Effects of realistic laser intensity and phase distribution on high-charge laser wakefield acceleration

Abstract

Laser wakefield acceleration (LWFA) can produce relativistic electron beams and various secondary particles in centimeter-long plasmas, making it a valuable particle source with important applications in many disciplines. In this work, we examine the effects of non-ideal transverse intensity and phase distribution of laser pulses on LWFA through both experimental measurements and particle-in-cell simulations. The complex transverse profile of the 75 TW laser pulses reduces the self-focused intensity in plasma compared with a transversely Gaussian laser. Furthermore, the sheath structure of the nonlinear plasma wake excited by realistic laser pulses is wider and more complicated than that of a Gaussian laser. These hinder the injection of the plasma electrons. As the laser pulse propagates through the plasma, its intensity profile gradually becomes elliptical and drives a plasma wake with a sharp sheath near the azimuths of the major axis, leading to an injection. When using a realistic laser profile in simulations, both the charge and energy of injected electrons closely match experimental results ($\sim200$ pC of charge and $\sim 200$ MeV peak energy), whereas the Gaussian laser simulations produce much higher charge ($\sim500$ pC). Our findings reveal the difference in injection dynamics between LWFA driven by non-ideal laser pulses and those driven by Gaussian pulses, and are useful for applications of LWFA which demand high-charge electron beams.

Figures (10)

• Figure 1: (a) Schematic of the experimental setup (not on scale). The inset is a typical spatial intensity profile of the laser pulse at the focus. A 10 $\micro\meter$ thick aluminum film is placed near the gas nozzle to block the laser pulse while allowing relativistic electrons to pass through it with minimal impact. (b) A typical plasma electron density distribution measured by the Michelson interferometer and the axial plasma density profile (red line). (c) The dependence of the plasma density on the backing pressure. The plasma density is measured at a height of 2 mm above the nozzle.
• Figure 2: Experimental results of the injected electron beams when scanning the plasma density $n_\mathrm{p}$. The dependence of the injected charge (a), the peak energy (b) and the relative FWHM energy spread on $n_\mathrm{p}$. Note $z_\mathrm{f}$ is fixed as $-1~\milli\meter$ in these experiments. The presented results are averaged over 5-15 shots at each $n_\mathrm{p}$. The orange squares show the PIC simulation results when using a transversely Gaussian laser pulse driver and the red triangle is the result with a reconstructed laser pulse.
• Figure 3: (a1)-(a3) The transverse intensity distribution of the lasers at the vacuum focus. (b) The plasma density profile. The black dashed line indicates the vacuum focal plane. The evolution of $a_0$ (c) and their axial locations $\xi_{a_0}$ (d) for three cases. The two insets shows the $\xi-x$ intensity profile of the Gaussian laser at different propagation distances, with white crosses marking the locations of the intensity peaks. (e) The evolution of the injected charge for three cases. (f1)-(f3) The energy spectra of the injected beam at the plasma exit for three cases.
• Figure 4: (a) The evolution of $\frac{\mathrm{d}\xi_\mathrm{t}}{\mathrm{d}z}$ for three cases. The black dashed lines is the theoretical value of $-\frac{v_\mathrm{etch}}{c}$. (b1)-(b3) The transverse intensity profile of the $\xi_{a_0}$-slice of the lasers at the propagation distances where they achieve the maximum $a_0$. (c1)-(c3) The plasma density distribution at the central slice of the corresponding nonlinear wakes. The white dashed lines in (b1), (b3), (c1), (c3) represent the major axis of the laser transverse intensity profile and the $x$-axis.
• Figure 5: (a) The Pearson correlation coefficient between the realistic laser profile and its elliptical fit. The plasma density distribution at the central slice of the nonlinear wakes for the realistic laser pulse at $z=-0.46~\milli\meter$ (b) and $z=0.46~\milli\meter$ (c). The dashed lines represent the major and minor axes of laser driver.