Towards controlling electron charge with nanoparticle assisted laser wakefield accelerators

TL;DR

The paper investigates nanoparticle-assisted injection in laser wakefield accelerators using particle-in-cell simulations to quantify how nanoparticle material and size control injected beam charge. It identifies a saturation threshold for the NP electric field, after which total beam charge scales with the nanoparticle's atom count rather than its ionized electron density, and shows injection across multiple plasma periods leading to higher charge but broader energy spread. Material and size effects strongly shape injection efficiency, wakefield loading, and the final spectra, with larger nanoparticles and higher-Z materials generally increasing charge while potentially reducing energy per electron. The work offers practical guidelines for experimental NA-LWFA, highlighting trade-offs between maximizing charge, maintaining energy, and robust injection under misalignment, and points to experimental strategies such as aerodynamic-lens nanoparticle delivery to tailor beam properties for applications.

Abstract

This study explores nanoparticle-assisted electron injection as a method for controlling beam charge in laser wakefield acceleration through particle-in-cell simulations. We systematically investigate how the material (Li through Au) and size (50-200 nm) of nanoparticles influence electron injection dynamics and beam charge. Our results demonstrate that beam charge (10-600 pC) can be effectively controlled by adjusting these parameters. We identify a saturation threshold in the nanoparticle electric field strength, beyond which beam charge depends on the total number of atoms in the nanoparticle rather than on the electron density after ionization. Significant electron injection occurs across multiple plasma wave periods with distribution patterns influenced by nanoparticle properties leading to increased beam charge but a broader energy spread. These findings offer practical guidelines for experimental implementation of nanoparticle-assisted injection in laser wakefield accelerators to tailor electron beam characteristics for various applications.

Figures (5)

• Figure 1: The scheme of the movement of electrons released from the nanoparticle after its ionization. The laser field is shown with the yellow color, while the electron density (plasma wave) is shown with green-blue-black. Nanoparticle ions and electrons are represented by red and blue dots respectively. (a) The situation before the nanoparticle (NP) ionization. (b) After the nanoparticle interacts with the laser pulse, two groups of electrons are identified: $e^-$$^{(1)}$ are electrons staying close to the nanoparticle ions and $e^-$$^{(2)}$ are electron dispersed into the surrounding plasma. (c) After the first plasma wave period passes the nanoparticle, we recognize two new electron groups: $e^-$$^{(3)}$ which are electrons temporarily trapped inside the laser electric field and $e^-$$^{(4)}$ which are electrons injected in the plasma wave.
• Figure 2: Panels (a), (d), and (g) display the laser pulse, the plasma wave and the electron macroparticles depicted by orange (nanoparticle) and yellow (plasma) dots for simulation with Li, Al and Cu nanoparticle, respectively, at $t$ = 710 $T_0$. Panels (b), (e), and (h) show the corresponding electron beam profile together with the injected charge. Panels (c), (f), and (i) depict the corresponding electron spectra, while the spectra obtained from all the four plasma wave periods are represented by the red curve. We also show the spectra from every plasma wave period displayed as the colored area under the red curve. The color scales are saturated.
• Figure 3: The effect of the nanoparticle (NP) material on the injected charge of the electron beam. (a) The beam charge dependency on the electron density of the ionized nanoparticle. In brackets [ ] is the average ionization level of the material atoms. (b) The dependency of the fraction of the beam charge comprising only of the electrons from the ionized nanoparticle on the electron density of the ionized nanoparticle (black) and the percentage of injected electrons created by the nanoparticle ionization (red). (c) The total charge distributed in the first four periods of the plasma wave. (d) The charge from nanoparticle electrons distributed in the first four periods of the plasma wave.
• Figure 4: Beam parameters dependency on the nanoparticle parameters (size and material). (a) Charge versus nanoparticle diameter (50-200 nm) for three different materials (Li, Al, Cu). (b) Maximum energy versus nanoparticle diameter. (c) Dependency of the fraction of the beam charge comprising only of the electrons from the ionized nanoparticle. (d) Mean energy dependency on the nanoparticle diameter, with error bars showing the root-mean-square energy spread. (e) Beam charge dependency on the number of atoms forming the nanoparticle (product of its volume and the solid state density). In the blue region solid density of the material still has some influence on the beam charge, in the red region this is no longer important.
• Figure 5: Results from the full 3D PIC simulations investigating the influence of transverse nanoparticle position on the beam properties. (a) The total beam charge and the charge formed only by the nanoparticle electrons. (b) Maximum and mean energy for different displacements.