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The development of linac–based narrow–band THz sources with sub–picosecond, μ J -level radiation pulses is in demand from the scientific community. Intrinsically monochromatic emitters such as coherent Smith–Purcell radiation sources appear as natural candidates. However, the lack of broad spectral tunability continues to stimulate active research in this field. We hereby present the first experimental investigation of coherent grating diffraction radiation (GDR), for which comparable radiation intensity with central frequency fine–tuning in a much wider spectral range has been confirmed. Additionally, the approach allows for bandwidth selection at the same central frequency. The experimental validation of performance included the basic spectral, spatial and polarization properties. The discussion of the comparison between GDR intensity and other coherent radiation sources is also presented. These results further strengthen the foundation for the design of a tabletop wide–range tunable quasi–monochromatic or multi–colour radiation source in the GHz–THz frequency range. [ABSTRACT FROM AUTHOR]

Success in design and construction of a compact, high-brightness accelerator system is strongly related to the production of ultra-short electron beams. Recently, the approach to generate short electron bunches or pre-bunched beams in RF guns directly illuminating a high quantum efficiency semiconductor photocathode with femtosecond laser pulses has become attractive. The measurements of the photocathode response time in this case are essential. With an approach of the interferometertype pulse splitter deep integration into a commercial Ti:Sa laser system used for RF guns, it has become possible to generate pre-bunched electron beams and obtain continuously variable electron bunch separation. In combination with a well-known zero-phasing technique, it allows us to estimate the response time of the most commonly used Cs2Te photocathode. It was demonstrated that the peak-to-peak rms time response of Cs2Te is of the order of 370 fs, and thereby, it is possible to generate and control a THz sequence of relativistic electron bunches by a conventional S-band RF gun. This result can also be applied for investigation of other cathode materials and electron beam temporal shaping and further opens a possibility to construct wide-range tunable, table-top THz free electron laser. [ABSTRACT FROM AUTHOR]

This paper numerically and experimentally demonstrates the influence of the space charge forces in short electron bunches on the spectrum of coherent transition radiation. To estimate and optimize dimensions of a short electron bunches produced by the KEK LUCX RF gun, ASTRA software was employed. Obtained results indicate that the modification of the femtosecond UV laser pulse transverse profile allows to decrease the space charge forces. The simulation results were confirmed experimentally with the good agreement. [ABSTRACT FROM AUTHOR]

Grating transition radiation (GTR) emitted by electrons interacting with a diffraction grating was experimentally confirmed. This radiation appears when a charge intersects the grating at some inclination angle θ between the main grating plane and the charge velocity vector. If the condition θ = 0 is fulfilled the GTR tends to the conventional Smith-Purcell radiation. In this report we present the experimental investigation of spectral and orientation dependencies of a coherent GTR in sub-THz wavelength region emitted by 8 MeV electron at KEK LUCX facility. The coherent GTR spectral line shapes up to 5th diffraction order were measured for different inclination angles ( θ = 0–25 deg) with the grating period 4 mm. A pair of room-temperature Schottky barrier diode (SBD) detectors with sensitivity bands 60–90 GHz and 320–460 GHz were used. The estimation of radiation frequency dependence on the inclination angle using the dispersion relation justifies the usage of different narrow-band SBD detectors to investigate different diffraction orders of coherent GTR spectral lines. It was shown that the bandwidth of the GTR spectral lines decreases with the diffraction order. The comparison with coherent transition radiation in the same frequency range showed that the brightness of both radiation mechanisms are comparable. [ABSTRACT FROM AUTHOR]

In this report we present experimental investigations of the THz radiation generated from a corrugated and a non-corrugated capillary with reflectors, using a femtosecond electron beam of LUCX accelerator at KEK, Japan. We discuss measurements of the radiation angular distributions and their comparison with Particle In Cell simulations, and also investigate an off-central propagation of the beam in the capillaries based on experimental measurements and simulations. [ABSTRACT FROM AUTHOR]

This paper demonstrates the detailed design concept, alignment, and initial testing of a Michelson interferometer for the THz spectral range. We present the first results on the measurement of a coherent transition radiation spectrum and describe the performance of a pair of ultra-fast broadband room temperature Schottky barrier diode detectors. We discuss the main criteria of interferometer beam splitter optimization, the alignment technique, the high-precision calibration and linearity check of the motion system. [ABSTRACT FROM AUTHOR]

We present an initial test of a new type of a pre-bunched beam pumped free electron maser based on Stimulated Coherent Diffraction Radiation (SCDR) generated in an open resonator. A fast Schottky Barrier Diode (time response < 1 ns ) has enabled us to investigate the properties of the radiation stored in the cavity as well as the intrinsic properties of the cavity itself. We observed a turn-by-turn SCDR generated by a multibunch beam. When the cavity length was exactly a half of the bunch spacing a clear resonance was observed. Moreover, turn-by-turn measurements revealed the cavity quality factor of 72.88, which was rather high for an open resonator in the wavelength range of 3–5 mm. An exponential growth of the photon intensity as a function of the number of bunches was also demonstrated. [ABSTRACT FROM AUTHOR]

Abstract: To obtain high-brightness quasi-monochromatic X-rays via inverse Compton scattering (ICS), an optical cavity for intensifying laser beams was designed and implemented in a new beam line at the KEK Superconducting RF Test Facility (STF) accelerator. The optical cavity adopts a planar configuration consisting of 4 mirrors. This confocal type resonator provides stable laser storage even with a long mirror distance, enabling head-on collision with the electron beams. To overcome the well-known astigmatism problems of the planar-type optical cavity, two forcibly bendable cylindrical mirrors were introduced instead of flat mirrors. With this new function for laser profile adjustment, an almost round laser profile at the waist point in the accelerator environment was successfully achieved. Estimated waist sizes were 43.7μm for the horizontal and 50.8μm for the vertical dimensions. The feedback control of this 4-mirror optical cavity worked with a stiff plate supporting all 4 mirrors. 1.7×103 finesse and 2.8-kW stored power for a 1-ms duration with 5Hz were achieved. [Copyright &y& Elsevier]

We represent the first experimental observation of the point spread function (PSF) of optical transition radiation (OTR) performed at KEK-Accelerator Test Facility extraction line. We have demonstrated that the PSF vertical polarization component has a central minimum with a two lobe distribution. However, the distribution width varied significantly with wavelength. We assume that we observed a severe effect from spherical or chromatic aberrations which are not taken into account in any existing theoretical model. We believe that the result of this work will encourage theoreticians to continue developing the theory as it is important for various transition radiation applications. Nonuniform distribution of the OTR PSF creates an opportunity for developing a submicrometer transverse beam size monitor. [ABSTRACT FROM AUTHOR]

Abstract: A fast room temperature microwave detection system based on the Schottky Barrier-diode detector was created at the KEK ATF (Accelerator Test Facility). It was tested using Coherent Synchrotron Radiation (CSR) generated by the 1.28GeV electron beam in the damping ring. The speed performance of the detection system was checked by observing the CSR from a multi-bunch (2.8ns bunch separation time) beam. The theoretical estimations of CSR power yield from an edge of bending magnet as well as new injection tuning method are presented. A very high sensitivity of CSR power yield to the longitudinal electron distribution in a bunch is discussed. [Copyright &y& Elsevier]

Abstract: Using the pre-bunched electron beam of 5MeV linear accelerator the coherent Smith–Purcell radiation (CSPR) from a flat periodic target (made of conductive layers separated by dielectric gaps) in millimeter wavelength region has been investigated. The angular distribution of this radiation was measured with a narrow-band detector and experimental data agree with our theoretical calculations for similar kind of targets. Such properties of Smith–Purcell radiation as strong dependence of radiation wavelength on the observation angle overlapping with coherent radiation effect may be used for a noninvasive bunch length measurement. The possibility of using the room temperature detectors for single bunch measurements is demonstrated. [Copyright &y& Elsevier]

The design of free-electron lasers (FELs) based on submillimeter-range undulators involves difficulties associated with the fabrication of undulators with a very small period. To do this also requires accelerators with high-energy electron beams (>50 MeV). It should also be taken into account that these facilities use precision and expensive magnetic systems. In some works, an alternative FEL mechanism based on the use of the Smith­Purcell radiation (SPR) was considered. To realize the feedback in such an FEL, one must use a basically different cavity scheme. This work reports the results of experimental study on the stimulated SPR in a ‘transverse’ cavity. [ABSTRACT FROM AUTHOR]

A free electron laser design is considered in which coherent radiation is generated during the flight of a bunched electron beam near a periodic metal target ("Smith-Parsell radiation). If the target (array) is placed in a special resonator, optical feedback can be realized. It is proposed to use an array made of thin conductive strips separated with vacuum gaps, since the radiation output in this case is much higher than that attainable with conventional arrays. Varying the array parameters, one can choose the necessary direction and required spectral distribution of the Smith-Parsell radiation. When radiation is generated in the direction normal to the trajectory of the electron beam, the array acts as one of the resonator mirrors. [ABSTRACT FROM AUTHOR]

Studies the coherent diffraction radiation of microtron electron beams. Energy of accelerated electrons; Macropulse duration and frequency; Current amplitude in macropulse.

Abstract: A laserwire is a non-invasive, high resolution particle beam size monitor based on Compton scattering that is required for future planned colliders and can also be used to estimate the size of the source in wakefield acceleration experiments. We present recent results from the high resolution laserwire transverse electron beam diagnostic installed at the Accelerator Test Facility 2 (ATF2) electron accelerator at KEK in Japan. Full characterisation of the propagation of the 150mJ, 77ps laser beam is used to deconvolve the transverse laserwire profile demonstrating the successful measurement of 1µm scale vertical electron beam sizes, even with extreme aspect ratios. We also present progress in the development of high energy photonic crystal fibre based laser systems for laserwire measurements at megahertz repetition rates, suitable for intra-train scanning for planned accelerators such as the International Linear Collider, or beam size measurement in laser or particle driven plasma accelerators. [Copyright &y& Elsevier]

Abstract: The KEK Accelerator test facility (ATF) extraction line laser-wire system has been upgraded last year allowing the measurement of micron scale transverse size electron beams. The last measurements using the upgraded system from recent operation at the ATF are presented, demonstrating raw measurements of order RMS. The main component contributions to this measurement are also discussed. [ABSTRACT FROM AUTHOR]

Abstract: A short electron bunch with length σ passing through a slit in the diffraction radiation (DR) target generates radiation with a broad spectrum. Optical part of the spectrum (incoherent radiation) may be used for beam size measurements, but in the wavelength range λ⩾σ radiation becomes coherent. The coherent DR spectrum per each electron in a bunch is equal to single electron spectrum times by the number of electrons in a bunch Ne and bunch form factor. For SLAC FFTB conditions (Ne∼1010, σ=0.7 mm, outer target size R∼10 mm, slit width h∼0.1 mm) we approximated coherent DR (CDR) spectrum by coherent transition radiation (TR) one because in the wavelength region λ∼σ≫h TR and DR spectra coincide with high accuracy. Changing the DR target by a TR target with projection on the plane perpendicular to electron beam as a circle with radius R⩽20 mm we calculated CDR spectra using simple model. Knowing the CDR spectrum we estimated the energy CDR emitting by each electron in the perpendicular direction (due to target inclination angle 45°). It means an electron receives the radiation recoil in this direction. In other words, electron has a transverse kick about 1 μrad that may be considered as permissible. [Copyright &y& Elsevier]

Abstract: 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 6MeV 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 the measured TR angular distribution shows a single maximum only (like DR one) in contrast to a lobe-shaped OTR distribution. [Copyright &y& Elsevier]

A cycle of measurements of the polarization characteristics of coherent transition radiation and coherent diffraction radiation at the LUCX accelerator has been performed. It has been shown that the results of measurements of the linear polarization component in both cases are well described by a theoretical model according to which the degree of elliptical polarization of diffraction radiation is close to 100%. This fact indicates a significant contribution (up to 90%) of the circularly polarized component of diffraction radiation for radiation angles with respect to the direction of mirror reflection exceeding γ−; (γ is the Lorentz factor). [ABSTRACT FROM AUTHOR]

With a low emittance and short-bunch electron beam at a high repetition rate realized by a superconducting linac, stimulated excitation of an optical cavity at the terahertz spectrum range is shown. The electron beam passes through small holes in the cavity mirrors without being destroyed. A sharp resonance structure which indicates wideband stimulated emission via coherent diffraction radiation is observed while scanning the round-trip length of the cavity. [ABSTRACT FROM AUTHOR]

Abstract A compact accelerator based super-radiant THz source is under development at Inter University Accelerator Centre (IUAC), New Delhi. The facility is based on the principle of pre-bunched Free Electron Laser (FEL) which will produce THz radiation in the range of ∼ 0.18 to 3 THz from a modulated electron beam. A photocathode electron gun will generate a short train of micro-bunches (a "comb" beam) driven by a fibre laser system capable of producing multi micro-pulse laser beam with variable separation ("comb" laser pulse). Upon acceleration, the electron beam will be injected in to a compact undulator magnet tuned to the same frequency as the separation of the electron micro-bunches. The paper discusses the process of enhancement of super-radiant emission of radiation due to modulation in the comb beam and the conditions required to achieve maximum enhancement of the radiation power. The feasibility study of generating a comb beam at the photocathode and its transport through the beamline while preserving its temporal structure has been reported. To evaluate the characteristics of the radiation emitted from the comb beam, a C + + based particle tracker and Liẽnard–Wiechert field solver has been developed. The conceptual understanding of the emission of radiation from comb beam is shown to conform with the numerical results. The code has been used to calculate the radiation pulse energy emitted into the central cone of undulator for various comb beam configurations. [ABSTRACT FROM AUTHOR]

We consider a charged particle bunch passing along the axis of a semi-infinite open-ended circular vacuum waveguide placed into a collinear infinite waveguide with larger radius. The rigorous solution of this problem is constructed using residue-calculus technique. Analytical expressions for the Fourier harmonics of the field components are obtained in each domain of the structure. Numerical integration over frequency is also performed and time-domain field dependencies are obtained. The analytical results are compared with the results of the numerical simulation and an excellent agreement is observed. In particular, we report on the effect of a “bunch image,” a radiation pulse is generated in the coaxial area and has the form of the transverse electric field correlating with the longitudinal charge distribution within the bunch. [ABSTRACT FROM PUBLISHER]

Micro-bunched particle beams are used for a wide range of research including wakefield-based particle acceleration and tunable sources of radiation. In all applications, accurate and non-destructive monitoring of the bunch-to-bunch separation is required. With the development of femtosecond lasers, the generation of micro-bunched beams directly from a photocathode becomes routine; however, non-destructive monitoring of the separation is still a challenge. We present the results of proof-of-principle experiments conducted at the Laser Undulator Compact X-ray accelerator measuring the distance between micro-bunches via the amplitude modulation analysis of a monochromatic radiation signal. Good agreement with theoretical predictions is shown; limitations and further improvements are discussed. [ABSTRACT FROM AUTHOR]

A project to construct a compact pre-bunched Free Electron Laser by using a normal conducting photocathode electron gun has been undertaken at Inter University Accelerator Centre (IUAC), New Delhi, India. In this facility, the short laser pulses from a high power laser system will be split into many pulses (2–16) commonly known as ‘Comb beam’ and will strike the photocathode material (metal and semiconductor) to produce electron beam bunches. The electrons will be accelerated up to an energy of ∼8 MeV by a copper cavity operated at a frequency of 2860 MHz and the beam will be injected into a compact, planar permanent undulator magnet to produce THz radiation. The radiation frequency designed to be tuned in the range of 0.15–3 THz by varying the magnetic field of the undulator and/or changing the energy of the electron. The separation of the laser micro-pulses will be varied by adjusting the path length difference to alter the separation of the electron micro-bunches and to maximise the radiation intensity. [ABSTRACT FROM AUTHOR]

The Cherenkov effect is a well-known phenomenon and its properties are widely used in many fields of physics. However, some features of the polarization characteristics of Cherenkov radiation that appears when charged particles pass near azimuthally asymmetric, finite dielectric targets are still poorly studied. This problem is solved in this work. The polarization characteristics of Cherenkov radiation in the case of a rectangular dielectric screen are analyzed using the Stokes approach. Owing to the azimuthal asymmetry of the target, radiation acquires an elliptic polarization whose rotation direction and inclination angle depend both on the direction of radiation propagation and on the dielectric properties of a substance. The results demonstrate that the Cherenkov effect can be used to create sources of elliptically polarized radiation with the controlled direction of polarization rotation. [ABSTRACT FROM AUTHOR]

Transition radiation appearing when a charged particle crosses the interface between two media with different dielectric constants, e.g., a metal-vacuum interface, has been well studied in a wide spectral range. However, primarily, radiation from smooth interfaces has been studied. Transition radiation from conducting gratings (grating transition radiation) is experimentally studied and theoretically analyzed in this work. In this case, it is possible to obtain monochromatic radiation with a tunable frequency depending on the rotation angle of the grating with respect to the electron momentum. Coherent grating transition radiation can be efficiently used as a source of terahertz radiation based on the use of a compact electron accelerator with an energy below 10 MeV and a bunch duration of ≤1 ps. [ABSTRACT FROM AUTHOR]

Quasi-monochromatic X-rays with high brightness have a broad range of applications in fields such as life sciences, bio-, medical applications, and microlithography. One method for generating such X-rays is via inverse Compton scattering (ICS). X-ray generation experiments using ICS were carried out at the superconducting RF test facility (STF) accelerator at KEK. A new beam line, newly developed four-mirror optical cavity system, and new X-ray detector system were prepared for experiments downstream section of the STF electron accelerator. Amplified pulsed photons were accumulated into a four-mirror optical cavity and collided with an incoming 40 MeV electron beam. The generated X-rays were detected using a microchannel plate (MCP) detector for X-ray yield measurements and a new silicon-on-insulator (SOI) detector system for energy measurements. The detected X-ray yield by the MCP detector was 1756.8±272.2 photons/(244 electron bunches). To extrapolate this result to 1 ms train length under 5 Hz operations, 4.60×10 5 photons/1%-bandwidth were obtained. The peak X-ray energy, which was confirmed by the SOI detector, was 29 keV, and this is consistent with ICS X-rays. [ABSTRACT FROM AUTHOR]

Abstract: A compact prototype four-mirror optical cavity is being constructed at KEK-ATF to measure low-emittance electron beams in the damping ring. Four-mirror-resonators reduce the sensitivity to the misalignment of mirrors in comparison to two mirror-resonators. The aspect ratio is important when constructing a compact resonator with a very small beam waist of less than 5μm. The total cavity length of a four-mirror resonator is matched according to the pulse repetition of mode-locked laser oscillator. Minimum beam waist is obtained in the sagittal plane using an IR pulsed laser. The advantage of such types of compact four-mirror-resonators is the total scanning time for measurement of the beam profile is much shorter in comparison to a CW laser wire system. By using a pulsed green laser that has been converted to the second harmonics from an IR pulsed laser, a minimum beam waist that has half the beam waist when using an IR laser oscillator can be obtained. Therefore, it is possible to obtain the beam waist of less than 5μm (σ value) that is required for effective photon–electron collision. We report on the development and performance studies for such types of compact four-mirror laser wire systems. [Copyright &y& Elsevier]

Abstract: Optical cavities have a variety of applications. However, a highly accurate and efficient method for measuring cavity beam waist size is urgently needed. In this article, we propose a simple approach to deduce the optical cavity beam waist size in terms of Gouy phase using a round-trip matrix for n-mirror cavities. Since Gouy phase can be easily and accurately obtained, this approach provides an alternative way of measuring the beam waist size. Experimentally, the effectiveness of this new method has been demonstrated in a 2-mirror cavity and a 4-mirror planar cavity. The results of beam waist size measurement show fairly good agreement with theoretically predicted values. This method is much simpler, and more convenient than other beam waist size measurement methods, e.g. methods based on pinhole photodiode or CCD. [Copyright &y& Elsevier]

A new laser-wire has been installed in the extraction line of the ATF at KEK. It aims at demonstrating that laser-wires can be used to measure micrometre scale beam size. In parallel, studies have been made to specify a laser suitable for the ILC laser-wires. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A novel scheme for the focusing of high-energy leptons in future linear colliders was proposed in 2001 [P. Raimondi and A. Seryi, Phys. Rev. Lett. 86, 3779 (2001)]. This scheme has many advantageous properties over previously studied focusing schemes, including being significantly shorter for a given energy and having a significantly better energy bandwidth. Experimental results from the ATF2 accelerator at KEK are presented that validate the operating principle of such a scheme by demonstrating the demagnification of a 1.3 GeV electron beam down to below 65 nm in height using an energy-scaled version of the compact focusing optics designed for the ILC collider. [ABSTRACT FROM AUTHOR]