High Efficiency Uniform Wakefield Acceleration of a Positron Beam Using Stable Asymmetric Mode in a Hollow Channel Plasma

Plasma wakefield acceleration in the blowout regime is particularly promising for high-energy acceleration of electron beams because of its potential to simultaneously provide large acceleration gradients and high energy transfer efficiency while maintaining excellent beam quality. However, no equivalent regime for positron acceleration in plasma wakes has been discovered to date. We show that after a short propagation distance, an asymmetric electron beam drives a stable wakefield in a hollow plasma channel that can be both accelerating and focusing for a positron beam. A high charge positron bunch placed at a suitable distance behind the drive bunch can beam-load or flatten the longitudinal wakefield and enhance the transverse focusing force, leading to high efficiency and narrow energy spread acceleration of the positrons. Three-dimensional quasistatic particle-in-cell simulations show that an over 30% energy extraction efficiency from the wake to the positrons and a 1% level energy spread can be simultaneously obtained. Further optimization is feasible.

Figure 1

Evolution of an asymmetric electron beam and wakefields in a hollow plasma channel. (a)–(c) Plasma and beam densities in $x-\xi$ plane (beam moves from left to right), and (d)–(f) at the cross section of beam center slice denoted by the gray dash line and transverse wakefields. Black arrows are the vector flow of transverse wakefield $-{\overrightarrow{W}}_{\perp }=-(E_x-c{B}_y)\widehat{x}-(E_y+c{B}_x)\widehat{y}$. Blue line is the lineout of $-{\overrightarrow{W}}_{\perp }$ at $y=0$. Red arrows indicate the spot size evolution of the drive beam. The propagation distances are 0, 2.5, 10 cm from left to right.

Figure 2

(a),(b) Plasma and beam densities of a symmetric and asymmetric electron beam with initial offset $0.1\mu \mathrm{m}$ in $x$ direction after 20 cm transportation in a hollow plasma channel. The dashed line is one-tenth of the maximum contour of the initial beam density. (c) The evolution of the average offset for the electron beams in $x$ and $y$ directions.

Figure 3

Unloaded wakefields. (a)–(d) is of a 100 pC driver and (e)–(h) is of a 2 nC driver. (a),(e) Beam and plasma densities after stabilization. (b),(f) $E_z$ field. Transverse wakefields in $x-\xi$ plane (c),(g) and $y-\xi$ plane (d),(h). The vertical lineouts of wakefields are at $\xi =315\mu \mathrm{m}$.

Figure 4

Positron beam-loading effects. (a) Loaded plasma and beam densities, the gray dashed line is the current profile of two beams. (b) Plasma density in region of interest. Left: unloaded, right: beam-loading case. The dashed line is one-tenth of the maximum contour of the initial positron bunch density. (c) Loaded wakefields in the region denoted by the orange dashed line in (a). The red lines represent the range of $\pm 2{\sigma }_{x,z}$ for the positron beam. (d) Normalized slice emittance of the witness beam at $\xi =315\mu \mathrm{m}$ vs propagation distances. (e) Evolution of the energy spread of the witness beam with $\xi >305\mu \mathrm{m}$. (f) Final energy spectra and longitudinal phase space of the witness beam, the dashed line represents energy of 13.8 GeV.