Smith-Purcell Free-electron Laser Research Articles

Improved Smith–Purcell free-electron laser based on quasi-bound states in the continuum

In this article, a method to enable efficient emission of coherent radiation by using an intense electron beam coupled with a quasi-bound state in the continuum (quasi-BIC) is investigated. We present an analytical solution providing an intuitive round-trip phase condition to explain the origin of quasi-BICs of dielectric gratings. Numerical study of the beam–wave interaction shows that the electrons can be bunched by the synchronous space harmonic enhanced by the quasi-BIC, resulting in self-excited coherent oscillation and consequently efficient Smith–Purcell radiation. This work presents an interesting solution for coherent radiation sources, and may find application in communications and physics.

Coherent terahertz Smith–Purcell free-electron laser with possible quantum effects

A quantum approach is proposed to describe the Smith–Purcell free-electron laser (SP–FEL) where the interacting electron is described quantum mechanically as a wave packet with a finite width. A quantum regime for the SP–FEL operation is newly defined when the electron wave packet width is larger or comparable to the interaction length. This quantum regime can be realized in SP–FELs in periodic nanostructures enabled by current nanofabrication advances. In this quantum regime, the gain linewidth is determined by the longitudinal size of the electron wave packet. The latter result can be used in measuring the spreading length of a free-electron quantum wave packet. It is found that as the width of the electron wave packet increases, the coherence of the emitted radiation is enhanced. Finally, whether in the classical or quantum regime, an analytical expression for the gain in a SP–FEL is derived. Then we propose an efficient scheme for a SP–FEL amplifier to generate terahertz radiation.

Hybrid (Oscillator-Amplifier) Free Electron Laser and New Proposals

The present work analyses a hybrid free electron laser (FEL) scheme where the oscillator is based on a radiation source operating with a slow-wave guiding structure as, for instance, a Cerenkov FEL or a Smith–Purcell FEL. Such devices, often running in transverse magnetic (TM) modes, present a longitudinal electric field which can easily affect the longitudinal electrons’ velocities, inducing an energy modulation on the beam. Such a modulation, properly controlled, can induce a strong radiation emission in a magnetic undulator properly designed to operate as a radiator. General considerations will be exposed together with a practical numerical example in the far infrared region of the spectrum.

Novel radiation sources using relativistic electrons - Applications

Novel sources of radiation using relativistic electrons in periodic structures are being briefly discussed based on different mechanisms and producing radiation of very different characteristic.In this article we will focus among others on the Smith-Purcell effect and the many efforts made since their discovery 55 years ago. Especially Smith-Purcell (SP) free- electron lasers (FELs) using low energy electron beam are being seen as attractive option for a compact source of coherent terahertz radiation. The latter refers to the production of radiation in the THz gap region associated with many applications including very promi- sing and interesting results on imaging techniques in Medicine as well as applications in Biology, Molecular Physics etc. We will present calculations of Radiation Factors based on the Smith-Purcell radiation production with very interesting characteristics and discuss the possibility of calculating and designing a complete system of coherent THz radiation production based on a Smith-Purcell FEL.

基于T形光栅的史密斯-珀塞尔自由电子激光器的研究

基于T形光栅的史密斯-珀塞尔自由电子激光器的研究

对史密斯-帕塞尔自由电子激光光栅的研究

对史密斯-帕塞尔自由电子激光光栅的研究

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.

Development of a Multialkali Photocathode Dc Gun for a Smith-Purcell Terahertz Free-Electron Laser

We have developed a photocathode dc gun for a compact Smith-Purcell free-electron laser in the terahertz wavelength region. The gun system consists of an alkali antimonide photocathode preparation chamber, a dc gun with a 250 kV-50 mA Cockcroft-Walton high-voltage power supply, and a downstream beamline with a water-cooled beam dump to accommodate a beam power of 5 kW. We fabricated a Cs3Sb photocathode with quantum efficiency of 5.8% at a wavelength of 532 nm and generated a 150 keV beam with current of up to 4.3 mA with a 500 mW laser. A vacuum chamber for the Smith-Purcell free-electron laser has been installed in the downstream beamline. We describe the present status of our work.

Coherent radiation at the fundamental frequency by a Smith-Purcell free-electron laser with dielectric substrate

This letter reports an approach to realize coherent radiation at the fundamental frequency of the wave that is resonant with the electron beam in a Smith-Purcell (SP) free-electron laser (FEL). We found that a two-dimensional grating structure equipped with a dielectric substrate could provide an electromagnetic mode which shows the property of a surface wave in the vacuum region where the beam-wave interaction occurs, and operates as a radiative wave in the dielectric region. Thus, an electron beam holding medium energy resonant with the mode could radiate at the fundamental frequency, which is different from the typical Smith-Purcell free-electron laser that radiates only at harmonics of the resonant frequency.

Radiation at 100 and 200 GHz From a Compact Planar Smith–Purcell Free-Electron Laser

In a recent experiment at 5 GHz, a Smith–Purcell free-electron laser equipped with lateral sidewalls emitted copious radiation at the fundamental frequency of the evanescent surface wave with 10% interaction efficiency. This was estimated using only the useful beam power, not the full beam power delivered by the generator. Theory shows that a rescaling of all grating parameters at constant energy leads to a rescaled wavelength for the emitted radiation. In this paper, we present the results of an experiment performed at CESTA at 100 GHz with a 1-mm period planar grating. The beam energy was 60 keV, beam current from 2 to 5 A. We estimated the output power at 100 GHz of about 200 W (for a 4-A beam), corresponding to an efficiency of 0.08%. Comparisons with the predictions from simulations are also included. The results of simulations to extend these results to higher frequencies are presented. This paper follows previous studies recently published in this journal with a cylindrical grating operating in the same range of frequency.

Three-dimensional analysis of the surface mode supported in Čerenkov and Smith-Purcell free-electron lasers

In \v{C}erenkov and Smith-Purcell free-electron lasers (FELs), a resonant interaction between the electron beam and the co-propagating surface mode can produce copious amount of coherent terahertz (THz) radiation. We perform a three-dimensional (3D) analysis of the surface mode, taking the effect of attenuation into account, and set up 3D Maxwell-Lorentz equations for both these systems. Based on this analysis, we determine the requirements on the electron beam parameters, i.e., beam emittance, beam size and beam current for the successful operation of a \v{C}erenkov FEL.

A Compact THz Source: 100/200 GHz Operation of a Cylindrical Smith–Purcell Free-Electron Laser

We report first operation in the terahertz regime of a cylindrical grating Smith–Purcell free-electron laser. Propagation of an annular electron beam in proximity to a cylindrical grating causes strong electron bunching due to a beam-surface wave interaction. Electromagnetic radiation results from the bunching (fundamental) and, at bunch harmonics, the Smith–Purcell effect. In the experiment, over 2.5 kW was generated at 100 GHz (fundamental) and over 100 W at 200 GHz (Smith–Purcell). The results illustrate the potential of this configuration for generation of high-power terahertz radiation.

基于带状束的史密斯-帕塞尔自由电子激光模拟与分析

基于带状束的史密斯-帕塞尔自由电子激光模拟与分析

First Lasing From a High-Power Cylindrical Grating Smith–Purcell Device

Over the past year, a collaborative effort between Advanced Energy Systems and Commissariat a l’Energie Atomique/Centre d’Etudes Scientifiques et Techniques d’Aquitaine has achieved the first lasing of a cylindrical Smith–Purcell free-electron laser (CSPFEL). These proof-of-principle experiments employed an 80-keV 45-A 200-ns full-width at half-maximum thin annular electron beam propagating just above a cylindrical grating. The interaction of the beam with the evanescent grating mode resulted in strong beam bunching and generation of high-power millimeter-wave radiation. First lasing of the CSPFEL was achieved with a grating period of 2 cm (5-GHz fundamental). Further experiments were performed with a 6-mm period grating (15-GHz fundamental) yielding 10 kW at the second harmonic, 30 GHz. This paper discusses only the second experiment. Comparison of the experimental results with particle-in-cell (PIC) simulations shows good agreement. An experiment at 100 GHz is in progress. The basic properties of Smith–Purcell radiation and the evanescent grating mode will be presented first. Then the experimental configuration and results will be discussed. The next section will cover the PIC simulations and comparison with the experiment. The final section will summarize the results and discuss the future directions for this research. The long-term goal is the development of compact terahertz devices with sufficient power to address applications that are impractical or impossible with the existing sources.

A quantum mechanical analysis of Smith–Purcell free-electron lasers

The paper presents a quantum mechanical treatment for analyzing the Smith–Purcell radiation generated by charged particles passing over a periodic conducting structure. In our theoretical model, the electrons interact with a surface harmonic wave excited near the diffraction grating when the electron velocity is almost equal to the phase velocity of the surface wave. Then, the surface harmonic wave is electromagnetically coupled to a radiation mode. The dynamics of electrons are analyzed quantum mechanically where the electron is represented as a traveling electron wave with a finite spreading length. The conversion of the surface wave into a propagating mode is analyzed using the classical Maxwell׳s equations. In the small-signal gain regime, closed-form expressions for the contributions of the stimulated and spontaneous emissions to the evolution of the surface wave are derived. The inclusion of the spreading length of the electron wave to the emission spectral line is investigated. Finally, we compare our results based on the quantum mechanical description of electron and those based on the classical approach where a good agreement is confirmed.

Three-dimensional theory of Smith-Purcell free-electron laser with dielectric loaded grating

A dielectric loaded rectangular grating for Smith-Purcell devices is proposed in this paper. Regarding the electron beam as a moving plasma dielectric, a three dimensional (3D) linear theory of beam-wave interaction is developed. The first and second order growth rates are calculated, which are obtained by expanding hot dispersion equation at synchronous point. The results show that the cutoff frequency is affected by grating width. The dispersion curve becomes flatter and shifts towards lower frequency by loading dielectric in grooves. The simulation results, which are obtained by a 3D particle-in-cell code, are in good agreement with theoretical calculations. Compared the first and second order growth rate, it shows that the discrepancy is large when beam parameters are selected with high values. In this case, it is necessary to apply the second order growth rate, which can accurately describe the process of beam-wave interaction.

The Output Characteristics of Smith-Purcell Free Electron Laser Based on an All-Feedback Resonator

We investigate the output characteristics of Smith-Purcell free electron laser based on an all-feedback resonator by theoretic analysis and particle-in-cell simulation method. The simulation results show that the output power spectrum density is different with different azimuthal angle range in this device. The output power spectrum density on azimuthal angle curve is a nonlinear curve. This output characteristics is important to design the port of this device.

Smith–Purcell free electron laser based on a multilayer metal-dielectric stack

We investigate a novel Smith–Purcell free electron laser composed of an electron gun, a quasi optical resonator, a multilayer metal-dielectric stack and collector. The characteristics of this device are studied by theoretical analysis and particle-in-cell simulation method. In this device, tunable coherent radiation with a high output peak power at millimeter wavelengths can be generated. The Smith–Purcell free electron laser compared with multilayer metal-dielectric stack is a promising substitute for developing a more compact, higher output peak power Smith–Purcell free electron laser compared with metallic reflecting grating.

Growth rate and start current in Smith–Purcell free-electron lasers

This letter reports a theory to calculate the growth rate and start current of a Smith–Purcell free-electron laser, which is a promising radiation source in the terahertz domain. A two-dimensional model was used to investigate the interaction between a sheet electron beam and the surface wave above a lamellar grating. After deriving the growth rate from the dispersion equation, the start current was carefully estimated by considering the power flow above the grating. The agreement between the predictions of our theory and the results from the particle-in-cell simulations is acceptable.

Observation of copious emission at the fundamental frequency by a Smith-Purcell free-electron laser with sidewalls

An experiment at microwave frequencies confirms the recent prediction that a Smith-Purcell [S. J. Smith and E. M. Purcell, Phys. Rev. 92, 1069 (1953)] free-electron laser equipped with sidewalls can emit radiation at the frequency of the surface wave. The power output is considerably greater than for the previously observed emission at the second harmonic, in agreement with three-dimensional simulations. The dependence of frequency on beam energy and emission angle is in good agreement with three-dimensional theory and simulations. Provided that a reduction in scale can be achieved, a path is open to coherent Smith-Purcell radiation at terahertz frequency.