High brightness, symmetric electron bunch generation in a plasma wakefield accelerator via a radially-polarized plasma photocathode

TL;DR

This work addresses the challenge of generating ultra-bright, symmetric electron bunches in plasma wakefield accelerators by employing a radially-polarized plasma photocathode. The authors perform start-to-end simulations combined with a multi-objective Bayesian optimization to map performance and stability of high-charge injection, revealing that azimuthally symmetric injection minimizes transverse emittance growth and maximizes 6D brightness in the optimally-loaded regime. They report final witness-bunch parameters around $Q_b \approx 38\,\mathrm{pC}$, mean energy $\approx 2.40\,\mathrm{GeV}$, $\varepsilon_{\mathrm{n},x/y} \approx 224\,\mathrm{nm}$, and $B_{6D,n} \approx 1.1 \times 10^{17}\ \mathrm{A\,m^{-2}\,0.1\%^{-1}}$ (projected), with slice brightness near $5.8 \times 10^{17}\ \mathrm{A\,m^{-2}\,0.1\%^{-1}}$. The study finds that $\langle \varepsilon_n \rangle$ scales as $\sqrt{Q_b}$ in the high-charge regime and demonstrates superior brightness and emittance symmetry relative to linearly-polarized injections, supporting potential deployment in future plasma-based light sources. Timing jitter between the drive beam and the ionizing pulse emerges as a key stability limiter, with mitigation strategies discussed (e.g., lower density operation, energy compression, and optimized wakefield matching). Overall, the radially-polarized plasma photocathode offers a practical route to high-brightness, compact injectors for next-generation FELs and other light sources.

Abstract

The plasma photocathode has previously been proposed as a source of ultra-high-brightness electron bunches within plasma accelerators. Here, the scheme is extended by using a radially-polarized ionizing laser pulse to generate high-charge, high-brightness electron bunches with symmetric transverse emittance. Efficient start-to-end modelling of the scheme, from ionization and trapping until drive bunch depletion, enables a multi-objective Bayesian optimisation routine to be performed to understand the performance of the radially-polarized plasma photocathode, quantify the stability of the scheme, and explore the fundamental relation between the witness bunch charge and its emittance. Comparison of plasma photocathodes driven by radially- and linearly-polarized laser pulses show that the former yields higher brightness electron bunches when operating in the optimally-loaded regime.

Figures (10)

• Figure 1: Snapshot from PIC simulation demonstrating the injection of dopant electrons into the wakefield driven by a high-density, ultra-relativistic electron bunch. This timestep corresponds to the focal location of the RPLP ($z = z_\mathrm{foc}$). Upper inset: The transverse intensity profile of the plasma photocathode at focus. Lower inset: The on-axis longitudinal electric field at $z = z_\mathrm{foc}$ (black) and $z = 1mm$ (grey, dashed), including the current profile of the trapped charge ($z = 1mm$). The electron number density, $n_e$, is coloured according to ionization source: pre-ionized hydrogen (grey), He (green/blue) and He$^+$ (pink/purple) and normalized to the background plasma density, $n_0 = 1e17cm^{-3}$. The normalized drive bunch density, $n_d$, is represented by the orange/black colormap, while the intensity profile of the RPLP is shown in red.
• Figure 2: The transverse phase space of the trapped electrons at (a) $z = 1mm$ and (b) $z = 90mm$, the depletion length of the drive bunch. The trapped charge from each ionization source (He: blue, He$^+$: pink) is displayed above the sub-figures. Logarithmic colorbars are used to improve visibility of the strongly-peaked distributions.
• Figure 3: (a) Longitudinal phase space of the witness bunch after propagation over 90mm. (b) Slice energy spread (grey, solid) and normalised transverse slice emittance (orange, dotted). In (b), (c) and (d) the plotted quantities are separated by ionization source: He (blue); He$^+$ (pink); black lines represent calculations over the entire bunch. Evolution of the (c) projected normalised transverse emittance, and (d) charge, of the witness bunch over the entire propagation distance. In (c), thick solid (dashed) lines represent $\varepsilon_{\mathrm{n},x}$ ($\varepsilon_{\mathrm{n},y}$) for each ionization source; the thin black line represents $\langle \varepsilon_\mathrm{n} \rangle = \sqrt{\varepsilon_{\mathrm{n},x} \cdot \varepsilon_{\mathrm{n},y}}$ for the witness bunch. The vertical dotted line at $z = 1mm$ marks the handover between simulation codes.
• Figure 4: (a) Evolution of the on-axis trapping potential, $\Delta \Psi (\zeta, z) = \Psi_i - \Psi_f$, indicating the region for which released electrons can become trapped, $\Delta \Psi \lesssim -1$ (black, hatched). (b) Comparison of $\Delta \Psi$ just after trapping begins ($z = 0.2mm$, black) and as the wakefield becomes over-loaded ($z = 0.5mm$, orange dashed).
• Figure 5: Evolution of the average transverse momentum, $\langle p_x \rangle$, of electrons ionized by the RPLP both (a) with and (b) without the presence of the wakefield. (c) Evolution of the transverse normalised slice emittance of electrons released from ionization of He (blue) and He$^+$ (pink) within a single simulation timestep, both with (solid) and without (dashed) the wakefield. Electrons slip backwards (towards $-\zeta$) before being captured at the rear of the wakefield cavity ($\zeta - \zeta_{0,L} \approx -38µ m$). The longitudinal profile of the RPLP is represented by the grey shaded region. In (c), a median filter has been applied to the data to remove high frequency oscillations due to the laser field, whose effect can clearly be seen in (a) and (b). This timestep corresponds to $z = 0.2mm$.