Relativistic Electron Bunches Research Articles

Nonlinear theory of wakefield excitation in a rectangular multizone dielectric resonator

A nonlinear self-consistent theory has been constructed and used to investigate numerically the wakefield excitation in multilayered dielectric resonators by relativistic electron bunches. Analytical expressions for solenoidal and potential components of an excited electromagnetic field have been derived. The excitation of a five-zone dielectric resonator by relativistic electron bunches was numerically investigated and comparison was made between the longitudinal distribution of an axial electric field and the results obtained previously for a corresponding problem in the waveguide formulation. The necessity of optimizing geometrical parameters of the resonator to reduce mode amplitudes nonresonant with a bunch, and to obtain a symmetric distribution of the longitudinal electric field component in the drive and accelerating channels, has been demonstrated.

Analysis and optimization of the performance of the electro-optic technique used for ultrashort relativistic electron bunches

The electro-optic (EO) technique used in the terahertz spectral technology is described. Response functions—which are used to describe the EO detecting efficiency—of five types of cubic EO crystals are calculated. Based on these results, the choice of different crystal thicknesses is discussed. In the EO detecting procedure, certain factors, such as reflection, dispersion, absorption and velocity mismatch, may distort the temporal profile of the electric field being detected; therefore, these factors are calculated. The relationship between the field distortion and the bunch duration as well as the crystal thickness is considered.

Ion Response to Relativistic Electron Bunches in the Blowout Regime of Laser-Plasma Accelerators

The ion response to relativistic electron bunches in the so called bubble or blowout regime of a laser-plasma accelerator is discussed. In response to the strong fields of the accelerated electrons the ions form a central filament along the laser axis that can be compressed to densities 2 orders of magnitude higher than the initial particle density. A theory of the filament formation and a model of ion self-compression are proposed. It is also shown that in the case of a sharp rear plasma-vacuum interface the ions can be accelerated by a combination of three basic mechanisms. The long time ion evolution that results from the strong electrostatic fields of an electron bunch provides a unique diagnostic of laser-plasma accelerators.

Inverse Compton scattering of coherent synchrotron radiation in an energy recovery linac

We propose inverse Compton scattering (ICS) of coherent synchrotron radiation (CSR) from a relativistic short electron bunch in energy recovery linacs (ERL) as a high-flux subpicosecond x-ray and $\ensuremath{\gamma}$-ray source. An advantage of the CSR scheme over a conventional ICS source is that no external laser is required, and synchronization between CSR pulses and electron bunches is obtained automatically. Moreover, higher-flux x rays can be generated from the ICS of CSR in an ERL operated at a high repetition rate, 100 MHz to 1.3 GHz. Using parameters of the Compact ERL at KEK, $1\ifmmode\times\else\texttimes\fi{}{10}^{13--14}\text{ }\text{ }\mathrm{phs}/\mathrm{s}$ b.w. 10% (the number of photons ${\mathrm{pulse}}^{\ensuremath{-}1}$ bandwidth ${\mathrm{unit}}^{\ensuremath{-}1}$) x ray with a 100 fs--1 ps pulse duration can be obtained, for an energy range from 0.04 to 4 keV. In the case of a 5-GeV ERL, $\ensuremath{\gamma}$ rays with energy around tens of MeV are generated with $1\ifmmode\times\else\texttimes\fi{}{10}^{8}\text{ }\text{ }\mathrm{phs}/\mathrm{pulse}$ b.w. 10% at a repetition rate of several hundreds of MHz.

Nonlinear interaction of charged particles with nonplane counterpropagating laser pulses of relativistic intensities

The nonlinear threshold phenomena of particle reflection and capture of electrons in the induced Compton process that have previously been revealed in the case of plane monochromatic counterpropagating waves, take place also with the actual nonplane laser pulses of ultrashort duration and lead to particle acceleration. In contrast to analogous phenomena in the induced Cherenkov and undulator processes, the Compton reflection-capture mechanism with laser pulses of relativistic intensities practically may be realized for arbitrary initial energies of particles. The acceleration effect for particles initially in rest is explored numerically, taking into account the significance of this case connected with the relativistic electron bunches of high densities, which currently may be realized by relativistic lasers on the ultrathin solid foils where the electrons initially are almost in rest.

Preliminary studies on fast particle diagnostics for the future fs-laser facility at PALS

The design, construction and development of proper diagnostics systems for multi-MeV proton/ion and relativistic electron beams, typically generated by fs-laser plasmas, are under study in our laboratories. Ultra-fast plasma ion detection through classical ion collectors (ICs) in time-of-flight (TOF) configuration, shielded by a thin metallic absorber, has been theoretically studied and preliminarily tested using the 300 ps, kJ-class iodine laser operating at PALS Research Centre, in order to cut the long photo-peak contribution and analyse the ultra-fast particle signal. Processing of obtained experimental IC-TOF data, including maximum proton/ion energy estimations, is reported. Design, modelling and simulations for relativistic electron bunch detection, by means of a proper magnetic electron spectrometer, are also included. The effort is mainly devoted to match the diagnostics systems with the peculiarity of fast particle streams which could be generated by the 20 TW Ti:sapphire laser (40 fs, 1 J, 10 Hz) recently installed at PALS.

Numerical and theoretical study of the generation of extreme ultraviolet radiation by relativistic laser interaction with a grating

The generation of harmonics by the interaction of a femtosecond, relativistic intensity laser pulse with a grating of subwavelength periodicity was studied numerically and theoretically. For normal incidence, strong, coherent emission at the wavelength of the grating period and its harmonics is obtained, nearly parallel to the target surface, due to relativistic electron bunches emanating from each protuberance. For oblique incidence (30°), only even harmonics of the grating periodicity are seen, but with an even higher intensity. This is due to constructive interference of the emission from the grating protuberances. The emission along the grating surface is composed of trains of attosecond pulses; therefore there is no need to use a filter. An efficiency greater than 10−4 is obtained for the 24th harmonic. The conversion efficiency is fairly constant when the similarity parameter S=ne/(a0nc)(∝neλL/IL1/2) is held fixed, and is optimum when S≃4. Here, ne and nc are the electron density and the critical density; a0=eEL/(meωLc) is the quiver momentum in the laser field EL normalized to mec.

High-rate deformation and spall fracture of hadfield steel under action of high-current nanosecond relativistic electron beam

Features of the plastic deformation and dynamic spall fracture of Hadfield steel under conditions of shock wave loading at a straining rate of ∼106 s−1 have been studied. The shock load (∼30 GPa, ∼0.2 μs) was produced by pulses of a SINUS-7 electron accelerator, which generated relativistic electron bunches with an electron energy of up to 1.35 MeV, a duration of 45 ns, and a peak power on the target of 3.4 × 1010 W/cm2. It is established that the spalling proceeds via mixed viscous-brittle intergranular fracture, unlike the cases of quasi-static tensile and impact loading, where viscous transgranular fracture is typical. It is shown that the intergranular character of the spall fracture is caused by the localization of plastic deformation at grain boundaries containing precipitated carbide inclusions.

Upconversion of a relativistic Coulomb field terahertz pulse to the near infrared

We demonstrate the spectral upconversion of a unipolar subpicosecond terahertz (THz) pulse, where the THz pulse is the Coulomb field of a single relativistic electron bunch. The upconversion to the optical allows remotely located detection of long wavelength and nonpropagating components of the THz spectrum, as required for ultrafast electron bunch diagnostics. The upconversion of quasimonochromatic THz radiation has also been demonstrated, allowing the observation of distinct sum- and difference-frequency mixing components in the spectrum. Polarization dependence of first and second order sidebands at ωopt±ωTHz, and ωopt±2ωTHz, respectively, confirms the χ(2) frequency mixing mechanism.

Emission of relativistic electron bunches in a circular diaphragmatic waveguide

The longitudinally uniform circular diaphragmatic waveguide (CDW) was considered. Expressions for determining the loaded Q-factor of lossy (QL1) and lossless (QL10) cells and the loaded Q-factor QL of a section of length l were derived. An expression for determining the electrical amplitude of the radiation field produced by a relativistic bunch with charge q, moving along the axis of CDW with series resistance Rs was derived. The electron beam energy, radiation power, and electronic efficiency were calculated.

Relativistic Attosecond Electron Bunches from Laser-Illuminated Droplets

The generation of relativistic attosecond electron bunches is observed in three-dimensional, relativistic particle-in-cell simulations of the interaction of intense laser light with droplets. The electron bunches are emitted under certain angles which depend on the ratios of droplet radius to wavelength and plasma frequency to laser frequency. The mechanism behind the multi-MeV attosecond electron bunch generation is investigated using Mie theory. It is shown that the angular distribution and the high electron energies are due to a parameter-sensitive, time-dependent local field enhancement at the droplet surface.

Pump and amplification dynamics of gamma rays in a nuclear medium with the hidden population inversion

The features of the pump dynamics of isomeric nuclei excited by X-rays of a repetitively pulsed relativistic electron beam followed by the production of a medium with the negative absorption of gamma quanta are analysed. In the extended nuclear medium, the pump excites a travelling hidden-population-inversion wave with the anisotropic gamma amplification, which becomes positive in the case of the excess over the critical pump parameter equal to the product of the peak spectral power density of the X-ray source and the relative duration of an ultrashort relativistic electron bunch. In the alternative scheme with orthogonal directions of pumping X-rays and a flux of amplified gamma quanta, the absence of the amplification anisotropy opens up the possibility for constructing a standard two-mirror resonator with Bragg single-crystal reflectors. The critical peak value of the spectral pump power density is compared with the known characteristics of relativistic-electron X-ray sources by examples of some nuclides.

Energy spectra from electromagnetic fields generated by ultra-relativistic charged bunches in a perfectly conducting cylindrical beam pipe

The spectrum of electromagnetic fields satisfying perfectly conducting boundary conditions in a segment of a straight beam pipe with a circular cross-section is discussed as a function of various source models. These include charged bunches that move along the axis of the pipe with constant speed for which an exact solution to the initial-boundary value problem for Maxwell's equations in the beam pipe is derived. In the ultra-relativistic limit all longitudinal components of the fields tend to zero and the spectral content of the transverse fields and average total electromagnetic energy crossing any section of the beam pipe are directly related to certain properties of the ultra-relativistic source. It is shown that for axially symmetric ultra-relativistic bunches interference effects occur analogous to those that occur due to coherent synchrotron radiation in cyclic machines despite the fact that in this limit the source is not accelerating. The results offer an analytic description of the fields showing how enhanced spectral behaviour depends on the geometry of the source, its location in the beam pipe and the details of the stochastic distribution of the source structure. The results are illustrated for different situations associated with the motion of on-axis ultra-relativistic bunches. The field energy spectra associated with a source containing identically charged ultra-relativistic pulses, each with individual longitudinal Gaussian profiles distributed according to a uniform probability distribution with compact support, is compared with that generated by charged bunches containing a distribution with 2n + 1 peaks in a region with compact support (modelling micro-bunches). These results are of relevance for the experimental determination of properties of the longitudinal charge distribution of short relativistic electron bunches with micro-structure in straight segments of a beam pipe, from observation of the associated electromagnetic energy spectra.

Numerical simulation of harmonic generation by relativistic laser interaction with a grating

The interaction of a femtosecond relativistic intensity laser pulse with a grating of subwavelength periodicity was simulated numerically. Strong coherent emission at the wavelength of the grating period and its harmonics was seen, nearly parallel to the target surface, due to relativistic electron bunches emanating from each protuberance. Normal and oblique incidence (30 degrees ) gave rise to trains of attosecond pulses and an efficiency greater than 10;{-4} was obtained for the 24;{th} harmonic (lambda approximately 16.7 nm) . Similarity theory gives optimum conditions for harmonic emission.

Excitation of wake fields by lengthy electron bunches in a dielectric resonator

Theoretical research and numerical simulation of the wake-field excitation in a dielectric resonator by a regular sequence of relativistic electron bunches is carried out. A nonlinear system of equations for the multimode excitation of the wake field in the dielectric resonator is derived. The numerical simulation is carried out, and the dependences of the maximum amplitude of the longitudinal electric field on the bunch length and charge are investigated and physically interpreted.

Characteristics of relativistic electron mirrors generated by an ultrashort nonadiabatic laser pulse from a nanofilm

For controllable generation of an isolated attosecond relativistic electron bunch [relativistic electron mirror (REM)] with nearly solid-state density, we proposed [V. V. Kulagin, Phys. Rev. Lett. 99, 124801 (2007)] to use a solid nanofilm illuminated normally by an ultraintense femtosecond laser pulse having a sharp rising edge (nonadiabatic laser pulse). In this paper, the REM characteristics are investigated in a regular way for a wide range of parameters. With the help of two-dimensional (2D) particle-in-cell (PIC) simulations, it is shown that, in spite of Coulomb forces, all of the electrons in the laser spot can be synchronously accelerated to ultrarelativistic velocities by the first half-cycle of the field, which has large enough amplitude. For the process of the REM generation, we also verify a self-consistent one-dimensional theory, which we developed earlier (cited above) and which takes into account Coulomb forces, radiation of the electrons, and laser amplitude depletion. This theory shows a good agreement with the results of the 2D PIC simulations. Finally, the scaling of the REM dynamical parameters with the field amplitude and the nanofilm thickness is analyzed.

Generation of Attosecond X-ray and gamma-ray via Compton backscattering

The generation of an isolated attosecond gamma-ray pulse utilizing Compton backscattering of a relativistic electron bunch has been investigated. The energy of the electron bunch is modulated while the electron bunch interacts with a co-propagating few-cycle CEP (carrier envelope phase)-locked laser in a single-period wiggler. The energy-modulated electron bunch interacts with a counter-propagating driver laser, producing Compton back-scattered radiation. The energy modulation of the electron bunch is duplicated to the temporal modulation of the photon energy of Compton back-scattered radiation. The spectral filtering using a crystal spectrometer allows one to obtain an isolated attosecond gamma-ray.

High-Power Picosecond Pulse Recirculation for Inverse Compton Scattering

In the next generation of linear colliders, inverse Compton scattering (ICS) of intense laser pulses on relativistic electron bunches will enable a mode of operation based on energetic γe and γγ collisions, with a significant complementary scientific potential. The efficiency of γ -ray generation via ICS is constrained by the Thomson scattering cross section, resulting in typical laser photon-to- γ efficiencies of <10 −9 . Furthermore, repetition rates of the state-of-art high-energy short-pulse lasers are poorly matched with those available from electron accelerators. Laser recirculation has been proposed as a method to address those limitations, but has been limited to only small pulse energies and peak powers. We propose and experimentally demonstrate an alternative, non-interferometric method for laser pulse recirculation that is uniquely capable of recirculating short pulses with energies exceeding 1 J [ I. Jovanovic, M. Shverdin, D. Gibson, and C. Brown, Nucl. Instrum. Methods A 578 160 (2007)]. ICS of recirculated Joule-level laser pulses is compatible with the proposed pulse structure for ILC and has a potential to produce unprecedented peak and average γ -ray brightness in the next generation of sources.

Synchronization of wakefield modes in the dielectric resonator

Multimode excitation of a wakefield in a planar dielectric resonator by a regular sequence of relativistic electron bunches is studied analytically and numerically. It is shown that, if the wakefield being excited consists of a large number of radial modes, the fields from the regular sequence of the bunches add together. Conditions under which this sequence provides synchronization of the dielectric resonator modes are found. Acceleration of a test bunch in the wakefield of the bunch sequence is studied.

Transverse dynamics and bunch-to-bunch energy exchange in a dielectric-filled accelerating structure

The self-consistent transverse dynamics of high-current relativistic electron bunches used for generating wakefields in multiple-bunch schemes of wakefield acceleration, which are applied in dielectric-filled structures, is studied. The flight range of the electron bunch under no-focusing conditions and the energy transferred to the bunch being accelerated in schemes with profiled (nonuniform) and uniform distributions of the charge over a sequence of generator bunches are determined. Requirements are formulated for a focusing system in which the profiled distribution offers an advantage over the uniform distribution for the efficiency of energy transfer from generator bunches to that being accelerated.