Coherent Diffraction Radiation Research Articles

On the Effect of Focusing of Coherent Diffraction Radiation by a Semi-Parabolic Target

On the Effect of Focusing of Coherent Diffraction Radiation by a Semi-Parabolic Target

Radiation transport line for Terahertz Coherent Diffraction Radiation at ERL Test Accelerator in KEK

Coherent radiation from a short bunch of electron beams is expected to be utilized as a terahertz (THz) radiation source. We have conducted an investigation on a terahertz source based on coherent diffraction-radiation (CDR) that possesses a unique characteristic of radial polarization and is potentially useful for certain applications. Particularly, with the high repetition beam of an energy-recovery linac, it is possible to achieve a watt-class high power source. We have designed a CDR setup and THz transport line, and subsequently measured the THz beam profile at the end of the line. The results confirmed the preservation of the distinctive characteristics of CDR within the experimental area.

Longitudinal charge distribution measurement of nonrelativistic ion beams using coherent transition radiation

Longitudinal charge profile measurements or bunch shape measurements is a challenge for temporally short non-relativistic bunches. The field profile has a larger longitudinal extent compared to the charge profile for such beams. This affects the ability of field sensing devices such as phase pick-ups or wall current monitors to measure charge distribution. Here we evaluate the feasibility for usage of coherent transition and diffraction radiation from non-relativistic beams for bunch shape measurements.

Bunch Compression Monitor Based on Coherent Diffraction Radiation at European XFEL and FLASH

Bunch Compression Monitor Based on Coherent Diffraction Radiation at European XFEL and FLASH

Method of Nonperturbing Measurements of the Electron Bunch Length Based on Coherent Diffraction Radiation

A simple nonperturbing technique for the electron relativistic bunch diagnostics has been proposed which does not require the use of an interferometer for spectral measurements. The interferometer functions are performed by a slit target in which diffraction radiation is generated. The measurements of electron bunch length are carried out by shifting one half of the target relative to the other. A good agreement with the results of measuring the coherent radiation spectrum of relativistic electron bunches has been obtained.

Multimillijoule terahertz radiation from laser interactions with microplasma waveguides

When a relativistic, femtosecond laser pulse enters a waveguide, the pulse energy is coupled into waveguide optical modes. The longitudinal laser field effectively accelerates electrons along the axis of the channel, while the asymmetric transverse electromagnetic fields tend to expel fast electrons radially outwards. At the exit of the waveguide, the ∼nC, ∼10 MeV electron bunch converts its energy to a ∼10 mJ terahertz (THz) laser pulse through coherent diffraction radiation. In this paper, we present 3D particle-in-cell simulations and theoretical analyses of the aforementioned interaction process. We investigate the process of longitudinal acceleration and radial expulsion of fast electrons, as well as the dependence of the properties of the resulting THz radiation on laser and plasma parameters and the effects of the preplasma. The simulation results indicate that the conversion efficiency of energy can be over 5% if the waveguide length is optimal and a high contrast pump laser is used. These results guide the design of more intense and powerful THz sources.

Diffractive shadowing of coherent polarization radiation

We report on the study of shadowing of electromagnetic fields radiated in the Terahertz (THz) region from two consecutive sources of coherent diffraction and transition radiation. In these conditions, the formation length is predicted to be ≲100 m, and shadowing effects should result in an almost complete suppression of radiated fields within distances of the order of tens of centimeters. We experimentally measured that shadowing effects disappear for distances significantly shorter than those predicted. We propose a new model that explains our experimental observations by taking into account 3D diffraction effects. These findings will have a positive impact on the beneficial use of consecutive radiators both for the generation of intense electromagnetic radiation and for beam diagnostics using coherent polarization radiation from ultra-relativistic charged particles.

Ultra-monochromatic far-infrared Cherenkov diffraction radiation in a super-radiant regime

Nowadays, intense electromagnetic (EM) radiation in the far-infrared (FIR) spectral range is an advanced tool for scientific research in biology, chemistry, and material science because many materials leave signatures in the radiation spectrum. Narrow-band spectral lines enable researchers to investigate the matter response in greater detail. The generation of highly monochromatic variable frequency FIR radiation has therefore become a broad area of research. High energy electron beams consisting of a long train of dense bunches of particles provide a super-radiant regime and can generate intense highly monochromatic radiation due to coherent emission in the spectral range from a few GHz to potentially a few THz. We employed novel coherent Cherenkov diffraction radiation (ChDR) as a generation mechanism. This effect occurs when a fast charged particle moves in the vicinity of and parallel to a dielectric interface. Two key features of the ChDR phenomenon are its non-invasive nature and its photon yield being proportional to the length of the radiator. The bunched structure of the very long electron beam produced spectral lines that were observed to have frequencies upto 21 GHz and with a relative bandwidth of 10–4 ~ 10–5. The line bandwidth and intensity are defined by the shape and length of the bunch train. A compact linear accelerator can be utilized to control the resonant wavelength by adjusting the bunch sequence frequency.

Noninvasive THz spectroscopy for bunch current profile reconstructions at MHz repetition rates

X-ray free-electron lasers based on superconducting accelerator technology deliver ultrashort photon pulses with unprecedented peak brilliance at high repetition rates. Continuous and noninvasive monitoring of the current profile of the electron bunches is essential for the operation and control of the accelerator. Longitudinal diagnostics based on coherent radiation have already shown their potential at various free-electron laser facilities, and the multi-GeV electron beams of x-ray free-electron lasers are powerful sources for the generation of broadband coherent diffraction radiation. We present noninvasive current profile measurements with a few femtoseconds resolution based on spectroscopy of coherent diffraction radiation in the frequency range 0.7--58 THz. The current profiles, reconstructed from the spectroscopic data with an advanced phase retrieval method, are compared with measurement results obtained with a transverse deflecting structure. For the first time, bunch-resolved current profiles have been recorded simultaneously to user operation at European XFEL for all bunches in the bunch train at MHz repetition rates.

Relativistic terahertz radiation generated by direct-laser-accelerated electrons from laser-foil interactions

A novel scheme for generating powerful terahertz (THz) radiation based on laser-solid interactions is proposed. When a $p$-polarized femtosecond laser impinges obliquely on a plane solid target and the target partially blocks the laser energy, surface electrons are extracted out and accelerated by the laser fields, forming a low-divergence electron beam. A half-cycle THz radiation pulse is emitted simultaneously as the beam passes by the edge of the target, due to coherent diffraction radiation. Our particle-in-cell simulations show that the relativistic THz pulse can have an energy of a few tens of millijoule and the conversion efficiency can be over 1$\%$ with existing $\sim$J level femtosecond laser sources.

Diagnostics for Electron Pulse Trains at SwissFEL Obtained by Energy Modulation in a Laser-Driven Dielectric Structure

Electron pulse trains with sub-femtosecond spike length offer novel possibilities to ultra-fast pump-probe experiments in free-electron lasers. The pulse train can be generated by an energy modulation which is converted to temporal bunching in a magnetic chicane. The source of the energy modulation is typically a resonant interaction with a co-propagating laser in a wiggler magnet. Alternatively, the modulation can be obtained by a dielectric laser accelerator (DLA). The implementation of the DLA modulation scheme at SwissFEL would be enabled by using the experimental chamber installed in the framework of the Accelerator-on-a-Chip International Program (ACHIP) and a magnetic chicane placed afterwards. In this contribution, we will focus on electron beam diagnostics for the DLA-modulated and compressed electron bunch train. Our simulation results predict that measuring the energy spread of the modulated beam and coherent diffraction radiation emitted from a foil with a hole provides a precise tool to characterize and optimize the laser-electron modulation. An absolute measurement of the longitudinal phase-space with a temporal rms-resolution of 350 as is possible with the planned X-band transverse deflecting cavity in Athos.

First demonstration of coherent resonant backward diffraction radiation for a quasi-monochromatic terahertz-light source

We proposed coherent resonant backward diffraction radiation (CRBDR), which generates wavelength-tunable quasi-monochromatic lights using a compact diffractor assembly in an accelerator facility of high-energy electron beams, as a unique intense terahertz (THz) light source. Superimposing the coherent backward diffracted radiation emitted by periodically arranged hollow diffractors, it is possible to amplify the frequency components satisfying a resonant condition, and make the radiation monochromatic. We demonstrated the CRBDR using the L-band linac at the Institute for Integrated Radiation and Nuclear Science at Kyoto University. It was observed that the coherent backward diffraction radiation was amplified more than three times at a frequency which was the fundamental resonant frequency in the CRBDR theory. Moreover, the number of diffractors at the saturation of the radiation power was consistent with the number estimated from the electron distribution in a bunch. The experimental results show that the CRBDR is useful as a quasi-monochromatic light source in the THz band.

Noninvasive bunch length measurements exploiting Cherenkov diffraction radiation

We present the observation and the detailed investigation of coherent Cherenkov diffraction radiation (CChDR) in terms of spectral-angular characteristics. Electromagnetic simulations have been performed to optimize the design of a prismatic dielectric radiator and the performance of a detection system with the aim of providing longitudinal beam diagnostics. Successful experimental validations have been organized on the CLEAR and the CLARA facilities based at CERN and Daresbury laboratory respectively. With ps to sub-ps long electron bunches, the emitted radiation spectra extend up to the THz frequency range. Bunch length measurements based on CChDR have been compared to longitudinal bunch profiles obtained using a radio frequency deflecting cavity or coherent transition radiation (CTR). The retrieval of the temporal profile of both Gaussian and non-Gaussian bunches has also been demonstrated. The proposed detection scheme paves the way to a new kind of beam instrumentation, simple and compact for monitoring short bunches of charged particles, particularly well-adapted to novel accelerator technologies, such as dielectric and plasma accelerators. Finally, CChDR could be used for generating intense THz radiation pulses at the MW level in existing radiation facilities, providing broader opportunities for the user community.

Stimulated emission of THz coherent diffraction radiation in an optical cavity by a multibunch electron beam

Accelerator-based terahertz (THz) radiation has been expected to realize a highpower broad-band source. Employing a low-emittance and short-bunch electron beam at a high repetition rate, a scheme to resonantly excite optical cavity modes of THz spectrum range via coherent diffraction radiation has been proposed. The confocal cavity design is the special case that resonance conditions of all the eigen modes coincide, resulting in realizing broad-band excitation. But in general cases of non-confocal cavities, the resonance condition depends on the mode, and the resonance peak becomes wide and weak. We performed an experiment with a non-confocal cavity as a follow-up experiment of that we have done with a confocal cavity. The result confirmed that the confocal design is the key for a broad-band source.

Coherent Diffraction Radiation of Relativistic Terahertz Pulses from a Laser-Driven Microplasma Waveguide.

We propose a method to generate isolated relativistic terahertz (THz) pulses using a high-power laser irradiating a microplasma waveguide (MPW). When the laser pulse enters the MPW, high-charge electron bunches are produced and accelerated to ∼100 MeV by the transverse magnetic modes. A substantial part of the electron energy is transferred to THz emission through coherent diffraction radiation as the electron bunches exit the MPW. We demonstrate this process with three-dimensional particle-in-cell simulations. The frequency of the radiation is determined by the incident laser duration, and the radiated energy is found to be strongly correlated to the charge of the electron bunches, which can be controlled by the laser intensity and microengineering of the MPW target. Our simulations indicate that 100mJ level relativistic-intense THz pulses with tunable frequency can be generated at existing laser facilities, and the overall efficiency reaches 1%.

Measurement of the Polarization Characteristics of Coherent Diffraction Radiation in the Subterahertz Range

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).

Diffraction and transition radiation on conducting sphere and hemispherical bulge in conducting plane: Further development

The radiation resulting from the uniform motion of a charged particle near (or through) a perfectly conducting sphere as well as near a hemispheric bulge on a metal plane has been considered previously using the method of images known from electrostatics. Here we discuss three topics: polarization of the diffraction radiation on the sphere as well as on the hemispherical bulge, transition radiation on the hemispherical bulge, and the coherent diffraction radiation from a bunch of charged particles.

High-efficiency broadband THz emission via diffraction-radiation cavity

Accelerator-based terahertz (THz) radiation has been expected to realize a high-power broadband source. Employing a low-emittance and short-bunch electron beam at a high repetition rate, a scheme of coherent diffraction-radiation in an optical cavity layout is proposed. The scheme's stimulated radiation process between bunches can greatly enhance the efficiency of the radiation emission. We performed an experiment with a superconducting linac constructed as an energy recovery linac (ERL) test facility. The electron beam passes through small holes in the cavity mirrors without being destroyed. A sharp THz resonance signal, which indicates broadband stimulated radiation correlated with beam deceleration, was observed while scanning the round-trip length of the cavity. This observation proves the efficient beam-to-radiation energy conversion due to the stimulated radiation process.

Stimulated Excitation of an Optical Cavity by a Multibunch Electron Beam via Coherent-Diffraction-Radiation Process.

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.

First indication of the coherent unipolar diffraction radiation generated by relativistic electrons

As is generally known, the integral of the electric field strength over all time for usual (bipolar) radiation is zero. The first demonstration of the possibility of unipolar radiation generation has been considered theoretically by Bessonov in 1981 [E.G. Bessonov, Zh. Eksp. Teor. Fiz. 80 (1981) 852]. According to this work, the unipolar radiation (or strange electromagnetic waves) is radiation for which the integral of the electric field strength over the entire duration of a pulse differs significantly from zero. Later, several theoretical papers devoted to this phenomenon have appeared in the literature, where authors investigated mainly synchrotron radiation. However, despite the critical interest, the experimental investigations ignored this effect. In this paper we present results of the first experimental investigation of the unipolar radiation generated by a relativistic electron beam. To detect the unipolar radiation the detector that is sensitive to the selected direction of the electric field strength has been elaborated and tested. We used a designed detector to observe the coherent backward diffraction radiation appearing when a bunched electron beam travels in the vicinity of a flat conductive target. The asymmetry of the electric field strength of the coherent backward diffraction radiation has been demonstrated.