Plasma-Wakefield Acceleration of an Intense Positron Beam

Plasma wakefields are both excited and probed by propagating an intense 28.5 GeV positron beam through a 1.4 m long lithium plasma. The main body of the beam loses energy in exciting this wakefield while positrons in the back of the same beam can be accelerated by the same wakefield as it changes sign. The scaling of energy loss with plasma density as well as the energy gain seen at the highest plasma density is in excellent agreement with simulations.

Figure 1

Longitudinal electric field (solid red curve) excited in a plasma by a positron beam which is moving right to left in the figure (dashed blue curve). The field is averaged over one ${\sigma }_r$. A peak decelerating field of $50\mathrm{M}\mathrm{V}/\mathrm{m}$ occurs 1 ps ahead of the centroid and a peak accelerating field of $64\mathrm{M}\mathrm{V}/\mathrm{m}$ occurs 3 ps behind the centroid.

Figure 2

3D particle-in-cell simulation of a positron beam in a plasma. The image depicts the phase space of the beam after it has traversed 1.4 m of plasma. Beam parameters in the simulation were identical to experimental parameters. The simulation predicts an energy loss of 64 MeV and an energy gain of 79 MeV.

Figure 3

Energy loss of the positron beam’s centroid after propagating through 1.4 m of plasma. The energy loss of the centroid increases with increasing plasma density. The red triangles are the 3D PIC simulation predictions of peak energy loss. The blue circles are the experimentally measured energy losses at different plasma densities. At a density of $1.8\times {10}^{14}{\mathrm{c}\mathrm{m}}^{-3}$, the energy loss is $68\pm 8\mathrm{M}\mathrm{e}\mathrm{V}$.

Figure 4

Time slice analysis of the energy dynamics within a single positron bunch. The plasma off (blue triangles) shows a slight head-tail energy chirp of $\sim 20\mathrm{M}\mathrm{e}\mathrm{V}$. When the plasma is on (red squares), the front half of the beam loses energy driving the plasma wave, while the back half of the beam is accelerated by the plasma wave. The black line is the charge distribution within the bunch