Wake Phase Velocity Research Articles

Effect of pulse group velocity on charge loading in laser wakefield acceleration

Total charge and energy evaluations for the electron beams generated in the laser wakefield acceleration (LWFA) is the primary step in the determination of the required laser and target parameters. Particle-in-cell (PIC) simulation is an efficient numerical tool that can provide such evaluations unless the effect of numerical dispersion is not diminished. The numerical dispersion, which is specific for the PIC modeling, affects not only the dephasing lengths in LWFA but also the total amount of the self-injected electrons. A numerical error of the order of 10−4−10−3 in the calculation of the speed of the driving laser pulse results in a significant error in the total injected charge, energy gain and other critical parameters of the accelerated electron bunches. In the standard numerical approach, the numerical correction in the speed of the laser pulse either requires infinitely small spatial grid resolution (which needs large computation platform) or force to compromise with the numerical accuracy. Here, we present a simple and easy to implement numerical scheme to suppress the numerical dispersion of the electromagnetic pulse in PIC simulations even with a modest spatial resolution, and without any special treatments to the core structure of the numerical algorithm. Evaluated charges of the self-injected electron bunches become essentially lower owing to the better calculations of the wake phase velocity.

Efficient Narrow-Band Terahertz Radiation from Electrostatic Wakefields in Nonuniform Plasmas.

It is shown that electrostatic plasma wakefields can efficiently radiate at harmonics of the plasma frequency when the plasma has a positive density gradient along the propagation direction of a driver. The driver propagating at a subluminal group velocity excites the plasma wakefield with the same phase velocity. However, due to the positive density gradient, the wake phase velocity steadily increases behind the driver. As soon as the phase velocity becomes superluminal, the electrostatic wakefield couples efficiently to radiative electromagnetic modes. The period of time when the phase velocity stays above the speed of light depends on the density gradient scale length. The wake radiates at well-defined harmonics of the plasma frequency in the terahertz band. The angle of emission depends on the gradient scale and the time passed behind the driver. For appropriate plasma and driver parameters, the wake can radiate away nearly all its energy, which potentially results in an efficient, narrow-band, and tunable source of terahertz radiation.

Reversal of laser wake phase velocity generates high-power broadband Cherenkov signal

As a femtosecond terawatt laser pulse propagates along a positive density gradient within a stratified plasma column, it drives a low-frequency electromagnetic wake wave, the period of electron fluid oscillations in the wake gradually shrinking. Phase velocity of the wake promptly exceeds the vacuum speed of light, setting in near-forward emission of terahertz Cherenkov radiation. As the wake accelerates further, the Cherenkov emission ray rotates by 180∘. Emission from a given plasma locality is sustained for a finite interval of time, in the middle of which the wake experiences a ‘reversal,’ its phase velocity becoming singular and changing sign (Zhang C J et al 2017 Phys. Rev. Lett. 119 064801) At this instant, the electromagnetic energy flows at 90∘, the emission power reaching its peak. After the reversal, the wake keeps radiating into the rear hemisphere until its phase velocity becomes subluminal. Experimentally capturing evolution of the Cherenkov signal may thus shed light onto the plasma wake dynamics. Far away from the plasma, the radiation fills an expanding, almost spherical shell, the shell thickness increasing with an increase in the observation angle. The length of the terahertz signal sampled in the wave zone thus ranges from zero (forward emission) to a few tens of picoseconds (backward emission). The signal is positively chirped, its frequency increasing from the Langmuir frequency at the foot of the column to the Langmuir frequency at the top. Theoretical estimates for the regimes involving 10 TW-class drive pulses promise a few-kW emission power; the energy conversion efficiency, from optical to terahertz, of order 10−7; and an MV m−1-scale electric field strength centimeters away from the plasma.

Generating high quality ultrarelativistic electron beams using an evolving electron beam driver

A new method of controllable injection to generate high quality electron bunches in the nonlinear blowout regime driven by electron beams is proposed and demonstrated using particle-in-cell simulations. Injection is facilitated by decreasing the wake phase velocity through varying the spot size of the drive beam and can be tuned through the Courant-Snyder (CS) parameters. Two regimes are examined. In the first, the spot size is focused according to the vacuum CS beta function, while in the second, it is focused by the plasma ion column. The effects of the driver intensity and vacuum CS parameters on the wake velocity and injected beam parameters are examined via theory and simulations. For plasma densities of $\sim 10^{19} ~\text{cm}^{-3}$, particle-in-cell (PIC) simulations demonstrate that peak normalized brightnesses $\gtrsim 10^{20}~\text{A}/\text{m}^2/\text{rad}^2$ can be obtained with projected energy spreads of $\lesssim 1\%$ within the middle section of the injected beam, and with normalized slice emittances as low as $\sim 10 ~\text{nm}$.

Laser plasma accelerators

An ultrafast intense laser pulse drives coherent wakefields of relativistic amplitude with a high phase velocity robustly supported by the plasma. The structures of wakes and sheaths in plasma are contrasted. While the large amplitude of wakefields involves collective resonant oscillations of the eigenmode of the entire population of plasma electrons, the wake phase velocity ~c and ultrafast nature of the laser pulse introduce the wake stability and rigidity. When the phase velocity decreases, wakefields turn into sheaths, which are more suitable for ion acceleration. This short review reports on 4 decades of discoveries on laser plasma accelerators.

Amplitude enhancement of the self-modulated plasma wakefields

Seeded Self-modulation (SSM) has been demonstrated to transform a long proton bunch into many equidistant micro-bunches (e.g., the AWAKE case), which then resonantly excite strong wakefields. However, the wakefields in a uniform plasma suffer from a quick amplitude drop after reaching the peak. This is caused by a significant decrease of the wake phase velocity during self-modulation. A large number of protons slip out of focusing and decelerating regions and get lost, and thus cannot contribute to the wakefield growth. Previously suggested solutions incorporate a sharp or a linear plasma longitudinal density increase which can compensate the backward phase shift and therefore enhance the wakefields. In this paper, we propose a new plasma density profile, which can further boost the wakefield amplitude by 30%. More importantly, almost 24% of protons initially located along one plasma period survive in a micro-bunch after modulation. The underlying physics is discussed.

Electron self-injection in the proton-driven-plasma-wakefield acceleration

The self-injection process of plasma electrons in the proton-driven-plasma-wakefield acceleration scheme is investigated using a two-dimensional, electromagnetic particle-in-cell method. Plasma electrons are self-injected into the back of the first acceleration bucket during the initial bubble formation period, where the wake phase velocity is low enough to trap sufficient electrons. Most of the self-injected electrons are initially located within a distance of the skin depth c/ωpe to the beam axis. A decrease (or increase) in the beam radius (or length) leads to a significant reduction in the total charges of self-injected electron bunch. Compared to the uniform plasma, the energy spread, emittance and total charges of the self-injected bunch are reduced in the plasma channel case, due to a reduced injection of plasma electrons that initially located further away from the beam axis.

Phase Velocity and Particle Injection in a Self-Modulated Proton-Driven Plasma Wakefield Accelerator

It is demonstrated that the performance of the self-modulated proton driver plasma wakefield accelerator is strongly affected by the reduced phase velocity of the plasma wave. Using analytical theory and particle-in-cell simulations, we show that the reduction is largest during the linear stage of self-modulation. As the instability nonlinearly saturates, the phase velocity approaches that of the driver. The deleterious effects of the wake's dynamics on the maximum energy gain of accelerated electrons can be avoided using side-injections of electrons, or by controlling the wake's phase velocity by smooth plasma density gradients.

Plasma-Density-Gradient Injection of Low Absolute-Momentum-Spread Electron Bunches

Plasma density gradients in a gas jet were used to control the wake phase velocity and trapping threshold in a laser wakefield accelerator, producing stable electron bunches with longitudinal and transverse momentum spreads more than 10 times lower than in previous experiments (0.17 and 0.02 MeV/c FWHM, respectively) and with central momenta of 0.76+/-0.02 MeV/c. Transition radiation measurements combined with simulations indicated that the bunches can be used as a wakefield accelerator injector to produce stable beams with 0.2 MeV/c-class momentum spread at high energies.

Improvement of electron beam quality in optical injection schemes using negative plasma density gradients

Enhanced electron trapping using plasma density down-ramps as a method for improving the performance of laser injection schemes is proposed and analyzed. A decrease in density implies an increase in plasma wavelength, which can shift a relativistic electron from the defocusing to the focusing region of the accelerating wakefield, and a decrease in wake phase velocity, which lowers the trapping threshold. The specific method of two-pulse colliding pulse injector is examined in detail using a three-dimensional test particle tracking code. A density down-ramp with a change of density on the order of tens of percent over distances greater than the plasma wavelength leads to an enhancement of charge by two orders in magnitude or more, up to the limits imposed by beam loading. The accelerated bunches are ultrashort (fraction of the plasma wavelength--e.g., approximately 5 fs), high charge ( > 20 pC at modest injection laser intensity approximately 10(17) W/cm(2)), with a relative energy spread of a few percent at a mean energy of approximately 25 MeV, and a normalized root-mean-square emittance of the order of 0.5 mm mrad.