Energy-Spread Preservation and High Efficiency in a Plasma-Wakefield Accelerator

Energy-efficient plasma-wakefield acceleration of particle bunches with low energy spread is a promising path to realizing compact free-electron lasers and particle colliders. High efficiency and low energy spread can be achieved simultaneously by strong beam loading of plasma wakefields when accelerating bunches with carefully tailored current profiles [M. Tzoufras et al., Phys. Rev. Lett. 101, 145002 (2008)]. We experimentally demonstrate such optimal beam loading in a nonlinear electron-driven plasma accelerator. Bunches with an initial energy of 1 GeV were accelerated by 45 MeV with an energy-transfer efficiency of $(42\pm 4)\%$ at a gradient of $1.3\mathrm{GV}/\mathrm{m}$ while preserving per-mille energy spreads with full charge coupling, demonstrating wakefield flattening at the few-percent level.

Figure 1

(a) A notch collimator with an adjustable width and position, located in a dispersive section, was used to create two bunches from a chirped electron bunch. (b) The resulting current profiles were measured with a downstream TDS. (c) A discharge capillary was used to form a plasma channel. The plasma density was measured to decay exponentially (orange trendline) after the initial discharge—the density was varied by adjusting the beam arrival time. Measurements shortly after the discharge (shaded area) may be inaccurate due to temperature effects [28]. (d) Energy spectra were measured with a dipole spectrometer and a set of quadrupoles for point-to-point imaging. (e) 3D parameter scan of plasma density (1) versus notch position (2) as a function of notch width (3). Each row of plots shows, from the top, measurements at each step of the transformer ratio ($T$), energy-transfer efficiency ($\eta$) and energy-spread-to-gain ratio (${\sigma }_{\delta }$), which were combined into an overall optimization parameter $\mathrm{\Omega }$ [Eq. (2)]. A full characterization was performed at the optimal operating point (red circle).

Figure 2

(a) Spectrometer images at the optimal operating point [red circle in Fig. 1], as well as the corresponding energy spectra, for shots with and without plasma interaction. The initial energy spread of the trailing bunch is preserved. (b) High stability is observed across 5000 consecutive shots—the energy gain is stable to within 3% rms. (c) In 6.4% of these shots, the energy spread is lower than or equal to the initial energy spread (dotted line). (d) Simultaneously, high energy-transfer efficiency is observed, distributed between 30% and 50%.

Figure 3

Comparison of incoming trailing-bunch charge (gray points) and accelerated charge (blue points) in a tail-collimator scan. Charge is lost until position $\xi \lesssim -360\mu \mathrm{m}$, after which full charge coupling is observed. This transition is caused by the defocusing field of plasma electrons crossing the axis: beam particles behind the axis crossing are lost, whereas particles ahead remain focused. The incoming trailing charge is calculated by subtracting the mean driver charge from the total charge, as measured by a toroid, where the error bars represent the standard error of the mean. The accelerated charge is measured on the spectrometer, where the error bars represent the standard deviation. Data in Fig. 2 were taken at the optimal tail-collimator position (dotted line).

Figure 4

(a) Longitudinally averaged wakefield measured using a tail-collimator scan, both for optimal beam loading (blue points) and for a full bunch (gray points)—in excellent agreement with PIC simulations (blue and gray solid lines). A simulation with no trailing bunch (blue dotted line) indicates that the wakefield was flattened by beam loading. The beam currents of the collimated and the full bunch (blue and gray areas) were measured with a TDS. (b) Snapshot showing cross sections of beam (orange) and plasma electron density (blue) for a simulation of the optimal operating point, on a logarithmic color scale. The simulated flat-top plasma density is $7.2\times {10}^{15}{\mathrm{cm}}^{-3}$.