Superradiant Smith-Purcell Radiation Research Articles

A Terahertz Vortex Beam Emitter With Tunable Topological Charge and Harmonic Excitation

Terahertz (THz) vortex beams carrying orbital angular momentum (OAM) with high purity and tunable topological charge (TC) will undoubtedly bring extraordinary capacities for advanced imaging or communication systems. We propose a convenient and efficient method to generate a tunable and broadband vortex beam that is excited by superradiant Smith-Purcell radiation (SSPR) on a helical grating. This scheme fully explores the advantages of natural broadband evanescent wave carried by the electron beam and the Bloch's theorem regulated helical periodic systems. An explicit relation is established between the index of the spatial harmonic wave on the grating and the topological charge of the vortex beam. The electron energy in the SSPR can be customized to ignite the specific spatial harmonics and manipulate the OAM beam, accordingly. The separated radiation region also promises the high purity of the OAM spectrum. The harmonic excitation reduces the communality between the wavelength and the device size and alleviates the difficulty of device fabrication in the THz band. The proposed vortex scheme can not only be handily scaled to microwave and mid-infrared regions, but also bring possibilities to applications based on compact tunable vortex beam sources.

Superimposed-harmonic Smith-Purcell free-electron lasers driven by periodic electron-bunches

To meet the requirements of high power and broad-tunable terahertz sources, we proposed and investigated a modified Smith-Purcell free-electron laser (SP-FEL) driven by periodic electron-bunches (PEBs). We first designed an electron-gun with a photocathode, which generates a train of PEBs with tunable macro-bunching frequency. These PEBs then interact with surface waves on a grating and are further micro-bunched. The macro-bunching frequency together with its harmonics is superimposed on the micro-bunching one such that the multi-color superradiant Smith-Purcell radiation is achieved at the superimposed harmonics. By adjusting the macro-bunching frequency, the superradiant frequency can be tuned from 0.5 to 1 THz—a region hard to reach by conventional SP-FELs and vacuum electron devices. Compared to DC-beams, the PEBs have higher peak current density and generate higher peak power. Thus, the proposed scheme can be promisingly developed as high power and broad-tunable terahertz sources.

Super-radiant Smith–Purcell radiation from periodic line charges

Smith–Purcell radiation occurs when an electron passes close to the surface of a metallic grating. The radiation becomes coherent when the length of the electron bunch is smaller than the wavelength of the radiation. A train of periodic bunches can enhance the spectral intensity by changing the angular and spectral distribution of the radiation. This is called super-radiant Smith–Purcell radiation, and has been observed in experiments and particle-in-cell simulations. In this paper, we introduce a new method to study this effect by calculating the reflected waves of an incident evanescent wave from periodic line charges. The reflection coefficients are numerically computed, and the spectral distributions of the super-radiant radiation are demonstrated. These analytical results are in agreement with those obtained through part-in-cell simulations.

Output of Super-radiant Smith-Purcell radiation: A theoretical analysis

A two-section grating system for Smith-Purcell free-electron lasers is investigated. The system is composed of two open gratings with different geometrical parameters. It is expected that the originally continuous electron beam is bunched in the first section by interaction with the evanescent wave near the grating surface, and enhanced radiation can be extracted at the second section in the form of super-radiant Smith-Purcell radiation at a designed angle. With the help of particle-in-cell simulation, the electron beam bunching and directed output of the fundamental radiation and higher order harmonics are demonstrated.

Small-signal theory of a grating-based free-electron laser in three dimensions

We present an analytic theory for the small-signal operation of a grating-based free-electron laser that includes the effects of transverse diffraction on the evanescent wave. In this device, the electron beam interacts with an evanescent wave of the grating that bunches the beam and creates superradiant Smith-Purcell radiation. We find that the evanescent wave is guided by the electron beam, giving an optical-mode width that depends on the gain. We consider the cases of very wide and very narrow electron beams. For a wide electron beam, the cubic dispersion relation previously found for slow-wave structures is recovered. When the electron beam is narrow, so that gain guiding is important, a fifth-order dispersion relation is found instead. Diffraction in a system where the group velocity is very different (sometimes negative) from the phase velocity leads to unexpected results. The Brillouin zone subdivides into four regions; only two physically allowed (gain-guided) roots are obtained in the regions near the center of the Brillouin zone, but three are found in the regions away from the center. In the left half of the Brillouin zone, corresponding to high electron energy, the device operates on a convective instability, as an amplifier. In the right half of the Brillouin zone, where the group velocity is negative, the device operates on an absolute instability, as an oscillator. In the region where only two guided modes exist, oscillator operation will be more difficult.

Enhanced Superradiant Smith–Purcell Radiation in a Three-Mirror Quasi-Optical Cavity

Smith-Purcell (SP) radiation has been widely used for the generation of radiation from microwave to optical frequencies, and it has recently become an attractive candidate for a Terahertz (THz) radiation source. Superradiant SP radiation has sharp angular and spectral distributions due to the bunching of the electron beams, and a three-mirror quasi-optical cavity (TMQOC) which has selectivity at both angles and frequency has been designed to enhance this process. The radiation power over 700 W/m at the 99.4 GHz are obtained through using a 45-keV 4-A/m beam skip over a metal grating with period, width, and depth are 1.6, 1.0, and 0.8 mm, respectively. This results show that the TMQOC can greatly enhance the superradiant SP radiation, and one may utilize this effect to design a compact, tunable, and powerful THz radiation source working at room temperature.

Development of High-Current Sheet Beam Cathodes for Terahertz Sources

Generation of a sheet beam directly from a scandia-doped dispenser cathode has been investigated and optimized by means of beam-profile simulations and measurements. Rectangular beams 600 mum wide and 100 mum thick with current densities over 50 A/cm2 have been generated. The beams are stable for at least several hundred hours with the cathodes operating at 950degCb, making them promising candidates for application in the next generation of terahertz vacuum electron devices such as a 0.5-THz superradiant Smith-Purcell radiation source.

Smith–Purcell Radiation with Three-Dimensional Simulation

An investigation of Smith–Purcell radiation is carried out with a three-dimensional particle-in-cell simulation. The simulation model consists of a rectangular grating with limited length and width, which is driven by a single electron bunch, a train of periodic bunches, and a continuous beam. Smith–Purcell radiation is observed and distinguished from an evanescent wave, which travels near the surface of the grating. We found that the evanescent wave has a frequency lower than the allowed minimum Smith–Purcell frequency, and does not radiate until it reaches the ends of the grating. It is also shown that superradiant radiation can be excited by periodic electron bunches. With the present parameters, superradiant radiations are emitted at higher harmonics of the bunch frequency and at corresponding Smith–Purcell angles. The initially continuous beam can be bunched by the interaction with the evanescent wave when the beam current is above a certain threshold, and consequently induces the superradiant Smith–Purcell radiation. The threshold current is derived as well.

Three-dimensional simulation of super-radiant Smith-Purcell radiation

A simulation of coherent and super-radiant Smith-Purcell radiation is performed in the gigahertz regime using a three-dimensional particle-in-cell code. The simulation model supposes a rectangular grating to be driven by a single electron bunch and a train of periodic bunches, respectively. The true Smith-Purcell radiation is distinguished from the evanescent wave, which has an angle independent frequency lower than the minimum allowed Smith-Purcell frequency. We also find that the super-radiant radiations excited by periodic bunches are emitted at higher harmonics of the bunching frequency and at the corresponding Smith-Purcell angles.

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

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 $173\text{ }\ensuremath{\mu}\mathrm{m}$ 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.

Superradiant emission of Smith-Purcell radiation

Smith-Purcell (SP) radiation is emitted when an electron passes close to the surface of a metallic grating. The radiation becomes coherent (fluence proportional to the square of the number of electrons) when the electrons are in bunches whose dimensions are smaller than the wavelength of the radiation. This has been observed in experiments in which the electrons are prebunched by an rf linac. The enhancement of the spectral intensity is accompanied by large changes in the angular and spectral distribution of the radiation when the electrons appear in periodic bunches. This is called superradiance. Recently, superradiant SP radiation has been observed from a so-called Smith-Purcell free-electron laser (SP-FEL) in which the electrons are bunched by the lasing process. As in other slow-wave structures, the electron beam in a SP-FEL interacts with an evanescent wave for which the phase velocity matches the electron velocity and amplifies it. The frequency of this wave lies below the range of SP radiation and the wave is not radiated except from the ends of the grating. However, the bunching of the electrons by the interaction with the evanescent wave enhances the ordinary Smith-Purcell radiation and changes the angular and spectral distribution due to superradiant effects. In this article, we introduce a new method for computing the SP radiation in three dimensions, including the effects of finite grating length and superradiance due to periodic electron bunching at an arbitrary frequency. We show that the SP radiation develops spectrally and angularly narrow peaks at the harmonics of the bunching frequency. In rf linacs, where the bunches are widely spaced, several closely spaced harmonics lie under the spectral envelope of the emission from a single electron. In a SP-FEL the harmonics are widely spaced and the SP radiation appears in narrow cones at the SP angles corresponding to the harmonics of the bunching frequency. Finally, we calculate the angular spectral fluence radiated by an electron passing over a lamellar grating of finite length, examine its coherent enhancement in SP-FELs and rf linacs, and compare the results with numerical simulations and available experimental data.