam on the grating surface, is coupled to a backward going surface wave by a second order Bragg reflection process. This feedback process produces a saturated oscillator. Another plausible mechanism for explaining the experiment [3, 4] is Backward Wave Oscillation (BWO) with a surface wave, accompanied by second harmonic bunching of the e-beam at saturation, and consequent coherent (superradiant) Smith-Purcell radiation. We present analytical models for the two proposed mechanisms, and compare them. The models predict different emission frequencies and angles. The first model matches better the experimental findings.