Backward Transition Radiation Research Articles

Monochromatic backward transition radiation from a tilted grating with adaptive strips produced by an electron bunch train

Nowadays, Smith–Purcell radiation is used for bunch length diagnostics. This paper presents experimental results of investigating radiation from a specific tilted grating. We propose using a grating consisting of adaptive strips, allowing to change a grating profile. Such a grating offers additional advantages compared to the traditional Smith–Purcell radiation geometry and radiation from a tilted grating. The results of measurements on the electron bunch train show that this advantage consists in the ability to increase the radiation intensity for a given tilt angle of the grating.

Coherent THz transition radiation for polarization imaging experiments

THz radiation is reflected by conducting materials and has the capability to penetrate a wide variety of non-conducting materials. With such abilities, THz radiation has great potential for imaging applications, e.g., screening and inspection for packaged electronics chips and electronics circuit boards. High intensity THz radiation have been produced by the accelerator-based radiation source at the Plasma and Beam Physics Research Facility, Chiang Mai University, Thailand. The coherent THz transition radiation was generated from short electron bunches of around 8 MeV and an Al-foil tilted by 45°. Characterization of the backward transition radiation properties was conducted, including spatial distribution, polarization and radiation spectrum. The THz radiation was then used to create THz images of a print circuit board by reflective THz imaging setup. THz imaging with polarized transition radiation is demonstrated. With selected radiation polarization, reflective THz imaging can be improved.

A THz Spectroscopy System Based on Coherent Radiation from Ultrashort Electron Bunches

A spectroscopy system will be discussed for coherent THz transition radiation emitted from short electron bunches, which are generated from a system consisting of an RF gun with a thermionic cathode, an alpha-magnet as a magnetic bunch compressor, and a linear accelerator for post-acceleration. The THz radiation is generated as backward transition radiation when electron bunches pass through an aluminum foil. The emitted THz transition radiation, which is coherent at wavelengths equal to and longer than the electron bunch length, is coupled to a Michelson interferometer. The performance of the spectroscopy system employing a Michelson interferometer is discussed. The radiation power spectra under different conditions are presented. As an example, the optical constant of a silicon wafer can be obtained using the dispersive Fourier transform spectroscopy (DFTS) technique.

Coherent backward transition radiation from sub-fs “pancake-like” bunches as a tool for beam diagnostics

Coherent transition radiation (CTR), produced by short electron bunches, is a well-known radiation mechanism which is applied for bunch length monitoring. By analyzing the CTR spectrum it is possible to reconstruct the longitudinal bunch shape. Such a technique is widely used for accelerators with electron beams possessing bunch lengths σz much larger than their transverse beam sizes σx,y (with σx,y,z the rms transverse and longitudinal beam sizes). New developments in the field of accelerator technology allow to produce and accelerate bunches with a duration of less than 1 fs with a “pancake-like” bunch shape, i.e. with σz ≪ σx,y. Diagnostic schemes of this kind of bunches give rise to interesting effects, for example connected with a transverse asymmetry (σx ≠ σy), with a tilting of the bunches (i.e. the bunch axis does not coincide with the velocity direction), and even more. Spectral-angular distributions of coherent backward transition radiation (BTR) were simulated taking into account a possible bunch tilt. It is shown that spectral intensity measurements of coherent BTR emitted in backward direction allow to obtain information about the bunch length.

Transition radiation on a dynamic periodic interface.

We investigate the transition radiation on a periodically deformed interface between two dielectric media. Under the assumption that the dielectric permittivities of the media are close, a formula is derived for the spectral-angular distribution of the radiated energy in the general case of a nonstatic profile function for the separating boundary. In particular, the latter includes the case of surface waves propagating along the boundary. The numerical examples are given for triangular grating and for sinusoidal profile. We show that instead of a single peak in the backward transition radiation on a flat interface, for periodic interface one has a set of peaks. The number and the locations of the peaks depend on the incidence angle of the charge and on the period of the interface. The conditions are specified for their appearance.

Radiation of relativistic electrons in a periodic wire structure

We present in this work the experimental investigation of the interaction of relativistic electron field with periodic wire structures. We used two types of the targets in experiments: flat wire target and sandwich wire target that represent the right triangular prism. The measurements were done in millimeter wavelength region (10–40mm) on the relativistic electron beam with energy of 6.2MeV in far-field zone. We showed that bunched electron beam passing near wire metamaterial prism generates coherent Cherenkov radiation. The experiments with flat wire target were carried out in two geometries. In the first geometry the electron beam passed close to the flat wire target surface. In the second case the electron beam passed through the flat wire structure with generation of a coherent backward transition radiation (CBTR). The comparison of the Cherenkov radiation intensity and BTR intensity from the flat wire target and from the flat conductive target (conventional BTR) was made.

Angular Distributions of EUV Generated by Electrons with 5.7 MeV Energy in a Multilayer Mo/Si Structure

The article considers the dynamics of the changes in the intensity and shape of the angular distributions of the backward transition radiation and quasi-monochromatic radiation emitted by the periodic structure of a multilayer mirror in a vacuum ultraviolet region for electrons with the energy of 5.7 MeV, depending on the orientation of the target. It has been shown that the dynamics of the changes in the intensity and shape of the angular distributions of the backward transition radiation and those of the periodic structure radiation observed after directing the radiation through an absorption edge filter display opposite tendencies. These opposite tendencies may be used as an indicator in the experiments aimed at detecting the effect of the periodic structure radiation in the ultraviolet region of the analog of parametric X-ray in crystals.

Observation of quasimonochromatic radiation in the vacuum ultraviolet region generated by 5.7-MeV electrons in a multilayer mirror

Angular distributions of radiation in the vacuum ultraviolet region with a photon energy of about 70 eV, which have been measured for several angles of incidence of 5.7-MeV electrons on the surface of a periodic Mo/Si nanostructure of a multilayer mirror, have been analyzed. It has been shown experimentally that vacuum ultraviolet radiation in the direction of backward transition radiation has a quasimonochromatic component associated with coherent radiation of the periodic structure of the target.

Backward transition radiation in the extreme ultraviolet region as a tool for the transverse beam profile diagnostic

The present article summarizes the results of two experiments which were performed to study the radiation properties of backward transition radiation (BTR) in the extreme ultraviolet (EUV) region. This wavelength region is of particular interest for transverse beam profile imaging, because the spatial resolution is improved as a result of the reduced contribution in the imaging process of the fundamental diffraction limit. In addition, the influence of coherent effects in the transition radiation emission process, which have been observed in the visible region, might be mitigated. The first experiment, dedicated to the investigation of the BTR angular characteristics, indicates that the radiation yield in the EUV region is higher than theoretically expected and that it is even comparable to the yield in the visible region. The second measurement was devoted to transverse beam profile imaging based on quasimonochromatic BTR, in both the EUV and the visible region, and is a proof-of-principle experiment demonstrating that EUV BTR is a suitable candidate for standard beam profile diagnostics.

Investigation of the characteristics of EUV backward transition radiation generated by 5.7 MeV electrons in mono- and multilayer targets

This paper presents experimental results of a study of radiation in the vacuum ultraviolet and extreme ultraviolet spectral region, generated by electrons with an energy of 5.7 MeV in a multilayer X-ray mirror. The angular distribution of the radiation generated in the rear hemisphere from the input surface of the target was measured. The contribution to the radiation yield caused by the periodic structure of the multilayer target was experimentally investigated.

Simulations of Transition Radiation from a Flat Target using CST Particle Studio

Numerical simulations of Backward Transition Radiation originating from a flat ideally conducting target inclined at 45 degrees with respect to a charge propagation direction are presented in this paper. The simulation results are compared with the theory for the wave and pre-wave zones. The main characteristics of the simulated radiation distributions are discussed in detail.

Backward transition radiation in EUV-region as a possible tool for beam diagnostics

In this paper we discuss the possibility of proof-of-principle experiment to observe backward transition radiation in EUV region as a first step for beam diagnostics. We have carried out simulations of backward transition radiation angular distributions in the EUV region for molybdenum target for different geometries. The simulation results are promising for beam diagnostics purposes.

Backward transition radiation in EUV-region as a possible tool for beam diagnostics

In this paper we propose to use backward transition radiation in EUV region for micron and submicron beam diagnostics. We have carried out simulations of backward transition radiation characteristics in the EUV region for molybdenum target using two approaches. The simulation results show a good agreement for both approaches and are very promising for beam diagnostics purposes.

Transition radiation in the pre-wave zone for an oblique incidence of a particle on the flat target

Backward transition radiation of an electron with an arbitrary energy is studied when an ideally-conducting target is flat and its normal is not collinear with the particle velocity (oblique incidence). A model for radiation registered with a small flat detector placed at a finite distance (including the radiation in the pre-wave zone) in the vicinity of specular reflection direction is developed. Characteristics of the radiation in the far-field zone are in complete agreement with well-known results. The calculations for pre-wave zone show that the angular distribution of the radiation intensity is distorted compared to the far-field case, and the radiation asymmetry (having a place in the far-field for moderately relativistic energies) is also preserved in the pre-wave zone. Some numerical estimations of the radiation asymmetry at a finite distance are also given. The technique developed may be used for estimations of coherent transition radiation intensity in the mm-wavelength range.

Coherent transition and diffraction radiation from a bunched 6.1 MeV electron beam

The theory of transition radiation (TR) from an ideally conducting infinite target is well known. During the last several years the theory of diffraction radiation (DR) from simple geometry targets has further been developed. In a few experiments coherent TR and coherent DR were used to measure the electron bunch length. However, experimental investigations of coherent TR (or DR) look rather poor for finite size targets. The experimental results of investigation of coherent backward transition radiation (CBTR) and coherent backward diffraction radiation (CBDR) are presented. The intensity of CBTR and CBDR from a finite conducting target for different impact-parameters depending on the target inclination angle θ ( θ-scan) has been investigated using the 6 MeV electron beam of the Tomsk microtron. The model allowing to calculate both TR and DR characteristics for any impact-parameter h (the shortest distance between particle trajectory and target edge) was developed and tested by comparison with experimental results. The calculations were performed for the real experimental conditions. For γλ ≫ h ( γ – Lorentz-factor, λ – TR (DR) wavelength) and large detector bandwidth γ ( λ max - λ min ) h > 1 the measured TR angular distribution shows a single maximum only (like DR one) in contrast to a lobe-shaped OTR distribution.

Some features of diffraction and transition radiation at the distance less than γ2λ

Recently the projects on the creation of the beam diagnostic methods using backward transition radiation (BTR) and noninvasive diagnostic methods using backward diffraction radiation (BDR) of electrons, passing through a conductive slit, are in progress. Most of the BTR and BDR theoretical models assume that the size of the radiation source region is much less than the size of the image in the observation point. For the large electron energy this condition actually may not be satisfied. This problem was considered in [Phys. Lett. A 273 (2000) 135], but this model may be used actually for very simple targets. We suggest a simple model, which allows us to estimate with a good accuracy the radiation properties of transition and diffraction radiation in near field zone from the targets of different geometries. The samples for transition and diffraction radiation from a slit target are presented. Also the transition radiation focusing effect on the parabolic targets is shown.

Synchrotron radiation contributions to optical diffraction radiation measurements

If we try to measure the backward optical diffraction radiation (BODR) of high energy electrons from a conductive slit or a semi-infinite plate, the electron beam will pass through the bending magnet (BM) or steering magnet (SM) or magnet lenses before striking the target. The synchrotron radiation (SR) from these magnets can then obscure the BODR measurements. Analysis of the SR properties from these magnets is presented. A model based on the modified Lenar–Wikherd potentials was created, and the SR angular distribution from relativistic electrons in BM and SM for different conditions of radiation in the optical region was calculated. Analysis shows that for the parameters of the KEK ATF extraction line, the SR intensity exceeds that of the backward optical transition radiation (BOTR) from the conducting targets, and it is much larger than the BODR intensity. The SR intensity from the SMs depends on its tuning and may be comparable to BOTR. Thus, the problem of separating of BODR and SR in the BODR measurements is important. Two methods of resolving this problem are suggested.

Production of X-ray transition radiation with relativistic electrons propagating at grazing incidence

This paper is concerned with the production of X-ray transition radiation when a relativistic electron crosses the interface between two media of different permittivities at the grazing incidence. The production yields are derived analytically when a thick interface is considered and are compared with those obtained when the electron crosses the interface at normal incidence. The production of X-ray photons having an energy between two photoabsorption edges or close to a photoabsorption edge is also investigated. The main features of the transition radiation produced are then exhibited and it is shown that the intensity can be increased by many orders of magnitude. We also show that, at grazing incidence, the backward transition radiation has an intensity close to the forward emission one, contrary to the normal incidence case. The production of quasi-monochromatic radiation is also presented. Finally, the production of X-ray transition radiation using a multilayer radiator is considered and compared in both geometries.

Status of optical diffraction radiation experiment at KEK-ATF extraction line

At KEK-ATF extraction line detailed investigation of backward optical diffraction radiation (ODR) and its implementation to transversal beam parameter diagnostics is planned. We designed and constructed a target chamber with a precise target movement mechanism, a precise laser alignment system and an optical system. The first step to achieve success in the experiment is the tuning of the optical system using backward optical transition radiation (OTR). We believe that, if we are able to measure OTR spectral-angular characteristics with a proper accuracy, we may obtain good results for ODR measurements as the OTR effect is well experimentally and theoretically studied. Therefore, the first stage of our experiment assumes the measurements of backward OTR to analyze all possible types of radiation coming from the target, which could be obstacles for the ODR measurements. In the first series of experiments we have found that the angular distribution of the detected optical radiation is strongly asymmetric. One of the possible reasons for the asymmetry is the synchrotron radiation contribution from bending or steering magnets.

Effect of an alternating electric field on transition radiation

Considers the problem of transition radiation in the presence of an alternating electric field. It is assumed that the particle travels at a uniform relativistic velocity and that the alternating electric field is applied at the boundary between two media in a direction parallel to the velocity of the particle. The method of Fourier transforms is used to solve the modified Maxwell equations with appropriate boundary conditions. Assuming that modulation energy is small compared with mean particle energy and using a linear approximation, the relativistic formula specifying the intensity of transition radiation in the presence of the field is presented in a compact form. It shows that in the presence of the field, the backward transition radiation increases, but forward transition radiation decreases compared with the radiation without a field. The contribution to the radiation due to the field varies linearly with the field strength, but varies inversely with the field frequency.