Demonstration of a beam loaded nanocoulomb-class laser wakefield accelerator

Abstract

Laser-plasma wakefield accelerators have seen tremendous progress, now capable of producing quasi-monoenergetic electron beams in the GeV energy range with few-femtoseconds bunch duration. Scaling these accelerators to the nanocoulomb range would yield hundreds of kiloamperes peak current and stimulate the next generation of radiation sources covering high-field THz, high-brightness X-ray and γ-ray sources, compact free-electron lasers and laboratory-size beam-driven plasma accelerators. However, accelerators generating such currents operate in the beam loading regime where the accelerating field is strongly modified by the self-fields of the injected bunch, potentially deteriorating key beam parameters. Here we demonstrate that, if appropriately controlled, the beam loading effect can be employed to improve the accelerator's performance. Self-truncated ionization injection enables loading of unprecedented charges of ∼0.5 nC within a mono-energetic peak. As the energy balance is reached, we show that the accelerator operates at the theoretically predicted optimal loading condition and the final energy spread is minimized.Higher beam quality and stability are desired in laser-plasma accelerators for their applications in compact light sources. Here the authors demonstrate in laser plasma wakefield electron acceleration that the beam loading effect can be employed to improve beam quality by controlling the beam charge.

Fig. 1

Energy spectra of 15 consecutive shots. a Raw energy electron spectra. The color map represents the charge density (pC mm⁻²) on the detector. b Energy spectrum of the first shot from a. The filled area represents the charge within the FWHM, the yellow dashed line represents the mean peak energy and the black dashed line represents the maximum attained energy (E _max) at 0.1 pC MeV⁻¹. Obtained with a supersonic gas jet with a 1.6 mm-long plasma density plateau of 3.1 × 10¹⁸ cm⁻³, 1% nitrogen doping and 2.5 J laser energy in 30 fs FWHM duration. Line graphs of all shots shown in (a) can be found in Supplementary Fig. 2

Fig. 2

Electron energy dependency on both charge and plasma density. Connected data points show a set of equal plasma density, increasing charge with increasing nitrogen doping (see Supplementary Fig. 5). Electron energy is the E _max and charge within the FWHM of the energy peak is displayed. The error bars represent the s.e.m. The estimated optimal loaded charge according to Eq. (1) of 313 pC is indicated by the dashed vertical line. The inset shows the predicted acceleration gradient (right axis) according to Eq. (1) (dotted line) for a laser peak power P = 64 TW (equivalent to the experiment) under optimal loading conditions. The left axis shows the predicted electron energy, taking 0.8 mm effective acceleration distance. Data points represent measured data points at the predicted optimum

Fig. 3

Beam absolute energy spread dependency on charge. Shown is the FWHM energy spread. The dashed vertical line represents the optimal load expected from Eq. (1). The error bars represent the s.e.m. A graph showing the relative energy spread can be found in Supplementary Fig. 7

Fig. 4

Energy evolution and beam loading effects during the acceleration process. Results from PIC simulations. a Energy histogram (top) showing evolution of the electron energy throughout the acceleration process. Final injected charge in the peak is 60 pC. The bottom plot shows the on-axis laser strength evolution with the dotted lines representing the required field for ionization of the two nitrogen K-shell electrons. Additional injection of both helium and nitrogen electrons occurs in the density down-ramp of the gas jet, resulting in a low-energy background in the final energy spectrum. b The effect of beam loading on the accelerating field E _z (line graphs, right axis) and electron phase space (color scale, left axis) for 60 pC load in the peak (red) and 168 pC load (blue). Corresponding to z = 2.3 mm in a

Fig. 5

PIC simulation results illustrating beam loading effects. a–d Electron energy histograms for increasing injected charges within the energy peak (purple area). The green- and purple-shaded area is the contribution from electrons originating from nitrogen, and orange-shaded area from helium electrons. The black line indicates the cumulative spectrum. a Corresponds to the histogram seen in Fig. 4a. e Shown is how maximum attained energy E _max (left axis) and beam energy spread (right axis) depend on injected charge

Fig. 6

Scaling of charge within FWHM with laser power. Circles represent measured data points taken with a nitrogen doping of 1% at a plasma density of 3.1 × 10¹⁸ cm⁻³. The error bars represent the s.e.m. The red curve represents a fit following the expected $Q\propto \sqrt{\mathrm{P}}$ dependency. The relative energy spread measured to be ∼15% for all measurement points