Laser Wakefield Acceleration in a Capillary Gas Cell Producing GeV-Scale High-Quality Electron Beams

TL;DR

This work demonstrates, through end-to-end modeling, that a single-stage capillary gas cell with a short mixed-gas injection region and a longer pure-He acceleration section can sustain ionization-injection LWFA at GeV energies while controlling energy spread. Hydrodynamic OpenFOAM simulations optimize the two-zone density profile, which is then fed into SMILEI PIC simulations to evaluate beam quality under a 100 TW, $820$ nm laser. The results show ionization-injected electrons from N$^{5+}$ K-shells can reach mean energies around $1.0$ GeV with relative energy spreads in the low-to-mid percent range; the best-performing case (Case-2) yields $\sim$41 pC, $E\approx1.01$ GeV, $\text{FWHM}\approx2.4\%$, and $\theta_{y,z}\approx1.0$ mrad. These findings inform future EuPRAXIA/ELI Beamlines experiments by highlighting design strategies to minimize energy spread and manage self-injected electrons through tailored density tapering and injection length control.

Abstract

Laser Wakefield Acceleration (LWFA) is a promising approach for producing high-brightness electron beams in the GeV energy range, offering significant potential for compact next-generation accelerator facilities. In this work, we present a computational study of LWFA in a specially designed single-stage capillary gas-cell target aimed at producing high-quality, GeV-class electron beams. The capillary cell includes a short (~2 mm) injection region at the entrance filled with a helium (He) and nitrogen (N2 ) gas mixture. This is followed by a longer (~12 mm) pure He section, which provides the required acceleration length and limits continuous ionization injection, thereby significantly reducing the energy spread of the accelerated beam. Hydrodynamic simulations are performed to optimize the capillary geometry and generate the required two-section gas-pressure profile. The resulting gas-density distributions for various cases are then directly incorporated in Particle-In-Cell (PIC) simulations to study LWFA. In particular, our hydrodynamic simulations demonstrate how tailored density profiles with longitudinal density tapering in the acceleration section can be realized in a capillary gas cell, while the corresponding PIC simulations reveal how these profiles influence the acceleration process and the resulting beam quality. Using a 100 TW-class laser system with parameters relevant to the L2-DUHA laser at the ELI Beamlines Facility, the PIC results demonstrate electron acceleration to mean energies exceeding 1.0 GeV with high-quality beam properties. Self-injected He electrons are also observed, and their impact on the main beam quality is evaluated. The findings of this study provide valuable insights for upcoming LWFA experiments planned within the EuPRAXIA Project at the ELI Beamlines Facility.

Figures (7)

• Figure 1: (a)-(b) Schematic of a specially designed capillary gas cell target consisting of two sections: (i) a short injection section with a nitrogen-helium mixture for electron injection, and (ii) a longer acceleration section where the injected electrons are further accelerated. This design helps truncate injection and reduce the final energy spread of the accelerated electron beam.
• Figure 2: (a1)–(a4) Snapshots of the gas-filling process in the capillary cell for a particular case at simulation times $t = 100~\mu\text{s}$, $150~\mu\text{s}$, $250~\mu\text{s}$, and $400~\mu\text{s}$, respectively. Subplots (b) and (c) show the mole fractions of helium and nitrogen, respectively, at $t = 400~\mu\text{s}$. Here, Inlet-1 is supplied with a $90\%$ He and $10\%$ N$_2$ mixture, while Inlet-2 and Inlet-3 are fed with $100\%$ He.
• Figure 3: (a)–(b) On-axis partial pressure profiles of He and $N_2$ along the capillary axis. The different cases are as follows: For Case-1 to Case-4, the separation between Inlet-1 and Inlet-2 is 0.5 mm, 1.0 mm, 1.5 mm, and 2.0 mm, respectively. In these cases, Inlet-1 and Inlet-2 are maintained at 30 mbar, while Inlet-3 is set to 40 mbar. For Case-5, all three inlets are maintained at 30 mbar, but the separation between Inlet-1 and Inlet-2 is 1.0 mm. The pressure at both outlets is fixed at $1.0 \times 10^{-6}$ mbar (vacuum) for all cases. (c)–(d) Corresponding on-axis electron density profiles calculated from the partial pressures, assuming two electrons per He atom and ten electrons per $N_2$ molecule. These on-axis 1D profiles are extracted at the steady-state condition at a simulation time of $t = 400~\mu\mathrm{s}$.
• Figure 4: Electron density distributions on the 2D x–y plane at three different simulation times for (a1–a3) Case-2, (b1–b3) Case-4, and (c1–c3) Case-5, respectively. The electron density is normalized to the critical density $n_c$ corresponding to the laser frequency used in the simulation.
• Figure 5: On-axis longitudinal electric field $E_x$ and the corresponding energy distributions of accelerated electrons at three different simulation times are shown for (a1)–(a3) Case-2, (b1)–(b3) Case-4, and (c1)–(c3) Case-5, respectively. The longitudinal electric field $E_x$ is plotted as green solid lines. The color-mapped patches (see colorbar) represent electrons ionized from N$^{5+}$ ions, while blue scattered dots correspond to electrons originating from helium with energies exceeding 5.0 MeV.