Particle-in-cell simulation of coherent and superradiant Smith-Purcell radiation

Abstract

This paper presents a study of coherent and superradiant Smith-Purcell (SP) radiation with the help of a two-dimensional particle-in-cell (PIC) simulation. The simulation model supposes a rectangular grating with period length of 173 m to be driven by a single electron bunch, a train of periodic bunches and a continuous beam, respectively. We chose 40 keV as the initial energy of electrons and therefore the SP radiation frequency falls in the THz regime. From our single bunch simulation we distinguish the true SP radiation separated in time from the emission of the evanescent wave. The evanescent wave radiates from both ends of the grating and is characterized by an angle independent frequency lower than the minimum allowed SP frequency. In order to avoid the buildup of beam bunching from an initially continuous beam, we use a train of periodic bunches to excite the grating and observe the superradiant phenomenon. The repetition frequency of the spatially periodic bunches is assumed to be 300 GHz. We find that the superradiant radiation is only emitted at higher harmonics of this frequency and at the corresponding SP angles. This result conforms to the viewpoint of Andrews and co-workers. The simulation with a continuous beam shows the dependence of the output power on the beam current. The power curve shows two regimes, one for the incoherent SP radiation and the other for the superradiance, which resembles the Dartmouth experimental result. And furthermore, the frequency spectrum shows an apparent difference for the two regimes, which is in contrast to the observations of Urata and co-workers.

Small signal gain of a Smith-Purcell free electron laser

“…Recently, SP FEL dynamics has also been investigated using the 2D particle-in-cell (PIC) code [18,19]. The numerical results of [18] in general are in agreement with the analysis of [13,14], but the gain calculated in [18] is within a factor of 2 of that predicted in the work [13].…”

Enhancement of coherent Smith-Purcell radiation at terahertz frequency by optimized grating, prebunched beams, and open cavity

“…In this paper, by using particle-in-cell (PIC) simulations, we identify the optimized grating parameters for minimizing the threshold (starting) current for coherent SPR. Contrary to previous simulation work [28,34], we present the frequency spectrum of both the radiated waves and the beam current, which helps to directly verify the model of Andews et al [22]. A simple analytical dispersion relation is derived for the open grating Smith-Purcell structure, which accurately predicts the operating frequency of the (evanescent surface mode) backward wave oscillation and the consequent coherent (superradiant) harmonic SPR.…”

Interference of surface plasmons and Smith-Purcell emission probed by angle-resolved cathodoluminescence spectroscopy

“…This effect can be explained from classical electrodynamics, describing the electron as a point charge [2][3][4][5]. An interesting extension is observed when the beam is bunched, so that several electrons are placed together within a small region compared with the emitted light wavelength [6][7][8][9][10]. The SPR intensity is then proportional to the squared number of electrons, in contrast to conventional uncorrelated SPR, whose intensity scales linearly with the electron current.…”