Coherent Smith-Purcell Radiation Research Articles

Study of Coherent Smith–Purcell Radiation in the Terahertz Region Using Ultra-Short Electron Bunches

Smith–Purcell radiation (SPR) can be generated nondestructively, providing valuable applications in light sources and beam monitors. Coherent SPR is expected to enable single-shot measurements of very short bunch lengths on the fs scale. Since the reconstruction of the longitudinal bunch shape from the coherent SPR is based on the reliable SPR spectrum, a more detailed understanding of the properties of the radiation is important in this context. Employing a 100 fs ultrashort electron bunch at the t-ACTS test accelerator, the spectrum, angular distribution, and polarization of the produced coherent SPR were measured in the terahertz frequency region and compared with a model calculation. In addition to the widely known surface current model evaluation, the effect of the geometrical shading effect on induced currents on metal surfaces was evaluated using 3D numerical calculations. The obtained SPR characteristics are also presented. In the evaluation of the grating with a shallow blaze angle, it was found that the shading effect has a non-negligible effect on the generated SPR intensity; the measured angular distribution and polarization results were in good agreement with this result.

Grating Optimization for Smith–Purcell Radiation: Direct Correlation Between Spatial Growth Rate and Starting Current

Smith–Purcell radiation (SPR) is generated when electrons travel close to a metallic periodic grating. It was found that the starting current of SPR varies by orders of magnitude by simply varying the grating parameters (groove’s heights and widths) while keeping the grating period and the electron beam properties fixed. In this article, we demonstrate that this strong dependence of starting current on the grating parameters is directly related to the spatial growth rate of the SPR. Using the hot-tube dispersion relation, we optimize the grating parameters to minimize the starting current to excite coherent SPR.

MeV electron bunch compression and timing jitter suppression using a THz-driven resonator

THz-driven electron bunch compression and manipulation have significantly progressed and shown great potential in recent years. This paper presents a THz-driven resonator capable of bunch compression and timing jitter suppression with the inherent advantages such as field enhancement and laser synchronization. The resonance behavior of the resonator is studied to deduce the expression of the resonance frequency. Using the particle-tracking simulation, we study the performance of the THz-driven resonator in a MeV ultrafast electron diffraction beamline, and conclude that a bunch length (rms) of 15.40 ± 4.86 fs and timing jitter of 10.1 fs (rms) is achievable, which enables a temporal resolution below 25 fs in theory. Moreover, the THz-regime resonating field in the THz resonator allows for direct manipulation of each microbunch of a bunch train with picosecond bunch spacing. Particle-tracking simulation shows that simultaneous multi-bunch compression can be realized with a frequency-customized THz resonator. We expect this novel method to be applied in multi-bunch applications such as the coherent Smith–Purcell radiation.

Coherent Terahertz Emission Using Metasurfaces to Intercept a Flat Electron Beam

Terahertz electromagnetic radiation is one of the hottest research topics thanks to its promising applications in diverse scientific and technological fields. Using a uniformly moving electron beam to interact with a metallic or dielectric surface is an efficient tool for the generation of high-power terahertz waves. In the present paper, we demonstrate enhanced coherent terahertz emission theoretically and experimentally by using a metasurface formed by an array of Fabry-Perot-like resonators to obliquely intercept a low-energy flat electron beam. We illustrate that the mechanism is one of modified coherent Smith-Purcell radiation rather than the expected transition radiation. More notably, this metasurface overcomes the inherent restriction imposed by the decay length in conventional beam-wave interactions, greatly enhancing the radiation efficiency and intensity. This emission promises an attractive way of developing high-power terahertz sources.

Coherent Smith-Purcell radiation for minimally invasive bunch length measurement at the subpicosecond time scale

We have designed a tool to measure the bunch length of an electron beam in a minimally invasive way by means of coherent Smith-Purcell radiation (SPR). The technique has been employed successfully at a test apparatus for the Mainz Energy-recovery Superconducting Accelerator MESA, demonstrating it is possible to determine the bunch length while losing less than 0.6% of the electron beam. The impact of the space charge on the bunch length can be reduced while tuning the longitudinal bunch preparation system during a live measurement at beam currents up to 1 mA. Doing so, RMS bunch lengths of $70\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ can be achieved in a typical operating mode of the low-energy beam transport system of MESA. In addition to the bunch length measurements, typical properties of the generated SPR are demonstrated.

Single shot, nondestructive monitor for longitudinal subpicosecond bunch profile measurements with femtosecond resolution

Accurate knowledge of the charged particle bunch longitudinal (time) profile is important in the context of wakefield accelerators, Compton Light sources, x-ray SASE FELs and THz radiation sources. However, it is still a challenge to obtain this information for subpicosecond long bunches and microbunched beams with the required femtosecond (fs) resolution and nondestructively. Apart from determining the profile in a nondestructive manner, the ideal bunch diagnostic would enable extraction of all required information in a single shot, have a sufficiently high repetition rate to monitor each bunch, small footprint, good cost efficiency and reliability. In this paper we present the design of a longitudinal bunch profile monitor that can determine the charged particle bunch profile with femtosecond resolution, nondestructively and in a single shot via the spectral analysis of coherent Smith-Purcell radiation (cSPr). It is based on the simultaneous deployment of three gratings with different periodicities, each with its own set of detectors. The number of the frequency sampling points is equal to the number of the optical channels and the discrimination against background radiation is based on the different polarization properties of cSPr and background radiation. The rationales for the choices made to optimize the monitor operation will be presented and the criteria for determining the number of frequency sampling points will be discussed. The possible future developments of the monitor are also presented and discussed.

Efficient and Tunable Terahertz Emission by Transforming Surface Waves to Coherent Smith–Purcell Radiation

We propose and investigate an efficient terahertz radiation scheme using low-energy electron beams, including a single-electron bunch and a train of bunches, to excite cascaded gratings. It combines the mechanisms of wakefield radiation and of Smith–Purcell radiation (SPR) by transforming a series of surface waves into coherent SPR. Compared with the conventional wakefield radiation and SPR schemes, its radiation efficiency and power can be remarkably higher. In addition, it changes the conventional incoherent SPR into coherent emission with frequency being continuously tunable. Thus, it affords a promising way of developing the high power, coherent, and tunable terahertz radiation sources.

Improved terahertz Smith–Purcell radiation using side-opening cylindrical gratings

The Smith–Purcell radiation (SPR) is a promising way for generating terahertz emission, which has tremendous application prospects. Yet, the conventional SPR schemes generally utilize the Gaussian electron beam/bunch with round cross section (also called a pencil beam) to interact with a planar grating. The shape mismatching between the electron beam and the grating essentially restricts the interaction efficiency together with the radiation intensity. To counteract this effect, we propose to use side-opening cylindrical gratings (SOCGs) to replace planar gratings. Detailed theoretical analyses using the surface-current model and mode-matching method are performed for the first time. A train of premodulated electron bunches is proposed to drive the SOCG, on which the electron bunches are further micro-bunched by interacting with the surface waves. The multi-bunched electron beam then generates multi-color coherent SPR at the superimposed harmonics of the train of bunches. Compared with that from a planar grating, the radiation intensity from the SOCG can be significantly enhanced, indicating that it is a promising way of developing high-power terahertz wave sources.

Spectrum discretization of Smith–Purcell radiation for efficient multicolor terahertz emission

The Smith–Purcell radiation (SPR) is a promising way for developing compact and high power terahertz sources with broad application prospects. Yet, the conventional SPR is largely restricted by its low efficiency and incoherency. Here we illustrate an enhanced coherent SPR from a compound array of deep-narrow-rectangular-grooves (DNRGs). We show, by detailed theoretical analysis and simulations, that each DNRG is an effective resonant cavity with low radiating Q-factor and a cluster of closely adjacent DNRGs can get enhanced coherent emission. More notably, by setting clusters of DNRGs with gradient shapes to be periodically arranged in an array, we can change the conventional incoherent SPR to be a coherent emission with a series of specified frequencies at specified directions, namely, the radiation is discretized. The total radiation efficiency can be increased by more than an order of magnitude. Thus, it affords an efficient way for high power multicolor terahertz generations.

A tunable source of coherent terahertz radiation driven by the microbunched electron beam

The studies of coherent terahertz (THz) tunable Smith–Purcell radiation source driven by the microbunched electron beam are presented. We discuss the conceptual design of a high power and broadly tunable THz radiation source capable of bridging the THz technological gap. The results of the numerical and analytical studies using different software packages, models and approaches are shown and good agreement between the results is demonstrated. The spectra of the coherent Smith–Purcell radiation (cSPr) excited by a single bunch and a train of microbunches are compared and the excitation of the harmonics of cSPr is discussed. It is shown that varying the distance between microbunches the power and the frequency of the output cSPr can be controlled.

Generation of two-color terahertz radiation using Smith–Purcell emitter and periodic dielectric layers

This paper proposes and simulates three vacuum state devices each of which generates two-color terahertz radiation by Smith–Purcell emitter and dielectric layers. Each of the devices is a SiO2-made vacuum tube consisting of a Smith–Purcell emitter located at the bottom of the tube and also periodic dielectric layers located at the top. Smith–Purcell structure is a metal grating above which electron beam is generated closely by the cathode and are collected by a collector in another side of the grating. The emitter generates two terahertz radiations including evanescent and coherent Smith–Purcell radiations. While the evanescent radiation is the first harmonic of the emitted radiation, coherent Smith–Purcell radiation is the second and dominant one. The coherent Smith–Purcell radiation is emitted in a specific angle calculated by Smith–Purcell relation. The emitted coherent Smith–Purcell radiation coincides with periodic grating dielectric layers made of Si and SiO2 on top of the vacuum tube, and the transmitted pulse is transformed to two-terahertz pulses. The output of each device is a two-color terahertz pulse. Two generated terahertz wavelengths are very applicable in pump–probe experiments.

Enhanced Smith–Purcell radiation by coupling surface plasmons on meta-films array with resonator modes on a grating

We propose to use uniformly moving free-electrons to excite an array of alternatedly lined metamaterial thin-films (meta-films) and open-resonators. The surface plasmon polaritons on meta-films and the resonant modes within open-resonators are both excited, and are coupled to each other in the open-resonators. This coupling remarkably increases the intensities of resonant modes, which then generate enhanced coherent Smith–Purcell radiation (SPR). Compared with conventional SPRs, the present scheme combines the advantages of both material and structure profile, and its radiation power is more than an order of magnitude higher. Thus it offers a promising way for developing efficient terahertz sources.

Selective excitation and control of coherent terahertz Smith-Purcell radiation by high-intensity period-tunable train of electron micro-bunches

We report the observation and studies of selective excitation and control of terahertz (THz) coherent Smith-Purcell radiation (cSPr) from a train of sub-picosecond-period electron micro-bunches. The coherence of the radiation from such a train has been demonstrated. The spectrum of cSPr was measured and the selective excitation of the first and the second harmonics was observed, respectively. We also demonstrate experimentally that radiation pulses generated by each electron micro-bunch interfere coherently with the maximum intensity of cSPr observed at the frequency equal to the frequency of the micro-bunch spacing. The experimental results greatly contribute to the understanding of coherent Smith-Purcell radiation from the train of electron micro-bunches, as well as the development of THz diagnostics for accelerators.

The coherent THz Smith-Purcell radiation from a three-dimensional open holes array structure

Coherent Smith-Purcell radiation from multiple electron beam excitation in a three-dimensional open holes array structure has been proposed. The electron beam passes through the holes, which can be coupled with each other to enhance surface wave fields, so that the multiple electron beam injection can interact with the surface wave field efficiently. And the Smith-Purcell radiation will be enhanced due to an increasement in dipole oscillations occurring when the electrons pass through the holes. Therefore, a high beam-wave interaction efficiency is yielded and very strong coherent terahertz radiation can be generated. This multi-beam system is expected to provide a favorable way to excite the coherent Smith-Purcell radiation, which would be used to generate terahertz radiation.

Multi-color and multidirectional-steerable Smith-Purcell radiation from 2D sub-wavelength hole arrays

We proposed and investigated, by theoretical analyses and simulations, a multi-color and multi-directional radiation by using sheet electron-beams to drive two-dimensional (2D) sub-wavelength hole arrays (SHAs). Each sub-wavelength hole is a resonant and radiative unit, which is successively excited by the electron-beam, such that the 2D SHA becomes a 2D array of radiating units and generates coherent Smith-Purcell radiation in specific directions where the radiation from all the units constructively interferes. The multi-color radiation is obtained when several resonant modes are excited. Each resonant mode could simultaneously radiate at several directions due to the multiple periodicity of the 2D SHA, and the radiation could be steered to any directions in the three dimensional space by adjusting the arrangement of 2D SHA and the electron energy. This radiation could promisingly be developed as radiation sources and devices with broad applications.

A Fabry-Pérot interferometer with wire-grid polarizers as beamsplitters at terahertz frequencies.

The design of a compact Fabry-Pérot interferometer (FPi) and results of the experimental studies carried out using the device are presented. Our FPi uses freestanding wire-grid polarizers (WGPs) as beamsplitters and is suitable for use at terahertz (THz) frequencies. The FPi was studied at the LUCX facility, KEK, Japan, and an 8 MeV linear electron accelerator was used to generate coherent Smith-Purcell radiation. The FPi was designed to be easy to align and reposition for experiments at linear accelerator facilities. All of the components used were required to have a flat or well understood frequency response in the THz range. The performance of the FPi with WGPs was compared to that of a Michelson interferometer and the FPi is seen to perform well. The effectiveness of the beamsplitters used in the FPi is also investigated. Measurements made with the FPi using WGPs, the preferred beamsplitters, are compared to measurements made with the FPi using silicon wafers as alternative beamsplitters. The FPi performs well with both types of beamsplitter in the frequency range used (0.3-0.5 THz). The successful measurements taken with the FPi demonstrate a compact and adaptable interferometer that is capable of analyzing THz radiation over a broad frequency range. The scheme is particularly well suited for polarization studies of THz radiation produced in an accelerator environment.

Observation of coherent Smith-Purcell and transition radiation driven by single bunch and micro-bunched electron beams

Generation of coherent Smith-Purcell (cSPr) and transition/diffraction radiation using a single bunch or a pre-modulated relativistic electron beam is one of the growing research areas aiming at the development of radiation sources and beam diagnostics for accelerators. We report the results of comparative experimental studies of terahertz radiation generation by an electron bunch and micro-bunched electron beams and the spectral properties of the coherent transition and SP radiation. The properties of cSPr spectra are investigated and discussed, and excitations of the fundamental and second harmonics of cSPr and their dependence on the beam-grating separation are shown. The experimental and theoretical results are compared, and good agreement is demonstrated.

Proposal for Simultaneous Two-Color Smith–Purcell Terahertz Radiation Through Effective Surface Plasmon Excitation

Proximity interaction of a travelling electron beam near the surface of a structured metallic slab excites effective surface Plasmon at lower frequencies. Due to the existence of a velocity phase match, the Terahertz (THz) frequencies—known as Smith–Purcell radiation (SPR)—are produced. Here, we investigate two-color SPR characteristics of a ladder-type slab by means of the femtosecond pre-bunched perturbation (in parallel and counter propagating configurations). Our single-bunch test showed a successful two-color generation in the THz domain with variable frequency separations. At the allowed frequencies of SPR, an umklapp process in electron-Plasmon momentum redshifts the ESP's momentum to a strong THz generation regime—beyond the light zone. Calculating the three-dimensional (3D) dispersion relation, we find out that the slab width has a great effect on required beam energy and also velocity matching between phase of ESP and beam. Furthermore, by filling the slits with higher dielectric materials, a stronger and narrower SPR was achieved. The two-color device showed a strong coherent SPR when a long sequence of prebunched beams with variable bunch-to-bunch distances was launched to the system. Numerical methods—finite integral and particle-in-cell—are used to study the dispersion relation and the interaction of electrons with slab, respectively. These simulations showed complete agreement with the reported 2D and 3D results and fully supported the wavelengths predicted by the analytic equations.

PiC code KARAT simulations of Coherent THz Smith-Purcell Radiation from diffraction gratings of various profiles

Generation of coherent THz Smith-Purcell radiation by single electron bunch or multi-bunched electron beam was simulated for lamellar, sinusoidal and echelette gratings. The dependences of the CSPR intensity of the corrugation gratings depth were investigated. The angular and spectral characteristics of the CSPR for different profiles of diffraction gratings were obtained. It is shown that in the case of femtosecond multi-bunched electron beam with 10 MeV energy sinusoidal grating with period 292 μm and groove depth 60 μm has the uniform angular distribution with high radiation intensity.

Monochromaticity of coherent Smith-Purcell radiation from finite size grating

Investigation of coherent Smith-Purcell Radiation (SPR) spectral characteristics was performed both experimentally and by numerical simulation. The measurement of SPR spectral line shapes of different diffraction orders was carried out at KEK LUCX facility. A pair of room-temperature Schottky barrier diode (SBD) detectors with sensitivity bands of $60-90$~GHz and $320-460$~GHz was used in the measurements. Reasonable agreement of experimental results and simulations performed with CST Studio Suite justifies the use of different narrow-band SBD detectors to investigate different SPR diffraction orders. It was shown that monochromaticity of the SPR spectral lines increases with diffraction order. The comparison of coherent transition radiation and coherent SPR intensities in sub-THz frequency range showed that the brightnesses of both radiation mechanisms were comparable. A fine tuning feasibility of the SPR spectral lines is discussed.