Wakefield Excitation Research Articles

Self consistent hydrodynamic description of the plasma wake field excitation induced by a relativistic charged-particle beam in an unmagnetized plasma

A self-consistent nonlinear hydrodynamic theory is presented of the propagation of a long and thin relativistic electron beam, for a typical plasma wake field acceleration configuration in an unmagnetized and overdense plasma. The random component of the trajectories of the beam particles as well as of their velocity spread is modelled by an anisotropic temperature, allowing the beam dynamics to be approximated as a 3D adiabatic expansion/compression. It is shown that even in the absence of the nonlinear plasma wake force, the localisation of the beam in the transverse direction can be achieved owing to the nonlinearity associated with the adiabatic compression/rarefaction and a coherent stationary state is constructed. Numerical calculations reveal the possibility of the beam focussing and defocussing, but the lifetime of the beam can be significantly extended by the appropriate adjustments, so that transverse oscillations are observed, similar to those predicted within the thermal wave and Vlasov kinetic models.

Simulation study of wake field excitation in interaction of intense laser and magnetized plasma: Half-sine pulse shape (HSPS) and trapezoid pulse shape (TPS)

We have conducted particle-in-cell (PIC) simulations of a linearly polarized intensive laser pulse with two different envelopes propagating through a homogeneous fully ionized cold plasma. It is shown that the amplitude of the wake field depends on laser wavelength, pulse duration, electron number density and envelope shape. We have also simulated the effect of applying a longitudinal magnetic field on the wake field excitation process. It is observed that magnetic field enhances the wake field and increases its intensity in all cases. Our results are in agreement with the analytical results presented by Askari and Shahidani [Opt. Laser Technol.45, 613–619 (2013)] and can help choosing the optimum values of affecting laser and plasma parameters in order to reach high accelerating wake electric fields.

Excitation and Control of Plasma Wakefields by Multiple Laser Pulses.

We demonstrate experimentally the resonant excitation of plasma waves by trains of laser pulses. We also take an important first step to achieving an energy recovery plasma accelerator by showing that a plasma wave can be damped by an out-of-resonance trailing laser pulse. The measured laser wakefields are found to be in excellent agreement with analytical and numerical models of wakefield excitation in the linear regime. Our results indicate a promising direction for achieving highly controlled, GeV-scale laser-plasma accelerators operating at multikilohertz repetition rates.

Generation of high-power, tunable terahertz radiation from laser interaction with a relativistic electron beam

We propose a method based on the slice energy spread modulation to generate strong subpicoseond density bunching in high-intensity relativistic electron beams. A laser pulse with periodic intensity envelope is used to modulate the slice energy spread of the electron beam, which can then be converted into density modulation after a dispersive section. It is found that the double-horn slice energy distribution of the electron beam induced by the laser modulation is very effective to increase the density bunching. Since the modulation is performed on a relativistic electron beam, the process does not suffer from strong space charge force or coupling between phase spaces, so that it is straightforward to preserve the beam quality for further applications, such as terahertz (THz) radiation and resonant excitation of plasma wakefield. We show in both theory and simulations that the tunable radiation from the beam can cover the frequency range of 1-10 THz with high power and narrow-band spectra.

Plasma wakefield excitation in a cold magnetized plasma for particle acceleration

A numerical study has been done to find a travelling wave solution for a highly relativistic electron beam driven cold magnetized plasma. The presence of magnetic field has an effect to reduce the transformer ratio (the ratio of energy gain to the drive beam energy) from its unmagnetized value. The effects of the beam shape on the nonlinear structures of different dynamical variables are also discussed. The results owe its significance in the laboratory context of particle acceleration or in the study of generation of ultrahigh accelerating charged particles by strong plasma waves in astrophysical situations.

An experimental study of electron acceleration with detuning of the bunch repetition frequency from that of an excited wake field

We have experimentally studied the excitation of wake fields in a dielectric structure by a train of relativistic electron bunches and the acceleration of subsequent bunches of the same train due to detuning of the bunch repetition frequency relative to that of the wake field excited in the dielectric structure at the Cherenkov resonance. Electron bunches of the first (leading) part of the train excite the wake wave, and bunches of the second (trailing) part of this train are shifted to the accelerating phase of the wake wave so as to gain additional energy. The possibility of controlling the number (repetition frequency) of bunches exciting the wake field in the dielectric structure and the number of subsequently accelerated bunches has been investigated by changing the value of detuning.

Excitation of wakefields by relativistic electron bunches in the dielectric waveguide filled with radially inhomogeneous plasma

Excitation of wakefields by relativistic electron bunches in the dielectric waveguide filled with radially inhomogeneous plasma

INCREASE OF AMPLITUDE OF ACCELERATING WAKEFIELD EXCITED BY SEQUENCE OF SHORT RELATIVISTIC ELECTRON BUNCHES IN PLASMA AT MAGNETIC FIELD USE

Earlier, the authors found a mechanism for the sequence of short relativistic electron bunches, which leads to resonant excitation of the wakefield, even if the repetition frequency of bunches differs from the plasma frequency. In this case, the synchronization of frequencies is restored due to defocusing of the bunches which get into the bad phases with respect to the plasma wave. However, in this case, the bunches are lost, which as a result of this do not participate in the excitation of the wakefield. In this paper, numerical simulation was used to study the dynamics of electron bunches and the excitation of the wakefield in a magnetized plasma by a long sequence of short bunches of relativistic electrons. When a magnetic field is used, the defocussed bunches return to the region of interaction with the field after a certain time. In this case, the electrons of the bunches, returning to the necessary phases of the field, participate in the excitation of the wakefield. Also, the use of a magnetic field leads to an increase of the frequency of the excited wave relative to the repetition frequency of bunches. The latter increases the time for maintaining the resonance and, consequently, leads to an increase of the amplitude of the excited wakefield.

Design Considerations on Complementary Split Ring Resonator-Loaded Waveguides for Wakefield Generation

In this paper, we present design considerations of metamaterial (MTM)-based structures to address practical issues relating to operation in particle accelerators and at high power in general. To the best of our knowledge, this is the first time these problems have been addressed. We focus on the structure proposed in our earlier paper based on a complementary split ring resonator (CSRR)-loaded waveguide for applications in accelerator science, as a backward propagating Cherenkov detector. Through the modification of the metasurface thickness, CSRR ring width and ring curvature, a number of possible waveguides are investigated to increase structural integrity and robustness, maintain the dispersion relation, and reduce the number of hybrid modes. Accurate numerical simulations are performed to compare the different setups with the original structure. Both the longitudinal and transverse wakefield excitations are analyzed to investigate how these modifications affect beam coupling. A suitable design is successfully identified and offers a promising opportunity for the development of MTMs for high-power beam-based applications.

Nonlinear structure of the wakefield generated by relativistic intense ion bunch

The resonant excitation of the nonlinear wakefield by a single proton bunch is investigated with the parameters characteristic of the AWAKE experiment. It is shown that obtained structure of the wakefield at a distance more than twenty periods behind the driver proton bunch can be suitable for the side injection and further acceleration of the witness electron bunch in the wakefield.

Dynamika korotkykh elektronnykh z·hustkiv ta zbudzhenykh nymy kil'vaternykh poliv u plazmi za vidsutnosti ta za nayavnosti pozdovzhn'oho mahnitnoho polya

Using the computer simulation, the influence of a longitudinal magnetic field on the wakefield excitation in plasma by an initially cylindrical electron bunch with the length equal to the wake wavelength and the inverse influence of excited fields on the bunch dynamics have been considered. It is shown that the magnetic field can suppress the radial defocusing of the bunch and increase the length of the region, in which the wake waves are excited, as well as the amplitude of those waves.

Relativistic electron beam driven longitudinal wake-wave breaking in a cold plasma

Space-time evolution of a relativistic electron beam driven wake-field in a cold, homogeneous plasma is studied using 1D-fluid simulation techniques. It is observed that the wake wave gradually evolves and eventually breaks, exhibiting sharp spikes in the density profile and sawtooth like features in the electric field profile [Bera et al., Phys. Plasmas 22, 073109 (2015)]. It is shown here that the excited wakefield is a longitudinal Akhiezer-Polovin mode [A. I. Akhiezer and R. V. Polovin, Sov. Phys. JETP 3, 696 (1956)] and its steepening (breaking) can be understood in terms of phase mixing of this mode, which arises because of relativistic mass variation effects. Further, the phase mixing time (breaking time) is studied as a function of beam density and beam velocity and is found to follow the well known scaling presented by Mukherjee and Sengupta [Phys. Plasmas 21, 112104 (2014)].

Mul'tybanchevyy rezhym zbudzhennya kil'vaternoho polya v plazmovo-dielektrychniy strukturi

The main results of theoretical and experimental researches concerning the physical processes in a dielectric wakefield accelerator based on the accelerating wakefield excitation in a dielectric structure with the use of a long sequence of electron bunches are presented. The enhancement of the excited wakefield amplitude is achieved owing to the coherent summation of wakefields generated by separate bunches. The bunches are accelerated in the total wakefield by arbitrarily dividing the bunch sequence into the exciting and accelerated parts, by using the appropriate frequency detuning of the bunch repetition frequency with respect to the frequency of the excited principal transverse mode. The influence of plasma in the transit channel on the amplitude of excited wakefields and the focusing of driving and accelerated bunches is analyzed.

High quality electron beam acceleration by ionization injection in laser wakefields with mid-infrared dual-color lasers

For the laser wakefield acceleration, suppression of beam energy spread while keeping sufficient charge is one of the key challenges. In order to achieve this, we propose bichromatic laser ionization injection with combined laser wavelengths of 2.4 μm and 0.8 μm for wakefield excitation and triggering electron injection via field ionization, respectively. A laser pulse at 2.4 μm wavelength enables one to drive an intense acceleration structure with a relatively low laser power. To further reduce the requirement of laser power, we also propose to use carbon dioxide as the working gas medium, where carbon acts as the injection element. Our three dimensional particle-in-cell simulations show that electron beams at the GeV energy level with both low energy spreads (around 1%) and high charges (several tens of picocoulomb) can be obtained by the use of this scheme with laser peak power totaling sub-100 TW.

Two-color beam generation based on wakefield excitation

Several beam manipulation methods have been studied and experimentally tested to generate two-color photon beams in free electron laser facilities to accommodate the user requests. We propose to use the interaction of the beam with an oscillating longitudinal wakefield source to obtain a suitable electron beam structure. The bunch generates two subpulses with different energies and delayed in time passing through a magnetic chicane after its longitudinal phase space has been modulated by the wakefield source. According to this approach the power of the emitted radiation is not degraded compared to the monochromatic beam, and the setup in the machine is quite simple because the bunch is manipulated only in the high energy section, where it is more rigid. We present the design applied to SwissFEL. We identified the parameters and the corresponding range of tunability of the time and energy separation among the two subbunches.

Characterization of the equilibrium configuration for modulated beams in a plasma wakefield accelerator

We analyze the equilibrium configuration for a modulated beam with sharp boundaries exposed to the fields self-generated by the interaction with a plasma. Through a semi-analytical approach, we show the presence of multiple equilibrium configurations and we determine the one more suitable for wakefield excitation. Once pointed out the absence of confinement for the front of the beam and the consequently divergence driven by the emittance, we study the evolution of the equilibrium configuration while propagating in the plasma, discarding all the other time-dependencies. We show the onset of a rigid backward drift of the equilibrium configuration, and we provide an explanation in the increasing length of the first bunch.

Self-modulated dynamics of a relativistic charged particle beam in plasma wake field excitation

The self-modulated dynamics of a relativistic charged particle beam is provided within the context of the theory of plasma wake field excitation. The self-consistent description of the beam dynamics is provided by coupling the Vlasov equation with a Poisson-type equation relating the plasma wake potential to the beam density. An analysis of the beam envelope self-modulation is then carried out and the criteria for the occurrence of the instability are discussed thereby.

The concept of coupling impedance in the self-consistent plasma wake field excitation

Within the framework of the Vlasov–Maxwell system of equations, we describe the self-consistent interaction of a relativistic charged-particle beam with the surroundings while propagating through a plasma-based acceleration device. This is done in terms of the concept of coupling (longitudinal) impedance in full analogy with the conventional accelerators. It is shown that also here the coupling impedance is a very useful tool for the Nyquist-type stability analysis. Examples of specific physical situations are finally illustrated.

Emission of strong Terahertz pulses from laser wakefields in weakly coupled plasma

The present paper discusses the laser plasma interaction for the wakefield excitation and the role of external magnetic field for the emission of Terahertz radiation in a collisional plasma. Flat top lasers are shown to be more appropriate than the conventional Gaussian lasers for the effective excitation of wakefields and hence, the generation of strong Terahertz radiation through the transverse component of wakefield.

Semi-analytical fluid study of the laser wake field excitation in the strong intensity regime

We present an analytical and numerical study of the interaction of a multi-petawatt, pancake-shaped laser pulse with an unmagnetized plasma. The study has been performed in the ultrarelativistic regime of electron jitter velocities, in which the plasma electrons are almost completely expelled from the pulse region. The calculations are applied to a laser wake field acceleration scheme with specifications that may be available in the next generation of Ti:Sa lasers and with the use of recently developed pulse compression techniques. A set of novel nonlinear equations is derived using a three-timescale description, with an intermediate timescale associated with the nonlinear phase of the electromagnetic wave and with the spatial bending of its wave front. They describe, on an equal footing, both the strong and the moderate laser intensity regimes, pertinent to the core and to the edges of the pulse.