Figure 1. Schematic diagram of the experimental setup.

The FACET electron beam is focused by final focus quadrupoles inside a volume filled with argon gas and contained by two beryllium windows. After interacting with the plasma, electrons are transported by a quadrupole doublet towards the beam dump and deflected vertically by a dipole magnet. A Cherenkov detector is used to measure the electron energy spectrum (see the Methods for details).