Plasma Wave Period Research Articles

Space-Charge Field Assisted Electron Acceleration by Plasma Wave in Magnetic Plasma Channel

When the duration of a laser pulse is comparable to the plasma wave period, a large-amplitude plasma wave can be driven by the laser. In the presence of a guiding magnetic field, the large-amplitude plasma wave of finite transverse extent and large phase velocity resonantly accelerates the electrons to a higher energy level. For a small laser spot size, the laser exerts an axial as well as the radial ponderomotive force on the electrons that creates a density depression on the laser axis. This electron-depleted channel also creates a radial electric field (ion space-charge field). The acceleration of electrons in this channel is investigated, where the effect of ion space-charge field is considered, by solving the single particle dynamical equations. This paper shows that the space charge field plays an important role in electron energy gain during acceleration by a plasma wave in a magnetic plasma channel. This paper may be crucial in understanding the dynamics of particle motion and improving the quality of accelerated electron beam in the laser wakefield accelerators.

Direct Observation of the Injection Dynamics of a Laser Wakefield Accelerator Using Few-Femtosecond Shadowgraphy.

We present few-femtosecond shadowgraphic snapshots taken during the nonlinear evolution of the plasma wave in a laser wakefield accelerator with transverse synchronized few-cycle probe pulses. These snapshots can be directly associated with the electron density distribution within the plasma wave and give quantitative information about its size and shape. Our results show that self-injection of electrons into the first plasma-wave period is induced by a lengthening of the first plasma period. Three-dimensional particle-in-cell simulations support our observations.

Excitation of two-dimensional plasma wakefields by trains of equidistant particle bunches

Nonlinear effects responsible for elongation of the plasma wave period are numerically studied with the emphasis on two-dimensionality of the wave. The limitation on the wakefield amplitude imposed by detuning of the wave and the driver is found.

Laser wakefield acceleration using wire produced double density ramps

A novel approach to implement and control electron injection into the accelerating phase of a laser wakefield accelerator is presented. It utilizes a wire, which is introduced into the flow of a supersonic gas jet creating shock waves and three regions of differing plasma electron density. If tailored appropriately, the laser plasma interaction takes place in three stages: Laser self-compression, electron injection, and acceleration in the second plasma wave period. Compared to self-injection by wave breaking of a nonlinear plasma wave in a constant density plasma, this scheme increases beam charge by up to 1 order of magnitude in the quasimonoenergetic regime. Electron acceleration in the second plasma wave period reduces electron beam divergence by $\ensuremath{\approx}25%$, and the localized injection at the density downramps results in spectra with less than a few percent relative spread.

Utilizing asymmetric laser pulses for the generation of high-quality wakefield-accelerated electron beams

Employing temporally asymmetric laser pulses in the interaction with plasma has been recently proposed for controlling the pointing angle of an electron beam produced by a laser wakefield acceleration at low plasma density and moderate laser intensity. In this paper, results on the electron beam parameters for both symmetric and asymmetric laser pulses are presented. These results show that the highest-quality (well-pointed, well-collimated and bright) electron beams are generated in the current regime only using asymmetric laser pulses, which are longer than the plasma wave’s acceleration period, τ>λp/2c. The interaction between the laser pulse and the accelerated electron beam in the first plasma-wave period is extracted from the experimental results and observed in preliminary two-dimensional particle-in-cell simulation.

Implicit–explicit time integration of a high-order particle-in-cell method with hyperbolic divergence cleaning

A high-order implicit–explicit additive Rung–Kutta time integrator is implemented in a particle-in-cell method based on a high-order discontinuous Galerkin Maxwell solver for the simulation of plasmas. The method enforces Gauss law by a hyperbolic divergence cleaner that transports divergence out of the computational domain at several times the speed of light. The stiffness in the field equations induced by this high transport speeds is alleviated by an implicit time integration, while an explicit time integration ensures a computationally efficient particle update. Simulations on a plasma wave and a Weibel instability show that the implicit–explicit solver is computationally efficient, allowing for computations with high divergence transport speeds that ensure an accurate representation of the governing plasma equations. The high-order method only requires two time steps per plasma wave period. Numerical instability appears when the time step exceeds the plasma frequency time scale. A divergence transport speed of approximately ten times the speed of light is shown to be optimal, since it combines an accurate representation of Gauss law with a small influence of numerical noise on the solution.

Unphysical kinetic effects in particle-in-cell modeling of laser wakefield accelerators

Unphysical heating and macroparticle trapping that arise in the numerical modeling of laser wakefield accelerators using particle-in-cell codes are investigated. A dark current free laser wakefield accelerator stage, in which no trapping of background plasma electrons into the plasma wave should occur, and a highly nonlinear cavitated wake with self-trapping, are modeled. Numerical errors can lead to errors in the macroparticle orbits in both phase and momentum. These errors grow as a function of distance behind the drive laser and can be large enough to result in unphysical trapping in the plasma wake. The resulting numerical heating in intense short-pulse laser-plasma interactions grows much faster and to a higher level than the known numerical grid heating of an initially warm plasma in an undriven system. The amount of heating, at least in the region immediately behind the laser pulse, can, in general, be decreased by decreasing the grid size, increasing the number of particles per cell, or using smoother interpolation methods. The effect of numerical heating on macroparticle trapping is less severe in a highly nonlinear cavitated wake, since trapping occurs in the first plasma wave period immediately behind the laser pulse.

Finite-difference time-domain simulation of fusion plasmas at radiofrequency time scales

Simulation of dense plasmas in the radiofrequency range are typically performed in the frequency domain, i.e., by solving Laplace-transformed Maxwell’s equations. This technique is well-suited for the study of linear heating and quasilinear evolution, but does not generalize well to the study of nonlinear phenomena. Conversely, time-domain simulation in this range is difficult because the time scale is long compared to the electron plasma wave period, and in addition, the various cutoff and resonance behaviors within the plasma insure that any explicit finite-difference scheme would be numerically unstable. To resolve this dilemma, explicit finite-difference Maxwell terms are maintained, but a carefully time-centered locally implicit method is introduced to treat the plasma current, such that all linear plasma dispersion behavior is faithfully reproduced at the available temporal and spatial resolution, despite the fact that the simulation time step may exceed the electron gyro and electron plasma time scales by orders of magnitude. Demonstrations are presented of the method for several classical benchmarks, including mode conversion to ion cyclotron wave, cyclotron resonance, propagation into a plasma-wave cutoff, and tunneling through low-density edge plasma.

Wakefield generation by elliptically polarized femtosecond laser pulse in ionizing gases

Wakefield generation by a femtosecond laser pulse is described in the frame of the slowly varying amplitudes approximation. The amplitude of the wakefield A, is studied as a function of laser pulse and background gas parameters, and is compared with well-known results for preformed, completely ionized plasma A/sub p,i/. It is found that the ionization processes can increase A/sub p/ as compared to A/sub p,i/ at comparatively high laser peak intensities. It is shown that the increase of the wakefield amplitude due to gas ionization is more pronounced for circularly polarized laser pulses than for linearly polarized laser pulses. The strongest enhancement of A/sub p/ in comparison with A/sub p,i/ takes place for longer laser pulses with a duration in excess of the plasma wave period when the resonant conditions for ponderomotive excitation of the wakefield are not matched. Thus, ionization processes can expand the region of parameters for efficient generation of the laser wakefields.

Structure of the wake field in plasma channels

An equation is derived that describes the linear response of an underdense inhomogeneous plasma [ω0≫ωp(r), where ω0 and ωp(r) are the laser-carrier and plasma frequencies, respectively] during the propagation of a laser pulse along the axis of a plasma channel with a characteristic width Rch. For a wide channel, i.e., when Rch/λp0>1 (where λp0=2πc/ωp0 is the wavelength of the excited plasma wave and ωp0 is the plasma frequency at the channel axis), the structure of the wake field is studied analytically. It is shown that this structure changes with the distance from the trailing edge of the pulse. As a result, at a certain distance behind the pulse, the fraction of the plasma wave period in which the simultaneous focusing and acceleration of electrons are possible increases by a factor of 2. For a narrow channel (Rch/λp0<1), the structure of the wake field is studied numerically and it is shown that, in this case, the doubling of the phase interval of the wave where the simultaneous focusing and acceleration of electrons are possible also occurs; but, in contrast to a wide channel, a rapid reconstruction of the wake occurs, so that the amplitude of the axial (accelerating) field in the wake decreases while the radial (focusing) field increases with the distance from the pulse trailing edge. The numerical modeling of the laser pulse (90 fs, 2 TW) guiding and the excitation of plasma waves in a narrow plasma channel is carried out and the possibility of reaching GeV energies of accelerated electrons in an experiment is discussed.