Narrow bandwidth, low-emittance positron beams from a laser-wakefield accelerator

Abstract

The rapid progress that plasma wakefield accelerators are experiencing is now posing the question as to whether they could be included in the design of the next generation of high-energy electron-positron colliders. However, the typical structure of the accelerating wakefields presents challenging complications for positron acceleration. Despite seminal proof-of-principle experiments and theoretical proposals, experimental research in plasma-based acceleration of positrons is currently limited by the scarcity of positron beams suitable to seed a plasma accelerator. Here, we report on the first experimental demonstration of a laser-driven source of ultra-relativistic positrons with sufficient spectral and spatial quality to be injected in a plasma accelerator. Our results indicate, in agreement with numerical simulations, selection and transport of positron beamlets containing $N_{e+}\ge {10}^5$ positrons in a 5% bandwidth around 600 MeV, with femtosecond-scale duration and micron-scale normalised emittance. Particle-in-cell simulations show that positron beams of this kind can be guided and accelerated in a laser-driven plasma accelerator, with favourable scalings to further increase overall charge and energy using PW-scale lasers. The results presented here demonstrate the possibility of performing experimental studies of positron acceleration in a laser-driven wakefield accelerator.

Figure 1

Illustration of the experimental setup. showing the electron plasma accelerator, the converter, the emittance mask, scintillators for electrons (LE1 and LE2) and positrons (LP1 and LP2). Electron (red) and positron (blue) trajectories are also shown to guide the eye.

Figure 2

Primary electron beam characteristics. Typical (a) angularly resolved and (b) angularly integrated electron spectra of the LWFA electron beams (red). The average (black) and standard deviation (grey) of the integrated electron spectra are shown along with the approximation used as an input for the positron generation simulations (blue).

Figure 3

Narrow energy spread positron beams. Typical single-shot positron spectra measured after energy selection for different positions of the energy selection slit. Raw data has been background-subtracted and smoothed with a 10 MeV Gaussian filter, with the shaded region representing the local rms scatter of the data. The central energy and FWHM bandwidth of each spectrum is indicated in the figure legend.

Figure 4

Raw images of energy-resolved beam profiles with the emittance mask. Example modulated (a) positron and (b) electron spatial charge density as a function of position on the screens ($x_p,y_p,x_e,x_e$) for a single shot with a converter thickness of 8.0 mm and the emittance mask in the beam-line. The positions corresponding to the given particle energies in MeV are shown as vertical red dashed lines. The slight difference between the electron and positron raw data is due to the slightly different position of the scintillator screens (discussed in the text).

Figure 5

Positron properties as function of energy and converter thickness. Measured electron and positron beam properties as functions of particle energy for different converter thicknesses. The (a) spectra (charge per 5% bandwidth), (b) source size, (c) divergence and (d) geometric emittance are given for each converter length as shown by the color-bars at the side of the figure. For each converter thickness, only the shots resulting in the highest charge of the positron beams were used for the analysis; the lines shown are thus an average of 4, 6, 3 and 8 shots for converter lengths of 1.0, 2.0, 4.0 and 8.0 mm, respectively (rms variation of $\approx$ 43% in the spectrum and $\approx$ 20% in emittance, respectively). Each line results from a Gaussian weighting of each measurement point using a kernel width of ${\sigma }_E=25$ MeV. The typical input electron spectrum (black-dashed) is shown in a) for comparison.

Figure 6

Comparison with numerical simulations. Positron beam (a) spectrum (charge per 5% bandwidth), (b) source size, (c) divergence and (d) geometric emittance plotted for a 1.0 mm thick lead converter. The experimental data (red) is plotted alongside FLUKA simulations for: zero drift distance for the primary electron beam (black); including the drift distance and the primary electron beam divergence and source size (cyan dashed); and including the 12.6 mrad shielding aperture (blue).

Figure 7

Simulated post-acceleration of a laser-generated positron beam. (a, b) show the longitudinal electric fields of the plasma wakefield generated by the laser pulse (yellow orb) and the trailing positron bunch (density in a logarithmic colour-scale) at the beginning of the plasma and after 96 mm of propagation. Panels (c, d) show the positron longitudinal phase space before and after acceleration, with the energy spectra indicated by the red lines. (e) shows the average energy and normalised emittance of the trapped bunch, defined as being comprised of particles which remain within $\pm 50$ µm of the central axis.