Observation of High Transformer Ratio Plasma Wakefield Acceleration

Particle-beam-driven plasma wakefield acceleration (PWFA) enables various novel high-gradient techniques for powering future compact light-source and high-energy physics applications. Here, a driving particle bunch excites a wakefield response in a plasma medium, which may rapidly accelerate a trailing witness beam. In this Letter, we present the measurement of ratios of acceleration of the witness bunch to deceleration of the driver bunch, the so-called transformer ratio, significantly exceeding the fundamental theoretical and thus far experimental limit of 2 in a PWFA. An electron bunch with ramped current profile was utilized to accelerate a witness bunch with a transformer ratio of $4.6_{-0.7}^{+2.2}$ in a plasma with length $\sim 10\mathrm{cm}$, also demonstrating stable transport of driver bunches with lengths on the order of the plasma wavelength.

Figure 1

Schematic layout of the measurement setup with the plasma, the transverse deflecting structure (TDS), the movable YAG screen for bunch profile measurements, the 60° sector dipole spectrometer, and the LYSO screen for longitudinal phase space measurements.

Figure 2

Experimental and simulated electron bunch currents $I_b$ (solid lines) and slice energy changes $\mathrm{\Delta }{E}_{\text{slice}}$ (dashed lines) between plasma off and on cases in the comoving coordinate $\xi =z/c$. Blue cross and red circle indicate measured and simulated maximum witness energy gain, respectively. Total charge of ramped driver (right) and short, low charge Gaussian witness bunch (left) is $518\pm 16\mathrm{pC}$. The time resolution of the measurement is 0.6 ps.

Figure 3

(a) Simulated longitudinal electrical field distribution and (b) beam electron (color scale) and plasma electron (gray scale) densities after 64 mm of propagation into the $2.2\times {10}^{13}{\mathrm{cm}}^{-3}$ plasma. The red line in (a) indicates the bunch current profile which peaks at 38 A. The blue line shows the longitudinal electrical field on axis and shares the axis of the color map.

Figure 4

Longitudinal phase space of driver and witness bunches measured without (a) and with (b) plasma acceleration in a plasma of $2\times {10}^{13}{\mathrm{cm}}^{-3}$ electron density. Note that the upper (witness) bunch was imaged with a different camera gain than the driver. Dashed lines indicate the witness mean (green) and maximum (red) energy and the driver mean slice energies (white). The bunch current and the measured slice energy changes inside of the driver bunch, corresponding to this measurement, are shown in Fig. 2.