Laser-plasma acceleration promises compact sources of high-brightness relativistic electron beams. However, the limited stability often associated with laser-plasma acceleration has previously prevented a detailed mapping of the drive laser and electron performance and represents a major obstacle towards advancing laser-plasma acceleration for applications. Here, we correlate drive laser and electron-beam parameters with high statistics to identify and quantify sources of electron energy drift and jitter. Based on our findings, we provide a parametrization to predict the electron energy drift with subpercent accuracy for many hours from measured laser parameters, which opens a path for performance improvements by active stabilization. Our results are enabled by the first stable 24-h operation of a laser-plasma accelerator and the statistics from 100 000 consecutive electron beams, which, by itself, marks an important milestone.

The FLASHForward project at DESY is a pioneering plasma--wakefield acceleration experiment that aims to produce, in a few centimetres of ionised hydrogen, beams with energy of order GeV that are of quality sufficient to be used in a free--electron laser. The plasma wave will be driven by high-current density electron beams from the FLASH linear accelerator and will explore both external and internal witness--beam injection techniques. The plasma is created by ionising a gas in a gas cell with a multi--TW laser system, which can also be used to provide optical diagnostics of the plasma and electron beams due to the <30 fs synchronisation between the laser and the driving electron beam. The operation parameters of the experiment are discussed, as well as the scientific program.

Plasma-based accelerators offer the possibility to drive future compact light sources and high-energy physics applications. Achieving good beam quality, especially a small beam energy spread, is still one of the major challenges. Here, we propose to use a periodically modulated plasma density to shape the longitudinal fields acting on an electron bunch in the linear wakefield regime. With simulations, we demonstrate an on-average flat accelerating field that maintains a small beam energy spread. DOI: 10.1103/PhysRevLett.118.214801 Using the extreme field gradients supported by a plasma cavity [1], plasma-based accelerators [2] promise very compact sources of ultrarelativistic electron beams for a large variety of applications. Yet, especially the beam energy spread, which in plasma experiments is typically on the percent level [3], causes emittance growth during beam transport, and hence renders its applicability to novel freeelectron laser (FEL) schemes [4,5] or high-energy physics applications [6] very difficult. Controlling the beam energy spread is thus one of the major challenges in the field of plasma acceleration.Decoupling the generation of the beam from the acceleration allows us to independently control and optimize the dynamics of each process and thus improve the beam quality. Prominent examples of this approach are based on the linear or quasilinear wakefield regime [2], and include the external injection of a well-characterized and tuned electron beam into a plasma acceleration stage, the more general concept of a staged plasma accelerator [6], and the colliding pulse injection scheme [7]. The community has made great progress in the direction of decoupling injection and acceleration with the recent experimental demonstration of a staged plasma accelerator [8] and by discussing the preservation of beam quality coupling the beam into and out of a plasma [9][10][11][12].In a plasma wakefield the beam is typically located at the slope of the accelerating field (referred to as off-crest acceleration), such that it is simultaneously accelerated and focused by the plasma fields. However, this choice of accelerating phase also imprints a longitudinal energy correlation (chirp) onto the bunch-an intrinsic feature of virtually all plasma-acceleration schemes, and a major source of the undesired energy spread growth.Here, we propose a novel scheme based on a periodically modulated plasma density profile in the linear wakefield regime, which mitigates the energy chirp accumulation in the plasma. By modulating the plasma density, the bunch periodically experiences accelerating fields with opposite slope, which effectively suppresses the chirp evolution. Our concept of periodically modulating the plasma density allows us to actively shape the longitudinal fields inside the plasma wave with applications even beyond the removal of the linearly correlated energy spread.This Letter is structured as follows. First, we discuss the longitudinal and transverse fields in the plasma cavity and derive an expression for...