Search

We investigate parametric X-ray radiation (PXR) under condition of the extremely asymmetric diffraction, when the ultra-relativistic electron bunch is moving in \textit{vacuum} parallel to the crystal-vacuum interface, close to the crystal surface. This type of geometry coincides with the well known mechanism of generation of radiation, when the self-field of the particle beam interacts with the reflecting metal grating, namely the Smith-Purcell effect. We demonstrate that in this geometry the main contribution is given via a tail region of the beam distribution, which penetrates the crystal and X-rays are radiated along the normal to the crystal surface. We determine the electron beam characteristics, when this phenomenon can be observed. It is essential that in this geometry the majority of electrons does not undergo multiple scattering and consequently the characteristics of the particle beam are not changed, thus allowing the usage of the emitted X-rays for the purpose of non-destructive beam diagnostics, which can complement the traditional knife-edge method., Comment: 12 pages, 9 figures

The frequency-tunability for a Smith-Purcell free-electron laser oscillator in the millimeter wave region is enhanced with the aid of a quasi-periodic distributed Bragg reflector (DBR) made of metallic waveguide gratings with rectangular grooves. For a quasi-periodic DBR, the grating parameters are adjusted by means of genetic algorithms to get the desired reflection coefficient, thus compensating for the frequency dependence of the overall output characteristics. For a typical numerical example, flat output characteristics have been obtained over 10% bandwidth around the center frequency 121 GHz

A linearized field theory of a grating-coupled Smith-Purcell traveling-wave tube (TWT) is presented. The configuration describes the propagation of a magnetized, sheet electron beam through a parallel-plate waveguide in which two opposing faces of the waveguide consists of either 1) a pair of identical diffraction gratings (referred to as a double-sided configuration) or 2) a diffraction grating along one face and a flat plate along the opposing face (referred to as a single-sided configuration). This is a two-dimensional formulation since we assume the configuration is uniform in the direction parallel to the slots in the grating. For each of these configurations, we derive the linearized dispersion equations for the vacuum structures and the wave-particle interaction in both the infinitely strong magnetic field limit and for an arbitrary (solenoidal) magnetic field. A discussion of the extraction efficiency based on phase-trapping arguments is also presented. An example of a W-band Smith-Purcell TWT is discussed.

Frequency multiplication in the Smith-Purcell free electron laser (FEL) as a way to obtain powerful millimeter/ sub-millimeter stimulated radiation from a high-current, moderately relativistic electron beam is studied. It is shown that the main mechanism that provided the multiplication is mutual scattering of low-frequency and high-frequency waves on the electron beam. The use of this mechanism can provide the power level of tens of megawatts and the possibility of a relatively broadband (few percent) frequency tuning.

A relativistic electron beam with velocity /spl nu//sub b/z passing over a metallic grating, with deep periodic depressions of wavenumber k/sub 0/z, excites a slow wave (/spl omega/, k/sub z/) via Cerenkov interaction, /spl omega/=(k/sub z/+k/sub 0/)/spl nu//sub b/. The beam bunches induce currents in the fins of the grating that act as a phased radiating dipole array, thereby generating coherent radiation at a wavelength /spl lambda/=(2/spl pi//k/sub 0/l)(c//spl nu//sub b/-sin/spl theta/) where l is an integer and /spl theta/ is the angle of emission measured from the normal of the grating. The current threshold for the instability and radiation power scaling with beam current are discussed.

Absorption of a photon by an electron moving parallel to a rough surface is studied.In the weak scattering regime we have evaluated the absorption probability of absorption of a single photon. It is shown the absorption probability with diffusional contribution becomes large by a lin/l factor compared to the analogous result with the single scattering contribution. The maximum of probability takes place for the infrared wavelengths and strongly depends on the particle energy. We also discuss the case of two-dimensional periodical surface profile and indicate optimal conditions for maximal absorption probability. The results can be used in electron energy gain spectroscopy and in laser-driven acceleration., 5 pages,2 figures

The Smith-Purcell radiation is usually measured when an electron passes over the grating of metallic stripes. However, for high frequencies (exceeding the plasma frequency of the grating material) none material could be treated as a conductor, but ought to be considered as a dielectric with plasma-like permittivity. So for describing Smith-Purcell radiation in the range of high frequencies new theoretical approaches are needed. In the present paper we apply the simple variant of eikonal approximation developed earlier to the case of radiation on the set of parallel semi-infinite dielectric plates. The formulae obtained describe the radiation generated by the particles both passing through the plates (traditionally referred as "transition radiation") and moving in vacuum over the grating formed by the edges of the plates (traditionally referred as "diffraction radiation", and, taking into account the periodicity of the plates arrangement, as Smith-Purcell radiation)., Submitted to Journal of Physics: Conference Series

A few years ago, a new theory for producing coherent Smith-Purcell radiation from an initially continuous electron beam was proposed. It was 2-D, and it has been supported by particle-in-cell code simulations, although no experiments have, until now, provided confirmation. In fact, all experiments used a round beam, for which the 2-D theory may not be valid. We report here on a two-pronged attack on this problem. A 3-D simulation in the microwave domain indicates that coherent radiation, at a frequency near that of the 2-D simulation, should be produced. We are also preparing a demonstration experiment using an intense finite-width sheet beam to confirm the theory and simulations.

The inverse Smith–Purcell effect is a candidate for laser‐driven linacs utilizing the interaction between laser light and an electron beam traveling just in front of a metallic grating. We have performed experiments to study electron energy spread as a function of electron beam position above the grating. A submillimeter wave laser (CH3F, 496 μm) is used as a driving source. It is found that the energy spread characteristics show exponential decay of the interaction strength (field intensity) in the direction perpendicular to the grating surface, as a classical theory on the effect predicts.

An approach has been developed where the Smith-Purcell radiation (SPR), i.e. emission of electrons moving close to a periodic structure, is treated as the resonant diffraction radiation. Simple formulas have been designed for the SPR intensity for a grating having perfectly conducting strips spaced by a vacuum gap. The results have been compared with those obtained via other techniques. It has been shown that the intensity of radiation for the said gratings for a relativistic case sufficiently exceeds the SPR intensity for the grating made up by a periodically deformed continuous surface., 9 pages, LATEX, 3 Postscript figures, uses epsf.sty, submitted to Phys.Letters A

A review of the experimental study of single-mode oscillators based on stimulated bremsstrahlung and Cerenkov radiation of high-current relativistic electron beams is given. Three types of Cerenkov oscillators are investigated in detail: orotrons, surface wave oscillators and a flimatron (free electron maser (FEM) based on Smith-Purcell radiation). The bremsstrahlung oscillators studied are gyrotrons with TM modes, a ubitron operating at a quasi-critical frequency and cyclotron autoresonance masers. Electrodynamic and electron methods of mode selection provide stable radiation with a reproducible space structure of radiation in all oscillators under study. The radiation power attained 50-100 MW for long and 10-30 MW for short millimeter wavelengths at the efficiency up to 5-10 percent. Various types of oscillators are compared. Promising methods for increasing power and radiation frequency are discussed.

A rigorous solution is derived for the problem of radiation of the Smith–Purcell type (Smith and Purcell 1953) extracted from a radio-frequency bunched electric-current sheath, above a plane exhibiting a periodically varying surface reactance. The plane is regarded as a model of an optical reflection grating. The solution indicates that the presence of the grating will give rise to electromagnetic resonance effects which are closely related to the so-called Wood anomalies (Wood 1935; Twersky 1956; Hessel 1960; Hessel and Oliner 1962; Stewart and Gallaway 1962), encountered in optical-grating spectroscopy. Utilization of these resonances may be advantageous in the generation of millimeter waves.

Interest in the development of laser-driven linacs has been stimu-lated recently by the advent of the high-power lasers. The use of a laser to accelerate charged particles was first proposed by K. Shimoda in 19621. He noticed that high values of acceleration per metre (acceleration gradient) could be obtained with an intense electric field in the output of a high-power laser. The inverse Smith–Purcell (ISP) effect proposed by us2 is a candidate for laser-driven linacs with a metallic grating as an interaction circuit. In 1953 Smith and Purcell demonstrated that light is emitted when a high-voltage electron beam moves parallel and close to a metallic optical diffraction grating in a direction perpendicular to the grating rulings3. The dispersion relationship is a synchronous condition between the electrons and the wave on the grating. Therefore, the inverse Smith-Purcell effect (ISP), or the extended interaction between electrons and an incident light wave should occur when the same relation is satisfied between the electrons and the incident wave (Fig. 1)2. Lawson pointed out4 that this effect would fail to accelerate relativistic particles. This difficulty was solved by Palmer5, who showed that acceleration is possible either if the particles travel skew to the grating lines, or if the radiation is falling at a skew angle onto the grating. Several authors have described possibilities for producing a grating linac (ISP effect) with accelerating field of a few GeV m−1 in the infrared or optical wave region6. Here we report the first observational evidence for this effect using a submillimetre-wave laser as a driving source. The experimental results give good agreement with theoretical predictions.

In this paper, we present a three dimensional numerical study on a terahertz frequency multiplier. The process of frequency multiplication is proposed via study of spontaneous Smith-Purcell (SP) radiation. Through introducing a single electron bunch perturbation which is passing over above a metallic grating, and due to interaction of electron bunch with surface wave, the evanescent wave radiates from the ends of the grating and the SP radiation is generated in the span of 0.466∼1.1 THz. We found that the maximum of SP radiation amplitude is distributed around 90° with the centre frequency 0f ∼0.65 THz. Simulations on the generation of SP radiation at THz frequencies have been performed with the help of the 3D particle-in-cell (PIC) finite integral method and results agree very well with analytic ones.