Wakefield Excitation Research Articles

Design and characterisation of frequency selective conductive materials for electromagnetic fields control

To prevent the electromagnetic (EM) wakefields excitation, protect detectors from damage at a range of installations and facilities including particle accelerators the EM field control is required. Conductive foils or wires providing EM protection and required thermal and mechanical properties are normally used. We suggest novel composite materials with uniquely designed frequency selective conductivity enabling them to overcome the properties of the conventional materials, protect from EM fields and supress undesirable phenomena. Theoretical and experimental investigations are carried out and the conductivity of designed and composite (dual-layer) aluminium/graphene metamaterials as well as graphene and aluminium foils is studied. The EM properties of these materials are compared, and conditions of full and partial electromagnetic transparency are discussed. Results observed allow engineering materials capable of EM field control, instability suppression including those observed in high-intensity particle accelerators and enabling control of an EM field generating media including relativistic charge particle beams.

Transformer Ratio at Wakefield Excitation by Train of Electron Bunches with Linear Growth of Current in Dielectric Resonator Electron–Positron Collider

The efficiency of electron bunch acceleration by wakefield, excited in two-beam electron-positron dielectric resonator collider by train of electron bunches is determined by transformer ratio (TR). The train of homogeneous bunches, in which current grows linearly along a train, is considered. The length of homogeneous bunch equals to half of excited wavelength 0.5λ. Interval between bunches is ρλ, p = 1, 2,… The conditions have been formulated, when the wakefield pulses from all electron bunches are added. Bunch is injected in dielectric resonator, when the back wavefront of the wakefield pulse is on the bunch-injection boundary of the resonator/cavity. Bunch leaves the resonator, when the first wavefront of wakefield pulse is on the end of the resonator. The conditions for a large TR have been formulated. The wakefield and TR have been derived after N bunch. The computer simulation has been performed taking into account wakefield pulse dispersion spreading and electron bunch transition radiations. The results of analytical investigation are compared with the results of computer simulation. It has been shown that the dependence TR = 2N is performed. Advantage of this mode with large transformer ratio that some injected bunch with decreased current becomes witness. When this bunch is injected it gets in maximum accelerating field.The infinite periodical mode of accelerator operation has been proposed. In this mode, witness-bunches and driver-bunches are injected according to periodic mode. In the infinite periodical mode, short shaped trains of driver-bunches are alternated with witnesses. Large transformer ratio and identical decelerating field for all driver-bunches are provided by this long periodic train.

MAXIMUM TRANSFORMER RATIO AT WAKEFIELD EXCITATION AT BLOWOUT REGIME IN PLASMA BY ELECTRON BUNCH UNDER SEMI-GAUSSIAN CHARGE DISTRIBUTION

Using 2d3v code LCODE, the numerical simulation of nonlinear wakefield excitation in plasma by shaped relativistic electron bunch with charge distribution, which increases according to Gaussian charge distribution up to the maximum value, and then decreases sharply to zero, has been performed. Transformer ratio, as the ratio of the maximum accelerating field to the maximum decelerating field inside the bunch, and accelerating the wakefield have been investigated taking into account nonlinearity of the wakefield. The dependence of the transformer ratio and the maximum accelerating field on the length of the bunch was investigated with a constant charge of the bunch. It was taken into account that the length of the nonlinear wakefield increases with increasing length of the bunch. It is shown that the transformer ratio reaches its maximum value for a certain length of the bunch. The maximum value of the transformer ratio reaches six as due to the profiling of the bunch, and due to the nonlinearity of the wakefield.

EXCITATION OF THE WAKE WAVES BY A LASER PULSE IN ION DIELECTRICS

The process of excitation of Cherenkov electromagnetic radiation by a laser pulse in ion dielectric waveguide is investigated. Nonlinear electric polarization in isotropic ion dielectric medium and, accordingly, polarization charges and currents induced by a ponderomotive force of a laser pulse are determined. Frequency spectra of the excited wakefields in the infrared and microwave frequency ranges are obtained. The spatio-temporal structure of the wakefield in ion dielectric waveguide is obtained and studied. It is shown that the excited field consists of a potential polarization electric field, as well as a set of eigen electromagnetic waves of ion dielectric waveguide.

Semi-Lagrangian Vlasov simulation of the interaction between femtosecond chirped and double laser pulses with a thin plasma slab

The wakefield generation and electrons acceleration in the interaction of femtosecond chirped and double laser pulses with an underdense thin plasma slab is examined by numerical solution of the relativistic Vlasov-Maxwell system of equations using the backward semi-Lagrangian method (BSL). Unlike particle-in-cell methods, the BSL method is free of numerical noise and provides an accurate representation of the distribution function, even in very low density regions of phase space where acceleration and trapping occur. The effect of density fluctuations in low density regions of phase space are important, since they affect the acceleration processes over the following interactions. Therefore, a detailed structure of the distribution function is needed to investigate this process in these regions, which only can be obtained by Vlasov codes. Excitation of wakefields by chirped, chirp free, and double pulse lasers in an underdense plasma slab are simulated using the BSL code and the results are compared. It is shown that by making use of chirped laser pulses, the mean kinetic energy of plasma electrons can be enhanced up to 40% compared with the chirp free case. The investigation of wakefield generation by double laser pulses with a time repetition equal to one plasma period, exhibits amplification of wakefield amplitude after the entrance of the second laser pulse into plasma slab.

Laser wakefield generation by R, L, X and O modes in magnetized plasma

Short and intense laser pulse propagating in a plasma produces an electrostatic wakefield that is widely used in laser acceleration of charged particles. Amplitude of the wakefield depends on several factors, including the laser pulse polarization. Magnetized plasma is anisotropic for various laser polarization and in different condition, different modes can propagate in plasma. In this paper, we investigate the propagation conditions of each modes and with finding the governing equations, the excitation of wakefield due to the laser pulse with the different modes, including circular (R and L modes), elliptic (X mode) and linear (L mode) polarizations, will be determined. By using the forth-order Runge-Kutta algorithm, the differential equations, simultaneously have been solved. The results show that, the amplitude of the wakefield depends not only on the laser mode but also on the plasma density, external magnetic field and laser frequency.

CHERENKOV RADIATION OF THE RELATIVISTIC ELECTRON BUNCH IN ION DIELECTRIC MEDIA

The process of wakefields excitation by the relativistic electron bunch in dielectric media with ion type of chemical bond is studied. The spatio-temporal structure of the excited wakefield in ion dielectric waveguide is obtained and investigated. It is shown that the excited wakefield in the infrared and longer wavelength ranges consists of the field of longitudinal optical phonons and Cherenkov radiation as a set of eigen electromagnetic waves of the dielectric waveguide.

WAKEFIELDS AND DRIVE ELECTRON BUNCHES DYNAMICS IN THREE-ZONE DIELECTRIC RESONATOR AT ASYMMETRICAL INJECTION

A numerical study of the wakefield excitation in a rectangular dielectric-loaded resonator by a sequence of relativistic electron bunches in the case of non-axial injection is carried out. The effect of the shift of injected bunches on the particle dynamics, as well as on the spatial distribution of the components of the bunch-excited fields, is studied. The current losses of a sequence of relativistic electron bunches due to the particles deposition on the surface of the dielectric is determined.

Effect of external magnetic field on wakefield generation in underdense plasma slab using backward semi-Lagrangian Vlasov code

Laser wakefield excitation in the interaction of femtosecond laser pulse with an underdense thin magnetized plasma slab is studied by one dimensional relativistic Vlasov-Maxwell system of equations. The interaction is modeled by relativistic Vlasov equation in propagation direction of laser pulse and fluid equations in transverse direction. Backward semi-Lagrangian method (BSL) is used for numerical solution of Vlasov equation. Generation of wakefields by a right and left handed circularly polarized (RCP) and (LCP) laser pulses in the interaction with nonmagnetized and magnetized plasma slab are examined and compared. Two directions are considered for external magnetic field, parallel and antiparallel to direction of laser pulse propagation. The results show that applying magnetic field along (opposite to) the propagation direction of RCP (LCP) laser pulse increases amplitude of density steepening and consequently wakefields amplitude, and mean kinetic energy of electrons compared with nonmagnetized plasma. The results are in complete agreement with previous analytical and numerical reports.

Polarized electron acceleration in beam-driven plasma wakefield based on density down-ramp injection.

We investigate the precession of electron spins during beam-driven plasma-wakefield acceleration based on density down-ramp injection by means of full three-dimensional (3D) particle-in-cell (PIC) simulations. A relativistic electron beam generated via, e.g., laser wakefield acceleration, serves as the driving source. It traverses the prepolarized gas target and accelerates polarized electrons via the excited wakefield. We derive the criteria for the driving beam parameters and the limitation on the injected beam flux to preserve a high degree of polarization for the accelerated electrons, which are confirmed by our 3D PIC simulations and single-particle modeling. The electron-beam driver is free of the prepulse issue associated with a laser driver, thus eliminating possible depolarization of the prepolarized gas due to ionization by the prepulse. These results provide guidance for future experiments towards generating a source of polarized electrons based on wakefield acceleration.

CHERENKOV RADIATION OF A LASER PULSE IN ION DIELECTRICS

The process of excitation of Cherenkov electromagnetic radiation by a laser pulse in ion dielectric waveguide is investigated. Nonlinear electric polarization in isotropic ion dielectric medium and, accordingly, polarization charges and currents induced by a ponderomotive force of a laser pulse are determined. Frequency spectra of the excited wakefields in the infrared and microwave frequency ranges are obtained. The spatiotemporal structure of the wakefield in ion dielectric waveguide is obtained and studied. It is shown that the excited field consists of a potential polarization electric field, as well as a set of eigen electromagnetic waves of ion dielectric waveguide.

OPTIMAL RESONANT ASYMPTOTICS OF WAKEFIELD EXCITATION IN PLASMA BY NON-RESONANT SEQUENCE OF ELECTRON BUNCHES

Resonant asymptotics of wakefield excitation in plasma by non-resonant sequence of relativistic electron bunches has been numerically simulated. It has been shown that in resonant asymptotics at optimal parameters the wakefield is excited with the maximum growth rate and the amplitude of the excited wakefield is the largest.

CHERENKOV RADIATION OF THE ELECTRON BUNCH IN DIELECTRIC MEDIA WITH FREQUENCY DISPERSION

Excitation of the Cherenkov electromagnetic radiation by a relativistic electron bunch in a dielectric waveguide is considered taking into account the frequency dispersion of the dielectric permittivity. Electric polarization in an isotropic dielectric medium and, accordingly, polarization charges and currents induced by the Coulomb electric field of a relativistic electron bunch are determined. The spatial-temporal structure of the excited wakefield in a dielectric waveguide is obtained and investigated. It is shown that the excited field consists of a potential polarization electric field and a set of eigen electromagnetic waves of the dielectric waveguide.

Wakefield excitation by a powerful sub-nanosecond 28.6 GHz microwave pulse propagating in a plasma filled waveguide

High-power microwave pulse generation (∼1.2 GW, ∼0.4 ns, 28.6 GHz) by a super-radiant backward wave oscillator (SR-BWO) and the feasibility of wakefield-excitation with this pulse in a plasma-filled waveguide are presented. The SR-BWO is driven by an electron beam (∼280 keV, ∼1.5 kA, ∼5 ns) generated in a magnetically insulated foilless diode and propagating through a slow-wave structure in a guiding magnetic field of 8 T. The plasma produced by an array of flashboards filling a cylindrical wire-array waveguide attached at the exit of the SR-BWO is also characterized. 1D and 3D numerical simulations demonstrate that for the experimental parameters of the microwave pulse and the flashboard plasma filling the waveguide, a wakefield forms accompanied by significant periodic density modulations such that their radial location and depth can be controlled by the waveguide radius, plasma density, and microwave power.

Passive Ballistic Microbunching of Nonultrarelativistic Electron Bunches Using Electromagnetic Wakefields in Dielectric-Lined Waveguides.

Temporally modulated electron beams have a wide array of applications ranging from the generation of coherently enhanced electromagnetic radiation to the resonant excitation of electromagnetic wakefields in advanced-accelerator concepts. Likewise producing low-energy ultrashort microbunches could be useful for ultrafast electron diffraction and new accelerator-based light-source concepts. In this Letter we propose and experimentally demonstrate a passive microbunching technique capable of forming a picosecond bunch train at ∼6 MeV. The method relies on the excitation of electromagnetic wakefields as the beam propagates through a dielectric-lined waveguide. Owing to the nonultrarelativistic nature of the beam, the induced energy modulation eventually converts into a density modulation as the beam travels in a following free-space drift. The modulated beam is further accelerated to ∼20 MeV while preserving the imparted density modulation.

Improvement of Properties of Self-Injected and Accelerated Electron Bunch by Laser Pulse in Plasma, Using Pulse Precursor

The accelerating gradients in conventional linear accelerators are currently limited to ~100 MV/m. Plasma-based accelerators have the ability to sustain accelerating gradients which are several orders of magnitude greater than that obtained in conventional accelerators. Due to the rapid development of laser technology the laser-plasma-based accelerators are of great interest now. Over the past decade, successful experiments on laser wakefield acceleration of electrons in the plasma have confirmed the relevance of this acceleration. Evidently, the large accelerating gradients in the laser plasma accelerators allow to reduce the size and to cut the cost of accelerators. Another important advantage of the laser-plasma accelerators is that they can produce short electron bunches with high energy. The formation of electron bunches with small energy spread was demonstrated at intense laser–plasma interactions. Electron self-injection in the wake-bubble, generated by an intense laser pulse in underdense plasma, has been studied. With newly available compact laser technology one can produce 100 PW-class laser pulses with a single-cycle duration on the femtosecond timescale. With a fs intense laser one can produce a coherent X-ray pulse. Prof. T. Tajima suggested utilizing these coherent X-rays to drive the acceleration of particles. When such X-rays are injected into a crystal they interact with a metallic-density electron plasma and ideally suit for laser wakefield acceleration. In numerical simulation of authors, performed according to idea of Prof. T.Tajima, on wakefield excitation by a X-ray laser pulse in a metallic-density electron plasma the accelerating gradient of several TV/m has been obtained. It is important to form bunch with small energy spread and small size. The purpose of this paper is to show by the numerical simulation that some precursor-laser-pulse, moved before the main laser pulse, controls properties of the self-injected electron bunch and provides at certain conditions small energy spread and small size of self-injected and accelerated electron bunch.

Stable plasmon excitations in quantum nanowires

Stable electrostatic plasmon excitations due to propagation of Gaussian electron pulses are studied in a cylindrical nanowire of radius “a” and length “L”. For this purpose, we consider conduction electrons that follow the quantum hydrodynamical model to account for quantum statistics, quantum diffraction, and exchange-correlation effects in the presence of classical static ions. The use of the well-known Fourier transform technique and the Trivelpiece-Gould configuration leads to the generalized dispersion for plasmons with finite mode quantization effects via the radial and azimuthal mode indices, as well with quantum settings. The excitations of plasmon wakefield appear when the electron beam propagates with a constant velocity along the axial direction of nanowires. The stability conditions are also analyzed for the existence of wakefield excitations by solving three important inequalities using typical parameters of gold nanowires. The results may prove useful for understanding the plasmon excitations in quantum nanowires, where electrons can be trapped and accelerated in the wakefield to emit radiation in the extreme-ultraviolet range.

Efficient injection of radiation-pressure-accelerated sub-relativistic protons into laser wakefield acceleration based on 10 PW lasers

We propose a hybrid laser-driven ion acceleration scheme using a combination target of a solid foil and a density-tailored background plasma. In the first stage, a sub-relativistic proton beam can be generated by radiation pressure acceleration in intense laser interaction with the solid foil. In the second stage, this sub-relativistic proton beam is further accelerated by the laser wakefield driven by the same laser pulse in a near-critical-density background plasma with decreasing density profile. The propagating velocity of the laser front and the phase velocity of the excited wakefield wave are effectively lowered at the beginning of the second stage. By decreasing the background plasma density gradually from near critical density along the laser propagation direction, the wake travels faster and faster, while it accelerates the protons. Consequently, the dephasing between the protons and the wake is postponed and an efficient wakefield proton acceleration is achieved. This hybrid laser-driven proton acceleration scheme can be realized by using ultrashort laser pulses at the peak power of 10 PW for the generation of multi-GeV proton beams.

Electron acceleration in a homogeneous plasma by Bessel‐Gaussian and Gaussian pulses

The process of electron acceleration by both Bessel‐Gaussian (BG) and Gaussian (G) laser pulses has been investigated comparatively in a homogeneous plasma. Starting with the hydrodynamics fluid and the Maxwell's equations, the three corresponding equations could be acquired which allow us to evaluate electron density perturbations (), wakefield (Ew) and electron energy‐gain (ΔW) respectively for both pulses. Here, the pulse duration and the electron plasma period are taken the same, and by utilizing the perturbation theory the proposed equations have been solved. The analytical results show that the wakefield which leads to electron acceleration, depends on pulse intensity (I), pulse length (L), pulse wavelength (λ) and plasma density (n). Furthermore, by comparing the outcomes, the BG pulse is found to be most suitable for wakefield excitation. The pertinent electron could be accelerated up to 90 MeV if I = 4 × 1021W/m2, L = 20 µm and n = 5.318 × 1024 m−3. Our results are in a good agreement with experimental data reported in literature.

Two-color ionization injection using a plasma beatwave accelerator

Two-color laser ionization injection is a method to generate ultra-low emittance (sub-100 nm transverse normalized emittance) beams in a laser-driven plasma accelerator. A plasma beatwave accelerator is proposed to drive the plasma wave for ionization injection, where the beating of the lasers effectively produces a train of long-wavelength pulses. The plasma beatwave accelerator excites a large amplitude plasma wave with low peak laser electric fields, leaving atomically-bound electrons with low ionization potential. A short-wavelength, low-amplitude ionization injection laser pulse (with a small ponderomotive force and large peak electric field) is used to ionize the remaining bound electrons at a wake phase suitable for trapping, generating an ultra-low emittance electron beam that is accelerated in the plasma wave. Using a plasma beatwave accelerator for wakefield excitation, compared to short-pulse wakefield excitation, allows for a lower amplitude injection laser pulse and, hence, a lower emittance beam may be generated.