Ultra-bright electron bunch injection in a plasma wakefield driven by a superluminal flying focus electron beam*

We propose a new method for self-injection of high-quality electron bunches in the plasma wakefield structure in the blowout regime utilizing a "flying focus" produced by an electron drive-beam with an energy chirp. In a "flying focus" the speed of the density centroid of the drive bunch can be superluminal or subluminal by utilizing the chromatic dependence of the focusing optics. We first derive the focal velocity and the characteristic length of the focal spot in terms of the focal length and an energy chirp for the particle beam. We then demonstrate using multi-dimensional particle-in-cell simulations that a plasma wake driven by a superluminally propagating flying focus of an electron beam can generate GeV-level electron bunches with ultra-low normalized slice emittance (∼ 30 nm rad), high current (∼17 kA), low slice energy spread (∼0.1%) and therefore high normalized brightness (>10¹⁹ A/rad²/m²) in plasma of density ~10¹⁹ cm^-3. The injection process is highly controllable and tunable by changing the focal velocity and shaping the drive beam current. Near-term experiments using the new FACET II beam could potentially produce beams with brightness exceeding 10²⁰ A/rad²/m².