Wakefield Generation Research Articles

Kinetic modeling of wakefield generation in ultrahigh intensity laser-plasma interaction

Quasistatic model of wakefield generation in ultrahigh intensity laser-plasma interaction is presented. The model implies that the plasma wake is slowly changed in the laser pulse frame. 2D axially symmetrical quasistatic PIC code based on this model is developed. The code provides stable numerical computation in ultrahigh intensity regime and much shorter computational time than that of standard PIC codes. The numerical modeling of the plasma wakefield generated by high power laser pulse is presented. A comparison with simulation results obtained by fully 3D electromagnetic PIC code is done.

DIAGNOSTICS FOR GUIDED LASER PLASMA WAKEFIELD

The structure of the wakefield is studied in a gas filled capillary under conditions of a quasi-monomode laser pulse propagation in a capillary tube. Self-consistent numerical simulations show that in the central bulk part of a plasma where the laser intensity exceeds the ionization threshold, the wakefield structure is similar to that of an infinite homogeneous plasma. The efficient generation of a regular wakefield over long distances suitable for the laser wakefield accelerators is shown. The modification of the spectrum of a frequency doubled probe pulse traveling in a plasma wave created in the wake of a pump pulse guided inside a capillary tube is analyzed for the cases of narrow or broad chirped probe spectra. It is shown that in both cases the measurement of the spectrum can be used as a diagnostic for the amplitude of the plasma wave. The results of full scale numerical modeling confirm the developed analytical theory of wakefield diagnostics.

Pair production by a strong wakefield excited by lasers in a magnetized plasma

The production of electron-positron pairs due to accelerated electrons in a magnetized plasma is considered. The high-energy electrons are produced during laser-plasma interactions in the presence of spontaneously produced magnetic fields. By using a classical fluid description, we analyze the generation of electrostatic wakefields at the plasma wave-breaking limit and estimate the number of produced pairs for different situations. We note that, in some cases, the presence of the magnetic field can increase the number of produced pairs.

Optics in the relativistic regime

The advent of ultraintense laser pulses generated by the technique of chirped pulse amplification (CPA) along with the development of high-fluence laser materials has opened up an entirely new field of optics. The electromagnetic field intensities produced by these techniques, in excess of ${10}^{18}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$, lead to relativistic electron motion in the laser field. The CPA method is reviewed and the future growth of laser technique is discussed, including the prospect of generating the ultimate power of a zettawatt. A number of consequences of relativistic-strength optical fields are surveyed. In contrast to the nonrelativistic regime, these laser fields are capable of moving matter more effectively, including motion in the direction of laser propagation. One of the consequences of this is wakefield generation, a relativistic version of optical rectification, in which longitudinal field effects could be as large as the transverse ones. In addition to this, other effects may occur, including relativistic focusing, relativistic transparency, nonlinear modulation and multiple harmonic generation, and strong coupling to matter and other fields (such as high-frequency radiation). A proper utilization of these phenomena and effects leads to the new technology of relativistic engineering, in which light-matter interactions in the relativistic regime drives the development of laser-driven accelerator science. A number of significant applications are reviewed, including the fast ignition of an inertially confined fusion target by short-pulsed laser energy and potential sources of energetic particles (electrons, protons, other ions, positrons, pions, etc.). The coupling of an intense laser field to matter also has implications for the study of the highest energies in astrophysics, such as ultrahigh-energy cosmic rays, with energies in excess of ${10}^{20}\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The laser fields can be so intense as to make the accelerating field large enough for general relativistic effects (via the equivalence principle) to be examined in the laboratory. It will also enable one to access the nonlinear regime of quantum electrodynamics, where the effects of radiative damping are no longer negligible. Furthermore, when the fields are close to the Schwinger value, the vacuum can behave like a nonlinear medium in much the same way as ordinary dielectric matter expanded to laser radiation in the early days of laser research.

Seeded self-modulated laser wakefield acceleration

A new approach to laser-wakefield acceleration (LWFA) has been analyzed. A seed electron beam bunch precedes the laser pulse into the plasma. This seed bunch initiates formation of plasma waves via a plasma wakefield acceleration mechanism. The amplitude of the plasma waves is subsequently amplified by the laser pulse via a self-modulated LWFA (SM-LWFA) process. This method enables the generation of strong wakefields even when the laser pulse by itself has characteristics that are insufficient for driving resonant LWFA or SM-LWFA. Another advantage is the wakefield formation begins at the seed bunch and does not start from noise as typically occurs in SM-LWFA. This feature may be helpful when the phase of the wakefield must be accurately controlled, for example, when staging multiple LWFA devices in series.

Analysis of multimode wakefield generation in accelerating structures with conductive dielectric loading

Wakefield generation by short electron bunches traveling along the vacuum channel of an accelerating structure loaded with a conductive ceramic dielectric has been investigated. The dielectric response is defined as no dispersion of the real part of permittivity and linear increase of loss tangent versus frequency. This model is in good agreement with measured microwave properties of some materials tested for Dielectric Wakefield Acceleration experiments. It has been shown that the dielectric conductivity has an essential impact on the wakefield. The loss factor influences most strongly on short electron bunch generation of the multimode wakefields.

Electric and magnetic wakefields in a plasma channel

A detailed analytical study of plasma wakefield generation in a wide parabolic plasma channel is reported. A perturbative technique involving orders of the incident laser beam and the effects of inhomogeneity of the plasma density is used to obtain explicit electric as well as magnetic wakefields. The axial and transverse forces acting on a test electron due to the wakefields have been evaluated.

Wakefield generation as the mechanism behind spectral shift of a short laser pulse

An equation describing the dynamics of plasma wave generation by a short intense laser pulse is analyzed to find a relation between the difference in mean-square pulse frequency before and after laser-matter interaction and the electric field amplitude in the wakefield plasma wave generated by the laser pulse. This relation can be effectively used in systems for wakefield diagnostics. The relation is applied to several geometries of interaction between a pulse and an ionizing gas or preformed plasma.

Pair production by a strong wakefield excited by intense neutrino bursts in plasmas

We consider the production of electron–positron pairs due to accelerated electrons in a strong wakefield that is created by intense neutrino bursts in plasmas. By using a classical fluid description, we investigate the generation of electrostatic wakefields at the plasma wave-breaking limit, and estimate the number of pairs that is produced by a trident process. We find that the pair concentration produced is huge, and this result can be very important in studies of astrophysical plasmas and in intense laser-plasma interaction experiments which are aimed to understanding several astrophysical phenomena in laboratory.

Interaction of powerful laser pulse with magnetized plasma

In this paper the generation of electrostatic wake field and electron acceleration in this field by intense laser pulse propagating in plasma in the presence of external magnetic field is investigated. It is shown that if the change in the laser pulse wave amplitude (approximation of the constant laser pulse wave field) is not taken into account, it is possible by switching to Lagrangian variables to reduce the system of equations describing excitation of the wake field and electron acceleration to a simplified system of ordinary differential equations (ODE), which can be easily integrated. This makes it possible to find the energy, transferred to electron oscillations in the wake field for different angles between the direction of laser pulse propagation and magnetic field. Stability of the laser pulse propagating with arbitrary angle to external magnetic field towards excitation of parametric and decay instabilities with involvement of upper hybrid and lower hybrid waves is investigated, as well. Equations describing these instabilities are derived and growth rates are found analytically for a small amplitude wake field and with the help of numerical calculations in the case of the finite wake field amplitude.

A self-consistent theory of the plasma wakefield of an electron beam

The generation of the plasma wakefield of an electron beam is investigated in a self-consistent method. The effect of the distortion of the driving beam shape on the strength of the driven plasma wave is taken into account. In addition to the loss of the kinetic energy of the beam, the variation in the shape, as well as that in its electric potential energy, also influences the strength of the driven plasma wave. The dependence of the amplitude of the wakefield on some controllable parameters of the beam is investigated via numerical experiment.

Envelope equations and conservation laws describing wakefield generation and electron acceleration

Previous authors have proposed various envelope equations to describe the behavior of an electromagnetic pulse generating a wakefield. In general these retain second-order derivatives, the reason being that the eikonal contains the initial wave frequency. Here it is shown that if the evolution of the wave frequency is followed using ray-tracing equations, a first-order evolution equation is obtained. It can be shown with this formalism that wave action is conserved and the energy lost from the electromagnetic wave can be explicitly accounted for in terms of energy gained by the plasma. The energy balance equations suggest that an electron bunch which will extract energy efficiently from a wakefield can be at least as efficiently accelerated by direct interaction with the electromagnetic pulse.

A Coupled System for Nonlinear Wave Propagation Combining Wake Field Generation with a Kerr-Nonlinearity

Three cases of nonlinear wave propagation in plasmas are studied, involving different wave modes. In all cases a generic system of equations is found, combining a Kerr-like self nonlinearity with nonlocal effects due to the generation of low-frequency wake fields. Using the conservation laws of the system, explicit expressions for the soliton acceleration and frequency conversion rate are found.

VORPAL: a versatile plasma simulation code

VORPAL is a new plasma simulation code designed for maximum flexibility through use of advance C++ techniques. Through use of inheritance, VORPAL incorporates multiple models for the plasma and electromagnetic fields. The plasma models include both particle-in-cell and fluid models. Through C++ meta-template programming a single code can be used to simulate one-, two-, or three-dimensional systems with no loss of performance. VORPAL can also be run in either serial or parallel, with the latter using a general domain decomposition. A new fluid algorithm that allows for regions of zero density was developed and incorporated into the code. VORPAL simulation results for the generation of laser wake fields through laser–plasma interaction are presented.

Guided propagation of short intense laser pulses and electron acceleration

The dynamics of a guided laser pulse propagation and wakefield generation in plasma channels, ionizing gases and gas filled capillaries is investigated. The efficient generation of a regular wakefield over long distances suitable for laser wakefield accelerators is demonstrated. The different schemes of electron bunch injection and compression to provide monoenergetic electron acceleration to high energies are analysed. A possibility of effective multistage acceleration of short electron bunches with low emittance and energy spread is demonstrated. The analytical results are confirmed by the results of numerical modelling.

Cohesive acceleration and focusing of relativistic electrons in overdense plasma.

We describe our studies of the generation of plasma wake fields by a relativistic electron bunch and of phasing between the longitudinal and transverse fields in the wake. The leading edge of the electron bunch excites a high-amplitude plasma wake inside the overdense plasma column, and the acceleration and focusing wake fields are probed by the bunch tail. By monitoring the dependence of the acceleration upon the plasma's density, we approached the beam-matching condition and achieved an energy gain of 0.6 MeV over the 17 mm plasma length, corresponding to an average acceleration gradient of 35 MeV/m. Wake-induced modulation in energy and angular divergence of the electron bunch are mapped within a wide range of plasma density. We confirm a theoretical prediction about the phase offset between the accelerating and focusing components of plasma wake.

Dynamics of a plasma channel created by the wakefield of a short laser pulse

A new physical effect of a plasma channel formation by the ponderomotive force of a wakefield excited by a short laser pulse with duration of the order of electron plasma oscillation period ωp0−1 (ωp0 is the plasma frequency) is discussed. The hydrodynamic and particle numerical codes, including plasma ion response, are used to simulate the long-term wakefield behavior. It is found that the wakefield creates a channel with a radial profile depending on the laser pulse width. Particularly, for a narrow pulse, wherein the width is less than c/ωp0 (c is the speed of light), the channel has an annular form with on-axis density maximum. The depth of the channel increases with the distance from the pulse until fine-scale mixing arises and the wake starts to break. Particle simulations show that wave breaking results in emergence of fast electrons taking an essential part of the wake energy during a few plasma periods. Quasilinear fluid equations describing self-consistently, the laser wakefield generation, and plasma channel formation are derived. The wave-breaking conditions are obtained in the geometrical optics approximation. The results of numerical simulations for high-intensity laser pulses are in good agreement with theoretical predictions. The scaling laws for wave breaking are discussed.

Propagation of electromagnetically generated wake fields in inhomogeneous magnetized plasmas

Generation of wake fields by a short electromagnetic pulse in a plasma with an inhomogeneous background magnetic field and density profile is considered, and a wave equation is derived. Transmission and reflection coefficients are calculated in a medium with sharp discontinuities. Particular attention is focused on examples where the longitudinal part of the electromagnetic field is amplified for the transmitted wave. Furthermore, it is noted that the wake field can propagate out of the plasma and thereby provide information about the electron density profile. A method for reconstructing the background density profile from a measured wake field spectrum is proposed and a numerical example is given.

Propagation of short intense laser pulses in gas-filled capillaries.

The guided laser pulse propagation and wake-field generation are studied in a wide (in comparison with the laser spot size) gas-filled capillary with an on-axis gas density depletion, which can be produced by a rapid spin of the capillary around its axis or by radially propagating shock waves generated in a piezoceramic tube. A single equation for the wake-field potential, which describes the fully relativistic plasma response in the presence of optical field ionization (OFI) of a gas, is derived and used to demonstrate a guided propagation of a short intense laser pulse over many Rayleigh lengths in a leaky plasma channel produced by the pulse due to OFI in the capillary filled with a radially inhomogeneous gas. The efficient generation of a regular wake field over long distances suitable for the laser wake-field accelerators is shown.

Wakefield generation in one-dimensional LWFA

The generation of wakefields in near-critical density plasma by a short laser pulse is considered. The equations describing the plasma response to the acting laser pulse and the Hamiltonian of an electron in the wakefield are derived by using the Hamilton-Jacobi equation for the cases of vg≃vp≃w = c and w < c (c is the speed of light). The equations are solved analytically in two cases for a square-shaped laser pulse. The plasma potential inside the laser pulse and the wakefield potential excited by the laser pulse are obtained. The optimal length of the driving laser pulse and the wavelength of the wake are expressed by the group velocity.