Multi-GeV Energy Gain in a Plasma-Wakefield Accelerator

A plasma-wakefield accelerator has accelerated particles by over 2.7 GeV in a 10 cm long plasma module. A 28.5 GeV electron beam with $1.8\times {10}^{10}$ electrons is compressed to $20\mu \mathrm{m}$ longitudinally and focused to a transverse spot size of $10\mu \mathrm{m}$ at the entrance of a 10 cm long column of lithium vapor with density $2.8\times {10}^{17}\mathrm{\text{atoms}}/{\mathrm{cm}}^3$. The electron bunch fully ionizes the lithium vapor to create a plasma and then expels the plasma electrons. These electrons return one-half plasma period later driving a large amplitude plasma wake that in turn accelerates particles in the back of the bunch by more than 2.7 GeV.

Figure 1

(a) The measured energy spectrum (solid line) is plotted with the matching spectrum obtained using litrack simulations (dashed line). (b) The corresponding simulated longitudinal phase space and (c) reconstructed current profile The current profile indicates that the head of the incoming electron bunch has a long low-current leading edge and a Gaussian shaped high-current core near the back. (d) The current profile from litrack (solid line) end the energy change predicted by a 2D osiris simulation (dashed line).

Figure 2

Single bunch energy spectra downstream from the plasma for (a) the case of no plasma and (b) a 10 cm long $2.8\times {10}^{17}{e}^{-}/{\mathrm{cm}}^3$ lithium plasma. The no plasma case shows the $\sim 1\mathrm{GeV}$ energy spread typical of the incoming compressed pulses. At right, the core of the electron bunch has lost energy driving the plasma wake while particles in the back of the bunch have been accelerated to 2.7 GeV over the maximum incoming energy. The images are displayed with a saturated color map to highlight the 7% of the bunch particles accelerated to energies higher than the maximum incoming energy. The peak intensity in both figures is more than a factor of 4 below the saturation level of the camera.

Figure 3

(a) Energy spectra for a plasma on and off case. 25% of the electrons in the head of the bunch, which also have the highest energies in the plasma off case, are not affected by the lithium vapor. (b) The normalized cumulative charge sum is used to quantify the energy gain or loss of the highest or lowest energy 1% fraction of bunch particles.

Figure 4

(a) Binned contours of 1% and 99% of the cumulative normalized bunch charge for plasma on (red) and off (blue) vs the energy signal on the CTR detector. The vertical bars represent the minimum and maximum values for that bin. The measured energy spectrum, matching litrack simulation (b)–(d) and corresponding bunch current profiles (e) are shown for three cases: less compressed ($37\mu \mathrm{m}$), optimal ($27\mu \mathrm{m}$), and further compressed ($19\mu \mathrm{m}$), respectively.