Numerical study of electron acceleration by microwave-driven plasma wakefields in rectangular waveguides

Abstract

Plasma-based acceleration schemes have attracted sustained interest as a pathway toward compact particle accelerators, owing to the large electric fields supported by plasmas. Although recent studies have demonstrated the excitation of plasma wakefields using high-power microwave pulses in plasma-filled waveguides, the conditions required for efficient electron acceleration in such configurations remain insufficiently characterized. In this work, we investigate the acceleration of externally injected electrons by microwave-driven plasma wakefields in rectangular waveguides filled with low-density plasma. Three-dimensional particle-in-cell simulations are employed to analyze the dynamics of electron injection and energy gain under both reduced and fully self-consistent numerical models. The results show that electron acceleration is strongly dependent on the injection phase and initial velocity. Optimal acceleration is achieved when electrons are pre-accelerated to velocities close to the group velocity of the driving microwave pulse. For the parameters considered, energy gains of the order of $10^2 \mathrm{keV}$ are obtained over interaction lengths of the order of meters, while maintaining a quasi-monoenergetic energy distribution under suitable injection conditions. The influence of transverse dynamics and space-charge effects is also examined, revealing additional constraints on acceleration efficiency associated with the transverse electromagnetic field of the driving microwave pulse. These results provide a quantitative assessment of the acceleration stage in microwave-driven plasma wakefield schemes and support their evaluation as a viable platform for compact plasma-based accelerators.

Figures (10)

• Figure 1: Schematic of the rectangular plasma-filled waveguide configuration. A short, high-power TE$_{10}$-mode microwave pulse propagates along the $z$ axis, exciting a longitudinal plasma wakefield trailing the driving pulse. The transverse waveguide dimensions are denoted by $a$ and $b$, and the Cartesian coordinate system $(x,y,z)$ is indicated.
• Figure 2: One-dimensional schematic showing the longitudinal electric field $E_z$ of the microwave-driven plasma wakefield. The spatial coordinate $\xi$ denotes the position relative to the wake phase. A witness electron (green marker) is injected within the first accelerating region, corresponding to the negative phase ($E_z < 0$), where efficient trapping and energy gain occur.
• Figure 3: Energy gain of the witness electron as a function of the initial longitudinal velocity $v_{z0}$ for five representative injection positions $\xi$ within the first accelerating bucket of the microwave-driven plasma wakefield. For each injection phase, an optimal initial velocity is observed.
• Figure 4: Initial phase-space distributions of the injected electron bunch. (a) Transverse distribution in the $x$–$y$ plane, showing a circular cross section with radius $r_b = 2$ mm. (b) Longitudinal distribution characterized by $\sigma_z = 2$ mm. (c) Longitudinal velocity distribution with a relative spread of $0.1\%$. Gaussian profiles are adopted in all cases.
• Figure 5: (a) Mean energy gain (black line) and associated energy spread $\Delta K$ (blue shaded area) of the electron bunch in the test-particle regime. (b) Longitudinal wakefield $E_z$ (blue) and transverse microwave field $E_y$ (black) at the injection stage. (c) Same field components as in (b) at the end of the simulation. The red dashed line indicates the mean longitudinal position of the bunch, and the green shaded region denotes the effective accelerating phase of the wakefield.