A computational study is presented on laser wakefield acceleration (LWFA) in bubble regime with the use of ultrashort laser pulse propagating in an under-dense plasma. The Particle-In-Cell simulations are performed to investigate the bubble wakefield acceleration of electrons realized by the incidence of an intense laser beam on cold, under-dense plasma in two-dimensional geometry. Different simulations are carried out and the results are compared for the beams with trapezoidal and Gaussian temporal pulse profiles having almost equal but slightly different energy contents. Focus is given to plasma density modulation, wakefield strength, electrons self-injection, energy spectrum of accelerated electrons, the effect of an external longitudinal magnetic field and the study of pump depletion length and dephasing length in bubble regime with respect to these laser pulse profiles. Two limiting cases of the trapezoidal pulse, that is, triangular and rectangular pulses, are also discussed for better understanding of the role of steepness and plateau region in the laser pulse profile to the bubble wakefield acceleration. Since down ramp density gradient plays a crucial role for the generation of high-quality electron beam in plasma wakefield acceleration as well as in LWFA, three different adjustments on the down ramp length determining three different density gradients are discussed for uncovering the role of trapezoidal laser pulse in LWFA.

In the process of bubble wakefield acceleration highly nonlinear region is developed inside plasma, which intuitively suggests that nonuniform plasma density having gradients may be more suited to achieve large nonlinearity in the system. Moreover, when an intense laser pulse propagates in a plasma, it is subjected to various instabilities and these instabilities can be controlled by plasma density profiles which effectively control the energy and flux of the accelerated particles. Considering all these points we investigate in the present work the scaling effect of up-ramp and down-ramp regions in plasma density profile on the bubble wakefield. These regions are separated by a plateau region (maximum density) enabling the density to have trapezoidal profile. With this density profile, the bubble wakefield acceleration is examined considering four different lengths of up-ramp and plateau regions keeping a constant down-ramp length. Increasing steepness of up-ramp length (larger density gradient), i.e., lowering the length of up-ramp and increasing the plateau length creates a bubble having higher wakefield strength, resulting into higher accumulation of plasma electrons at its tail and higher energy spectrum with higher kinetic energy gradient and Poynting flux of accelerated electrons.

Laser wakefield acceleration (LWFA) is a promising technique to build compact and powerful particle accelerators. In such accelerators, the electric fields required to accelerate charged particles are sustained by electron density modulations in the plasma. The plasma wave modulating the electron density may be excited by an intense laser pulse. However, propagation of intense laser pulse in plasma is subject to various instabilities which result in significant losses of laser energy, reducing the efficiency of wakefield generation. Using a train of lower intensity pulses instead of a single higher intensity pulse appears to be a more efficient scheme for LWFA. Here we have studied this alternative scheme by applying an ultra-short femtosecond Gaussian laser beam consisting pulse train of a various number of pulses in different cases to underdense plasma. The plasma density modulation and strength of the resulting wakefield have been compared in various cases of multi-pulse and single-pulse lasers, for the same amount of input energies. Here we demonstrate that applying multi-laser pulses of optimally selected lower intensities and proper spacing leads to stronger wakefield generation and more efficient electron acceleration compared to the case of a single pulse of higher energy.

High-energy particle beam is used to probe the local or long range structure and properties of materials. Particle accelerators are also used in material science to understand radiation damage, particularly in studies of structural material to be applied in fusion power generator and satellites. Laser Wakefield Acceleration (LWFA) is a promising technique to build compact and powerful particle accelerators for generating multi-GeV electron beams. The efficient coupling of laser energy with plasma is very important to generate high energy electrons, which is the subject to optimum laser and plasma profiles. In this article, we have used high intensity ultra-short laser pulse and a polygon plasma density profile to analyse the behaviour of LWFA in the bubble regime, which corresponds to the evacuation of plasma electrons in the wake of the laser pulse. Many electrons get self-injected in this wakefield and get accelerated behind the laser pulse up to very high energies, producing high energy beam useful for material processing along with their other applications. We have diagnosed some parameters such as electron density variation, wakefield magnitude and energy spectrum of the electrons accelerated in this mechanism. Copyright (c) 2022 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Materials, Processing & Characterization.