Electron acceleration by an intense laser pulse inside a density profile induced by non-linear pulse evolution

Abstract

By sophisticated application of particle-in-cell simulations, we demonstrate the ultimate role of non-linear pulse evolutions in accelerating electrons during the entrance of an intense laser pulse into a preformed density profile. As a key point in our discussions, the non-linear pulse evolutions are found to be very fast even at very low plasma densities, provided that the pulse length exceeds the local plasma wavelength. Therefore, these evolutions are sufficiently developed during the propagation of typical short density scale lengths occurred at high contrast ratios of the pulse, and lead to plasma heating via stochastic acceleration in multi-waves. Further analysis of simulation data at different physical parameters indicates that the rate of evolutions increases with the plasma density leading to higher plasma heating and overgrown energetic electrons. In the same way, shortening the density scale length results into increase in the evolution rate and, simultaneously, decrease in the interaction time. This behavior can describe the observed optimum value of pre-plasma scale length for the maximum electron heating.

Numerical investigation of plasma heating during the entrance of an intense short laser pulse into a density profile

“…This wave-brake formed at the vacuum-plasma interface toward the vacuum region, which is electrostatic in nature [16], and can accelerate electrons and could be considered as a generic mechanism being active at the front of the target spatially for a sharp step-like density profile. In the case of the laser pulse with τ L = 60 fs, the boundary wave-brake cannot be isolated and therefore, at the very early time of interaction, electrostatic effects such as the longitudinal field's oscillations along with the boundary wake-break could be possible acceleration mechanisms even in the absence of electromagnetic pulse evolution [23]. The total radiation spectrum and spatiotemporal normalized vector potential for short pulse lengths striking the step-like density profile are depicted in figures 1(a) and (b), respectively, at times 0.1, 1.5 and 3 ps.…”

“…In our previous publication [23], we found that for long laser pulses, even in the absence of laser pulse backscattering from critical density and boundary wave-break, there are radiation scatterings at the early stage of the interaction, which are actively responsible for the initiation of electron stochastic acceleration in the pre-plasma.…”

“…The computational details of the method are out of the scope of the current paper, readers are instead referred to [25]. This method has been extensively used in relativistic laser plasma interactions [21][22][23]. For ultra-short laser pulses, the plasma is initially modeled as being fully ionized [14], i.e.…”

“…Here n e0 = 2n c is the undisturbed plasma density, x 0 is the starting point of the density's flat top, and L p denotes the preplasma density scale length [23]. For all the scale lengths, the time-integrated electron energy spectrum is calculated at the end of the flat top part of the density within a 1 μm space, to ensure that the relativistic critical density (γ OSC n c ) prevents laser pulse penetration to that point, and at the time that it becomes saturated.…”

Self modulation and scattering instability of a relativistic short laser pulse in an underdense plasma

“…Despite the latter, in the former the laser pulse is susceptible to scattering instabilities [16][17][18][19][20] and pulse breakup [17,[21][22][23][24]. In most applications, these effects on the laser pulse act as a doubleedged sword; they both restrict the pulse penetration into the plasma [25,26], and in the same time lead to enhanced particle acceleration [27][28][29][30][31][32][33][34]. Generally, scattering assisted electron acceleration is now considered as a generic mechanism for production of very-energetic/super-ponderomotive electron populations observed in high density plasma irradiation by intense pulses [13,27,[30][31][32][33][34].…”