N. Naseri

Self-channelling of relativistic laser pulses in large-scale underdense plasmas

Relativistic self-focusing and channelling of intense laser pulses have been studied in underdense plasma using two-dimensional particle-in-cell (PIC) simulations, for different laser powers and plasma densities. Analytical solutions for the stationary evacuated channels have been recovered in PIC simulations. It is shown that otherwise stable channels can accelerate electrons due to surface waves on the walls of the channels. Relativistic filaments with finite electron density are unstable to transverse modulations which lead in the nonlinear stage to the breakup of laser pulses into independent filaments. Different regimes of relativistic self-focusing and channelling, including electron heating, transverse instability, and break-up of the filaments, have been discussed and characterized using plasma density and laser power.

Axial magnetic field generation by intense circularly polarized laser pulses in underdense plasmas

Axial magnetic field generation by intense circularly polarized laser beams in underdense plasmas has been studied with three-dimensional particle-in-cell simulations and by means of theoretical analysis. Comparisons between analytical models and simulation results have identified an inverse Faraday effect as the main mechanism of the magnetic field generation in inhomogeneous plasmas. The source of azimuthal nonlinear currents and of the axial magnetic field depends on the transverse inhomogeneities of the electron density and laser intensity. The fields reach a maximum strength of several tens of megagauss for laser pulses undergoing relativistic self-focusing and channeling in moderately relativistic regime. Ultrarelativistic laser conditions inhibit magnetic field generation by directly reducing a source term and by generating fully evacuated plasma channels.

Self-channelling of intense laser pulses in underdense plasma and stability analysis

Self-channelling of intense laser pulses have been studied in underdense plasma using three-dimensional particle-in-cell simulations, for different laser powers and plasma densities as well as analytical theory. It is found that single channel solution occurs for laser powers above the threshold power ( ∼ 1.1 P c r) and for plasma densities n 33 P c r, ring structure, an evacuated ring enclosed by electron filament, was observed in the simulations as predicted by analytical model. The stability of ring structure against symmetric and asymmetric azimuthal perturbations have been discussed.

PPPS-2013: Fast and accurate simulations of 10 GEV-scale laser plasma accelerators

Because of their ultra-high accelerating gradient, laser plasma based accelerators (LPA) are contemplated for the next generation of high energy colliders and light sources. The upcoming BELLA project will explore acceleration of electron bunches to 10 GeV in a meter long plasma, where a wakefield is driven by a PW-class laser. Particle-in-cell (PIC) simulations are used to design the upcoming experiments. Simulations are challenging because of the disparity of length scale between the laser wavelength (~1 micron) that needs to be resolved and the simulation length (~ 1 m). We report on recent developments of the Laser Envelope Model, a reduced model for laser-plasma interactions that has previously demonstrated orders of magnitude speedup. In particular, we present the implementation of the model in cylindrical coordinates, allowing for quite rapid prototyping of laser acceleration stages. We discuss the performance benefits as well as the limitations and trade-offs of this model. In parallel, high frequency noise in PIC simulations makes it difficult to accurately represent beam energy spread and emittance. We show that calculating the beam self-fields using a static Poisson solve in the beam frame dramatically reduces particle noise, allowing for more accurate simulation of the beam evolution.

Relativistic electron generation in laser produced ion channels

This contribution is concerned with the channeling of a relativistic laser pulse propagating in an underdense plasma, and with the subsequent generation of fast electrons in the cavitated ion channels. Specifically, we study the interaction of laser pulses of duration of several 102 femtoseconds, having their intensity Iλ2 in the range [1019; 1020]Wcm−2μm2 and focused in underdense plasmas, with electron densities n0 such that the ratio n0=nc lies in the interval [10−3, 10−1], nc denoting the critical density. The laser power PL exceeds the critical power for laser channeling Pch = 1:09Pc, Pc denoting the critical power for relativistic self-focusing. The laser-plasma interaction under such conditions is investigated by means of three dimensional (3D) Particle-In-Cell (PIC) simulations. It is observed that the steep laser front gives rise to the excitation of a surface wave which propagates along the sharp radial boundaries of the electron free channel created by the laser pulse. The mechanism responsible for the generation of relativistic electrons observed in the PIC simulations is then analyzed by means of a 3D test particles code. The fast electrons are thus found to be generated by the combination of the electron acceleration caused by the surface wave and of the betatron mechanism. The maximum electron energy observed in the simulations is scaled as a function of PL/Pc; it reaches 350 - 400 MeV for PL/Pc = 70 - 140.

Generation of GeV energy electrons from laser wakefield acceleration via ionization induced injection

Summary form only given. Laser wakefield acceleration (LWFA) is a promising approach to realize table-top accelerators. The injection process into the wakefield bubble to some extent determines the charge, divergence, energy gain as well as the energy distribution of the accelerated electrons. Traditionally, self injection using pure helium or hydrogen gas as the interaction medium was employed to accelerate the electrons. However, very high laser powers are required to achieve self injection at the low densities which are compatible with acceleration to GeV energies. Recently, a new technique, ionization induced injection, which takes advantage of the large ionization potential difference between the inner and outer shell electrons of trace atoms in the plasma, has been demonstrated to generate electron beams beyond 1 GeV at lower threshold laser powers than self injection would require [1].

Wakefield acceleration of quasi-monoenergetic 200 MeV electrons in nitrogen and helium gas targets

Quasi-monoenergetic electron beams of energies over 200 MeV with high flux are generated from both nitrogen and helium gas with a modest laser power of 6.5 TW. 2D PIC simulations are in progress to compare to experiments.