Phase Space Dynamics of a Plasma Wakefield Dechirper for Energy Spread Reduction

Abstract

Plasma-based accelerators have made impressive progress in recent years. However, the beam energy spread obtained in these accelerators is still at ∼ 1% level, nearly one order of magnitude larger than what is needed for challenging applications like coherent light sources or colliders. In plasma accelerators, the beam energy spread is mainly dominated by its energy chirp (longitudinally correlated energy spread). Here we demonstrate that when an initially chirped electron beam from a linac with a proper current profile is sent through a low-density plasma structure, the self wake of the beam can significantly reduce its energy chirp and the overall energy spread. The resolution-limited energy spectrum measurements show at least a threefold reduction of the beam energy spread from 1.28 % to 0.41 % FWHM with a dechirping strength of ∼ 1 (MV/m)/(mm pC). Refined time-resolved phase space measurements, combined with high-fidelity three-dimensional particle-incell simulations, further indicate the real energy spread after the dechirper is only about 0.13 % (FWHM), a factor of 10 reduction of the initial energy spread.

Effect of fluctuations in the down ramp plasma source profile on the emittance and current profile of the self-injected beam in a plasma wakefield accelerator

“…The peak energy of this spike is 0.2 GeV with a rms energy spread of ∼3%. The bunch has a large negative chirp, which increases the projected energy spread; however, the slice energy spread remains as low as ∼0.5% throughout the whole bunch, and it has been demonstrated that the negative chirp of the bunch can be removed by a plasma dechirper [46][47][48] so that the projected energy spread of the bunch can be reduced to be comparable to the slice energy spread. In another simulation where the length of the plasma is 2 cm, the peak energy of the beam exceeds 1 GeV.…”

Compact Multistage Plasma-Based Accelerator Design for Correlated Energy Spread Compensation

“…4, the total energy spread has been reduced by a factor of ∼ 20 down to 0.12% with respect to a case with no chirp compensation. This corresponds to a dechirping strength of 1.1 GeV/µm/m, orders of magnitude higher than in other schemes [34][35][36][37][38]. The slice energy spread has also been reduced to just 2.8×10 −4 , when considering 0.1 µm slices.…”

“…The presented scheme therefore offers a path towards beams with ultra-low energy spread (∼ 10 −4 ) while reducing the sensitivity to issues such as the timing jitter for external injection and the hosing instability. The dechirping strength offered by this concept is orders of magnitude higher than in other proposals [34][35][36][37][38], and allows for dechirping while further accelerating the beam. The concept is ideally suited for PBAs with weakly or linearly beam-loaded wakefields where the beam develops a negative chirp before the magnetic chicane.…”

“…These include modulating [30] or tailoring [31] the plasma density profile as well as injecting a secondary bunch [32], but they show limited success or remain to be experimentally realized. Other dechirping techniques based on beam-induced wakefields have been proposed [33] and even demonstrated experimentally [34][35][36][37][38], although not for the extreme chirps imprinted by PBAs.…”

Intrinsic energy spread and bunch length growth in plasma-based accelerators due to betatron motion

“…Major efforts have therefore been dedicated towards this objective, including the development of controlled injection techniques [23][24][25][26][27] as well as concepts for mitigating the correlated energy spread arising from the steep slope of the accelerating fields. These include beam loading the wake [28][29][30] , either by the accelerated bunch itself 31 or the injection of a secondary one 32 , the use of modulated 33 or tailored 34 plasma profiles, the insertion of a magnetic chicane in a multistage PBA 35 , or taking advantage of the beam-induced wakefields [36][37][38][39][40] .…”