The use of Smith-Purcell radiation to measure electrons longitudinal profiles has been demonstrated at several facilities in the picosecond and sub-picosecond range. There is a strong interest for the development of non intercepting longitudinal profile diagnostics for high current proton beams. We present here results of simulations on the expected yield of longitudinal profile monitors using Smith-Purcell radiation for such proton beams.
Smith-Purcell radiation is used in several applications including the measurement of the longitudinal profile of electron bunches. A correct reconstruction of such profile requires a good understanding of the underlying model. We have compared the leading models of Smith-Purcell radiation and shown that they are in agreement within the experimental errors.
The simplest model of the magnetized infinitely thin electron beam is considered. The basic equations that describe the periodic solutions for a self-consistent system of a couple of Maxwell equations and equations for the medium are obtained.
For the grating, which has depth of grooves as a small parameter, the dispersion equation of the Smith-Purcell instability was obtained. It was found that the condition of the Thompson or the Raman regimes of excitation does not depend on beam current but depends on the height of the beam above grating surface. The growth rate of instability in both cases is proportional to the square root of the electron beam current.
In the framework of the earlier derived dispersion equation, we study induced Smith Purcell (SP) radiation of relativistic electron beam in the absence of a resonator. We offer a new method for calculation of coefficients for partial amplitudes in the case of rectangular grating. Using this method we calculate the growth rate of the SPFEL with a rectangular grating. The growth rate dependences on harmonic number, beam current, observation angle and relativistic factor are presented.
Smith-Purcell radiation and Transition Radiation are two radiative phenomenon that occur in charged particles accelerators. For both the emission can be significantly enhanced with sufficiently short pulses and both can be used to measure the form factor of the pulse. We compare the yield of these phenomenon in different configurations and look at their application as bunch length monitors, including background filtering and rejection. We apply these calculations to the specific case of the CLIO Free Electron laser.
The E-203 collaboration is testing a device on FACET at SLAC to measure the longitudinal profile of electron bunches using Smith-Purcell radiation. At FACET the electron bunches have an energy of 20~GeV and a duration of a few hundred femtoseconds. Smith-Purcell radiation is emitted when a charged particle passes close to the surface of a metallic grating. We have studied the stability of the measurement from pulse to pulse and the resolution of the measure depending on the number of gratings used.
Using the pre-bunched electron beam of 5-MeV 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 non…
Measurements of coherent radiation at accelerators typically give the absolute value of the beam profile Fourier transform but not its phase. Phase reconstruction techniques such as Hilbert transform or Kramers Kronig reconstruction are used to recover such phase. We report a study of the performances of these methods and how to optimize the reconstructed profiles.
In Čerenkov and Smith-Purcell free-electron lasers (FELs), a resonant interaction between the electron beam and the co-propagating surface mode can produce copious amount of coherent terahertz (THz) radiation. We perform a three-dimensional (3D) analysis of the surface mode, taking the effect of attenuation into account, and set up 3D Maxwell-Lorentz equations for both these systems. Based on this analysis, we determine the requirements on the electron beam parameters, i.e., beam emittance, beam size and beam current for the successful operation of a Čerenkov FEL.
Measurements of coherent radiation at accelerators typically give the absolute value of the beam profile Fourier transform but not its phase. Phase reconstruction techniques such as Hilbert transform or Kramers Kronig reconstruction are used to recover such phase. We report a study of the performances of these methods and how to optimize the reconstructed profiles.
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.
The Smith Purcell radiation as one promising THz radiation source has been studied for decades. Various structure designs and beam configurations have been proposed. In this paper, we report our recent exploration of Smith Purcell radiation driven by electron beam accelerated by plasma wakefield.
We consider radiation generated by an electron travelling parallel to a planar rectangular silicon grating: Smith-Purcell radiation to the vacuum side, internal Smith-Purcell radiation into the dielectric, and Cherenkov radiation into the dielectric. Internal Smith-Purcell radiation dominates over the other two radiation mechanisms in the range where conventional Smith-Purcell radiation is forbidden. This observation may lead to improved design of contactless particle beam monitors.
Radiation of a charged particle moving parallel to a inhomogeneous surface is considered. Within a single formalism periodic and random gratings are examined. For the periodically inhomogeneous surface we derive new expressions for the dispersion relation and the spectral-angular intensity. In particular, for a given observation direction two wavelengths are emitted instead of one wavelength of the standard Smith-Purcell effect. For a rough surface we show that the main contribution to the radiation intensity is given by surface polaritons induced on the interface between two media. These pola…
Terahertz (THz) radiation occupies a very large portion of the electromagnetic spectrum and has generated much recent interest due to its ability to penetrate deep into many organic materials without the damage associated with ionizing radiation such as x-rays. One path for generating copious amount of tunable narrow-band THz radiation is based on the Smith-Purcell free-electron laser (SPFEL) effect. In this Letter we propose a simple concept for a compact two-stage tunable SPFEL operating in the superradiant regime capable of radiating at the grating's fundamental bunching frequency. We demon…
Coherent Smith-Purcell radiation generated by bunched electron beam in the lamellar metal and dielectric gratings in the millimeter wavelength range was compared theoretically and experimentally. For theoretical estimation a simple model suitable for both dielectric and metal gratings was developed. Experimental comparison was carried out using extracted bunched 6.1 MeV electron beam of the microtron at Nuclear Physics Institute (Tomsk Polytechnic University). Both theoretical estimations and experimental data showed the difference of the radiation characteristics from the lamellar metal and d…
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…
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 a…
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 pla…
Smith-Purcell radiation is generated by a charged particle beam passing close to the surface of a diffraction grating which has a strong dependency of the emitted radiation intensity on the form of the grating profile. For relativistic electron beam, it is important to take into account the number of grating periods in practical SPR setups. In this paper, the theoretical investigations of Smith-Purcell radiation from a three-dimensional bunch of relativistic electrons that moves at constant speed parallel to an electrically perfectly conducting grating with finite rectangular grooves are carri…
The intensity of Smith-Purcell radiation from metallic and dielectric gratings (silicon, silica) is compared in a frequency-domain simulation. The numerical model is discussed and verified with the Frank-Tamm formula for Cherenkov radiation. For 30 keV electrons, rectangular dielectric gratings are less efficient than their metallic counterpart, by an order of magnitude for silicon, and two orders of magnitude for silica. For all gratings studied, radiation intensity oscillates with grating tooth height due to electromagnetic resonances in the grating. 3D and 2D numerical models are compared.
The radiation resulting from the uniform motion of a charged particle near a hemispheric bulge in a metal plane is considered. The description of the radiation process based on the method of images is developed for the case of non-relativistic particle and perfectly conducting target. The spectral-angular and spectral densities of the diffraction radiation on the single bulge (as well as the Smith-Purcell radiation on the periodic string of bulges) are computed. The possibility of application of the developed approach to the case of relativistic incident particle is discussed.
We have considered the possibility of using coherent Smith-Purcell radiation for the single-shot determination of the longitudinal profile of 50 fs (FWHM) long electron bunches. This length is typical for the bunches currently produced by Laser Wakefield Acceleration and is at the limit of what is achievable by alternative techniques, such as Electro-Optic sampling. It is concluded that there are no obstacles, either theoretical or experimental, in the implementation of this technique. A set of three gratings, with periods of 15, 85 and 500 micrometres, will produce detectable energy in the wa…
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 abso…
The diffraction and transition radiation under normal incidence of a particle to the semi-infinite dielectric plate had been considered previously using the eikonal approximation in the transition radiation theory. This approach is valid for the high radiation frequencies domain (where the dielectric permittivity of the plate material is close to unit), including X-ray domain. In the present report this method is applied to the case of oblique incidence of the particle on the plate (but parallel to the plate's edge) as well as to the periodic set of such plates (Smith-Purcell radiation). The o…
We investigate the radiation from a charge rotating around conductors with cylindrical symmetry. First the problem is considered with a charge rotating around a conducting cylinder immersed in a homogeneous medium. The surface charge and current densities induced on the cylinder surface are evaluated. A formula is derived for the spectral-angular density of the radiation intensity. In the second part, we study the radiation for a charge rotating around a diffraction grating on a cylindrical surface with metallic strips parallel to the cylinder axis. The effect of the grating on the radiation i…
We consider the radiation from an electron in flight over a surface wave of an arbitrary profile excited in a plane interface. For an electron bunch the conditions are specified under which the overall radiation essentially exceeds the incoherent part. It is shown that the radiation from the bunch with asymmetric density distribution of electrons in the longitudinal direction is partially coherent for waves with wavelengths much shorter than the characteristic longitudinal size of the bunch.
The Resonant Optical Polarization Radiation (ROPR) in the Smith-Purcell geometry and the one from the inclined grating at the Tomsk synchrotron and 6-MeV microtron have been investigated. The polarization radiation was observed at 4.2o from the 200 MeV electron beam and at 5o from the 6.2 MeV electron beam. Two methods of measurement of ROPR maxima in these two cases have been used. In the first case (the experiment on synchrotron) we have fixed the wavelength of radiation using an optical filter; the orientation dependence of this radiation was measured. In this dependence we have observed tw…
CLIO is a Free Electron Laser based on a thermionic electron gun. In its normal operating mode it delivers electron 8 pulses but studies are ongoing to shorten the pulses to about 1 ps. We report on simulations showing how the pulse can be shortened and the expected signal yield from several bunch length diagnostics (Coherent Transition Radiation, Coherent Smith Purcell Radiation).
In coherent radiation sources (diffraction radiation, Smith-Purcell effect, etc.) based on relativistic electrons passing by a material radiator, the electron self-field is partly shadowed after each part of the radiator over a distance of the order of the formation length g2l. This effect has been investigated on coherent diffraction radiation (DR) by electron bunches. An absorbing half-plane screen was placed at various distances L before a standard DR source (inclined half-plane mirror). The DR intensity was reduced when the screen was at small L and on the same side as the mirror. No reduc…
We outline the fundamental coherent radiation emission processes from a bunched charged particles beam. In contrast to spontaneous emission of radiation from a random electron beam that is proportional to the number of particles, a pre-bunched electron beam can emit spontaneously coherent radiation proportional to the number of particles - squared, through the process of (spontaneous) superradiance (SP-SR) (in the sense of Dicke's). The coherent SP-SR emission of a bunched electron beam can be even further enhanced by a process of stimulated-superradiance (ST-SR) in the presence of a seed inje…
