High-Gradient Plasma-Wakefield Acceleration with Two Subpicosecond Electron Bunches

A plasma-wakefield experiment is presented where two 60 MeV subpicosecond electron bunches are sent into a plasma produced by a capillary discharge. Both bunches are shorter than the plasma wavelength, and the phase of the second bunch relative to the plasma wave is adjusted by tuning the plasma density. It is shown that the second bunch experiences a $150\mathrm{MeV}/\mathrm{m}$ loaded accelerating gradient in the wakefield driven by the first bunch. This is the first experiment to directly demonstrate high-gradient, controlled acceleration of a short-pulse trailing electron bunch in a high-density plasma.

Figure 1

Layout of the double-bunch PWFA experimental setup.

Figure 2

Experimental and simulated energy spectra of the double-bunch beam after the 6 mm long capillary discharge at $4\times {10}^{15}{\mathrm{cm}}^{-3}$ plasma density (left column), and at $1\times {10}^{16}{\mathrm{cm}}^{-3}$ density (right column): (a),(f) raw energy spectrum without plasma; (b),(g) raw energy spectrum with plasma on; (c),(h) experimental energy profiles; (d),(i) simulated energy profiles; (e),(k) simulated plasma-wakefield and position of the bunches inside the wake.

Figure 3

Experimental data points for the energy shift of the witness electron-bunch centroid for a range of plasma densities. The solid curve represents 2D numerical calculations for the centroid energy shift.