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Register a new userA tunable plasma-based energy dechirper
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A tunable plasma-based energy dechirper has been developed at FLASHForward to remove the correlated energy spread of a 681~MeV electron bunch. Through the interaction of the bunch with wakefields excited in plasma the projected energy spread was reduced from a FWHM of 1.31$%$ to 0.33$%$ without reducing the stability of the incoming beam. The experimental results for variable plasma density are in good agreement with analytic predictions and three-dimensional simulations. The proof-of-principle dechirping strength of $1.8$~GeV/mm/m significantly exceeds those demonstrated for competing state-of-the-art techniques and may be key to future plasma wakefield-based free-electron lasers and high energy physics facilities, where large intrinsic chirps need to be removed.
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Synchronized, independently tunable and focused $mu$J-class laser pulses are used to release multiple electron populations via photo-ionization inside an electron-beam driven plasma wave. By varying the laser foci in the laboratory frame and the position of the underdense photocathodes in the co-moving frame, the delays between the produced bunches and their energies are adjusted. The resulting multibunches have ultra-high quality and brightness, allowing for hitherto impossible bunch configurations such as spatially overlapping bunch populations with strictly separated energies, which opens up a new regime for light sources such as free-electron-lasers.
The dynamic process of a laser or particle beam propagating from vacuum into underdense plasma has been investigated theoretically. Our theoretical model combines a Lagrangian fluid model with the classic quasistatic wakefield theory. It is found that background electrons can be injected into wakefields because sharp vacuum-plasma transitions can reduce the injection threshold. The injection condition, injection threshold as well as the injection length can be given theoretically by our model and are compared with results from computer simulations. Moreover, electron beams of high qualities can be produced near the injection thresholds and the proposed scheme is promising in reducing the injection threshold and improving the beam qualities of plasma based accelerators.
A near-hollow plasma channel, where the plasma density in the channel is much less than the plasma density in the walls, is proposed to provide independent control over the focusing and accelerating forces in a plasma accelerator. In this geometry the low density in the channel contributes to the focusing forces, while the accelerating fields are determined by the high density in the channel walls. The channel also provides guiding for intense laser pulses used for wakefield excitation. In certain regimes, both electron and positron beams can be accelerated and focused in a nearly symmetric fashion. Near-hollow plasma channels can effectively mitigate emittance growth due to Coulomb scattering for high-energy physics applications.
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-in-cell 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.
Plasma-based electron and positron wakefield acceleration has made great strides in the past decade. However one major challenge for its applications to coherent light sources and colliders is the relatively large energy spread of the accelerated beams, currently at a few percent level. This energy spread is usually correlated with particle position in the beam arising from the longitudinal chirp of the wakefield amplitude. Therefore a dechirper is highly desirable for reducing this spread down to $sim0.1%$ level, while at the same time for maintaining the emittance of the accelerated beam. Here we propose that a low-density hollow channel plasma can act as a near-ideal dechirper for both electrons and positrons. We demonstrate the concept through large-scale three-dimensional particle-in-cell simulations. We show that the initial positive correlated energy spread (chirp) on the beam exiting a plasma accelerator can be compensated by the nearly linear self-wake induced by the beam in the hollow channel from few percent level down to $leq 0.1%$. Meanwhile, the beam emittance can be preserved due to the negligible transverse field inside the channel. This passive method may significantly improve the beam quality of plasma-based accelerators, paving the way for their applications to future compact free electron lasers and colliders.
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