First Electron Acceleration in a Tunable-Velocity Laser Wakefield

Abstract

We present the first experimental confirmation that a laser-wakefield accelerator produced by a flying focus pulse is able to maintain the coherent structures necessary to accelerate electrons to relativistic energies. Through a combination of spatio-temporal near-field shaping of the beam and focusing with an axiparabola - a long-focal-depth mirror that produces a quasi-Bessel beam - the propagation velocity of the wakefield is tuned to control the maximum electron energy achievable. The experimental data are supported by advanced optical and particle-in-cell simulations and are aligned with a simplified analytical model. Together, the results significantly strengthen the case for the flying-focus wakefield as a strategy for mitigating dephasing in laser-wakefield acceleration.

Figures (3)

• Figure 1: (a) Schematic representation of the electron acceleration experiment. A laser pulse (red disk) is focused by the axiparabola (turquoise cylinder) onto a gas jet (gray emanating from gold jet), creating a wakefield (blue column) and accelerating an electron bunch (yellow dot). The laser pulse is also shown at focus over the gas jet demonstrating the development of the Bessel rings along the focal depth. After the gas jet, the electrons travel through a magnet, which gives an energy dependent trajectory to the electrons, which then impinge on a Lanex scintillator screen. A sample Lanex image with an electron spectrum is shown. (b) Normalized focal spot intensity over the focal depth, in vacuum. Solid line shows simulated value while markers show experimentally measured points. (c) Selected 2D focal spots images at different points along the focal depth. (d) Ansys Fluent simulation of gas density from the nozzle. Dotted white line shows laser height.
• Figure 2: (a) Measured velocity of intensity peak propagation in vacuum along the optical axis for the axiparabola focused beam. Shown for the $\alpha = -0.0045$ (green), $\alpha = 0.0055$ (blue), and $\alpha = 0.0190$ (orange) cases. The shaded area corresponds to the measurement error. (b--d) Three selected Lanex images for the $\alpha = -0.0045$ (b), $\alpha = 0.0055$ (c), and $\alpha = 0.0190$ (d) cases. The colorbar gives charge density, the x-axis provides energy information, and the y-axis gives the divergence. Total charge above 150 MeV is provided for each shot. (e) Simulated Lanex images for the $\alpha = -0.0045$ (top), $\alpha = 0.0055$ (middle), $\alpha = 0.0190$ (bottom) cases. (f) Simulated wakefield for $\alpha = -0.0045$ (top) and $\alpha = 0.0190$ (bottom) cases. Blue colorbar shows relative electron density distribution $n_e/n_0$ and red color shows the intensity of the axiparabola laser field. Line plots show the longitudinal electric field $E_z$ for the $\alpha = -0.0045$ (green) and $\alpha = 0.0190$ (orange) cases.
• Figure 3: (a--c) Electron spectra above $225$ MeV, averaged over 20 shots, for the $\alpha = -0.0045$ (b, green), $\alpha = 0.0055$ (c, blue), and $\alpha = 0.0190$ (d, orange) cases. The energy is shown on the x-axis and the charge density on the y-axis. The shaded area is the RMS shot-to-shot fluctuation. (d) Comparison of the three averaged spectra for the three cases. (e) Comparison of the spectra obtained from the PIC simulation for the three cases. (f) Plot of the maximum energy fluctuations for each of the three cases. Dotted horizontal line gives average maximum energy for each of the cases. (g) Plot of the charge (above $150$ MeV) fluctuations for each of the three cases. Dotted horizontal line gives average charge for each of the cases.