Controlling the characteristics of injected and accelerated electron bunch in corrugated plasma channel by temporally asymmetric laser pulses.

In laser-driven plasma wakefield accelerators, the accelerating electric field is orders of magnitude stronger than in conventional radio-frequency particle accelerators, but the dephasing between the ultrarelativistic electron bunch and the wakefield traveling at the group velocity of the laser pulse puts a limit on the energy gain. Quasi-phase-matching, enabled by corrugated plasma channels, is a technique for overcoming the dephasing limitation. The attainable energy and the final properties of accelerated electron beams are of utmost importance in laser wakefield acceleration (LWFA). In this work, using two-dimensional particle-in-cell simulations, the effect of the driving pulse duration on the performance of quasi-phase-matched laser wakefield acceleration (QPM-LWFA) is investigated. It is observed that for a pulse duration around half the plasma period, the maximum energy gain of the beam electrons finds its peak value. However, the results show that for a pulse of that duration the collimation of the bunch is much worse, compared to the case where the pulse duration is twice as long. Furthermore, the dynamics of the laser pulse and the evolution of the quality of the externally-injected electron bunch are studied for a symmetric pulse with sine-squared temporal profile, a positive skew pulse (i.e., one with sharp rise and slow fall), and a negative skew pulse (i.e., one with a slow rise and sharp fall). The results indicate that for a laser pulse with an appropriate pulse length compared with the plasma wavelength, the wakefield amplitude can be greatly enhanced by using a positive skew pulse, which leads to higher energy gain. Initially, this results from the stronger ponderomotive force associated with a fast rise time. Later, due to the distinct evolution of the three pulses with different initial profiles, the wakefield excited by the positive skew pulse becomes even stronger. In our simulations, the maximum energy gain for the asymmetric laser pulse with a fast rise time is almost two times larger than for the temporally symmetric laser pulse. Nevertheless, stronger focusing and defocusing fields are generated as well if a positive skew pulse is applied, which degrade the collimation of the bunch. These results should be taken into account in the design of miniature particle accelerators based on QPM-LWFA.

Quasi-phase-matched laser wakefield acceleration of electrons in an axially density-modulated plasma channel.

Quasi-phase matching in corrugated plasma channels has been proposed as a way to overcome the dephasing limitation in laser wakefield accelerators. In this study, the phase-lock dynamics of a relatively long electron bunch injected in an axially-modulated plasma waveguide is investigated by performing particle simulations. The main objective here is to obtain a better understanding of how the transverse and longitudinal components of the wakefield as well as the initial properties of the beam affect its evolution and qualities. The results indicate that the modulation of the electron beam generates trains of electron microbunches. It is shown that increasing the initial energy of the electron beam leads to a reduction in its final energy spread and produces a more collimated electron bunch. For larger bunch diameters, the final emittance of the electron beam increases due to the stronger experienced transverse forces and the larger diameter itself. Increasing the laser power improves the maximum energy gain of the electron beam. However, the stronger generated focusing and defocusing fields degrade the collimation of the bunch.

Rotating ion beam effects on temperature gradient instability in completely ionized plasmas.

The aim of this paper is to investigate the effects of a rotating ion beam on the temperature gradient instability (TGI) in completely ionized plasmas. The interplay of the temperature and density gradients provides the basis for experiencing an unstable inhomogeneous plasma medium due to TGI taken under consideration. The density and temperature gradients are considered perpendicular to the magnetic field where a nonrelativistic rotating ion beam such as O^{+} is present. By implementing the kinetic theory together with a zeroth-order approximation of geometrical optics, the dielectric permittivity tensor of the inhomogeneous plasma is obtained where by a suitable linear eikonal equation, the growth rate of the TGI in the collisional regime is calculated in the presence of a rotating ion beam. In such a configuration an unstable condition is experienced in regions with opposite electron density and temperature gradients, where it is destabilized by the temperature and plasma density gradients and the frequent electron collisions. As a consequence, the results reveal that the TGI can be damped or modified through interaction with the rotating ion beam depending on the characteristics of the ion beam, namely, velocity and density.