Influence of ion motion in a resonantly driven wakefield accelerator

TL;DR

This study investigates how ion motion influences the self-modulation of a long proton beam driving a resonantly excited wakefield, using PIC simulations aligned with AWAKE parameters. It identifies two ion-motion mechanisms—detuning of the drive with wakefields and enhanced transverse wavebreaking—that damp the wakefield and shorten the microbunch train, with detuning dominating at the beam head and wavebreaking at the tail. Both mechanisms share the same ion-mass scaling, $m_i^{-1/3}$, and are demonstrated across xenon, argon, and helium plasmas. The findings help guide ion selection and experimental design for efficient, long-distance plasma wakefield acceleration in projects like AWAKE.

Abstract

Several different schemes for plasma wakefield acceleration using a train of drivers have been pursued, based on the resonant excitation of a plasma wave. Since these schemes rely on the plasma electron wave surviving for many periods, the motion of the plasma ions can have a significant impact on the beam--plasma interaction. In this work, simulations are used to study the impact of this ion motion on the development of the self-modulation of a long beam, directly applicable to recent experiments. It is shown that two related but distinct effects contribute to the suppression of the wakefield excitation: the loss of resonance between the drive beam and the plasma wave it excites, and phase mixing due to transverse wavebreaking. Although only the latter has previously been investigated, we show that the two effects follow the same scaling with ion mass.

Figures (5)

• Figure 1: Time-resolved proton beam profiles from experiment (a-d) and simulation (e-h) after propagating for $10\,\mathrm{m}$ in vacuum (a,e), or a plasma of ionized xenon (b,f), argon (c,g) or helium (d,h). The images are captured using a streak camera positioned 3.5 m after the plasma source. For a full discussion of the experimental results, please refer to Turner2024.
• Figure 2: The plasma response to a proton beam undergoing self-modulation after a propagation distance of $s=$5 m. a) The beam density, b) the plasma ion density, and c) the longitudinal component of the wakefield are shown for xenon (top) and helium (bottom) plasmas.
• Figure 3: Lineouts showing key metrics of the beam--plasma interaction after 5 m propagation in xenon and helium plasmas. a) The effective current of the proton driver, $I_\mathrm{eff}$; b) the envelope, $\tilde{E_z}$, and c) the relative phase, $\Delta\varphi\left(E_z\right)$, of the on-axis longitudinal wakefield; and d) the plasma sheath field, evaluated at $k_pr=10.25$.
• Figure 4: Spatiotemporal evolution of the proton beam and plasma response over 10 m of plasma interaction, for xenon (upper of each plot) and helium (lower of each plot). Right to left shows the evolution along the beam, with the beam propagating to the right, while upper/lower edge towards the centre shows the evolution along the propagation length. a) the beam effective current; b) the amplitude and c) the relative phase of the longitudinal wakefield; and d) the plasma sheath field.
• Figure 5: Current profiles of the proton beam passing through a virtual aperture for different plasma species a) at the plasma exit ($z=10\,\mathrm{m}$) and b) after propagation to a screen placed 3.5 m after the plasma exit, as in \ref{['fig:ion_expstreak']}e-h. The cyan line indicates the end of the beam head, and the green line indicates the start of the tail.