Quasi-phase-matched acceleration of electrons in a corrugated plasma channel

Abstract

A laser pulse propagating in a corrugated plasma channel is composed of spatial harmonics whose phase velocities can be subluminal. The phase velocity of a spatial harmonic can be matched to the speed of a relativistic electron resulting in direct acceleration by the guided laser field in a plasma waveguide and linear energy gain over the interaction length. Here we examine the fully self-consistent interaction of the laser pulse and electron beam using particle-in-cell (PIC) simulations. For low electron beam densities, we find that the ponderomotive force of the laser pulse pushes plasma channel electrons towards the propagation axis, which deflects the beam electrons. When the beam density is high, the space charge force of the beam drives the channel electrons off axis, providing collimation of the beam. In addition, we consider a ramped density profile for lowering the threshold energy for trapping in a subluminal spatial harmonic. By using a density ramp, the trapping energy for a normalized vector potential of a 0 ¼ 0:1 is reduced from a relativistic factor 0 ¼ 170 to 0 ¼ 20.

Particle-in-cell simulations of quasi-phase matched direct laser electron acceleration in density-modulated plasma waveguides

“…The formation of density peaks in QPM of DLA 14,15 , or microbunches, becomes prominent when a long bunch is injected. Since most of the bunch electrons can be confined in the ion channel over a long distance, a sufficient time exists during the DLA process for this density modulation to be realized.…”

“…3(b) shows the variation of n pe (x) when electron bunches with τ d = 6.2 fs and τ d = 0 are injected. The bunch expels the electrons and an ion channel is gradually formed following the front edge of the bunch 15 . At this moment, the ion focusing force at both injection delays increases the peak density of the bunch to n b0 ∼ 3 × 10 18 cm −3 , thus fulfilling the condition for creating an underdense plasma lens.…”

“…12 Analogous to the "slow-wave" structures extensively used for RF waves, a periodic density structure in a plasma waveguide expands the laser axial field into several harmonics, for which quasiphase-matching (QPM) of DLA has been proposed and well studied. 14, 15 The results of prior simulations show that the harmonic axial field component, having a subluminal phase ve-locity v p < c, can effectively accelerate electrons along the waveguide when a proper density modulation period for the QPM condition, including any necessary density ramping, 15 has been prepared. The QPM process of DLA can also be understood in an alternative picture, by breaking the energy gain symmetry between the acceleration and deceleration phases, similar to the processes in quasi-phase-matched relativistic harmonic generation.…”

“…The hollow intensity distribution of the laser radial field is a particular case where the ponderomotive force pushes the background plasma electrons to concentrate in the center, which in turn produces a radial electrostatic force that can defocus the injected electron bunch. 15 The electron bunch also expands because of its finite emittance. Analogous to the Rayleigh length of a laser beam, the minimum β-function β * = γσ 2 y / N characterizes the beam size variation along the propagation, where γ is the Lorentz factor of the bunch, σ is the root-mean-square (RMS) bunch size and N is the normalized emittance.…”

“…For example, the electron bunch collimation can be improved by using a higher density for the injected electron bunch, as has been shown in a recent 2-D PIC study. 15 In this work, a 3-D PIC model has been developed and used to simulate QPM of DLA in density-modulated plasma waveguides. Sharp axial periodic structures with alternating waveguide and neutral gas regions are defined in the simulation to reproduce the properties of a laser-machined plasma waveguide.…”