A tunable electron beam source using trapping of electrons in a density down-ramp in laser wakefield acceleration

Abstract

One challenge in the development of laser wakefield accelerators is to demonstrate sufficient control and reproducibility of the parameters of the generated bunches of accelerated electrons. Here we report on a numerical study, where we demonstrate that trapping using density down-ramps allows for tuning of several electron bunch parameters by varying the properties of the density down-ramp. We show that the electron bunch length is determined by the difference in density before and after the ramp. Furthermore, the transverse emittance of the bunch is controlled by the steepness of the ramp. Finally, the amount of trapped charge depends both on the density difference and on the steepness of the ramp. We emphasize that both parameters of the density ramp are feasible to vary experimentally. We therefore conclude that this tunable electron accelerator makes it suitable for a wide range of applications, from those requiring short pulse length and low emittance, such as the free-electron lasers, to those requiring high-charge, large-emittance bunches to maximize betatron X-ray generation.

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

“…Interestingly, the data show that the emittance decreases almost exponentially as the down ramp length increases. We note that other numerical studies have shown contradictory results; i.e., the emittance of the injected beam either decreases [36] or increases [37] for a longer down ramp. This discrepancy as well as the exponential decrease of the emittance we observed are worth being checked in an experiment.…”

“…The data show that, even for the most abrupt down ramp (L ¼ 5 c=ω p ), the slice emittance of the beam is about 0.1 μm. We note that simulations of laser-driven down ramp injection show that the generation of beams with an emittance as low as ∼0.2 μm is achievable [36,37].…”

Numerical study of laser energy effects on density transition injection in laser wakefield acceleration

“…A semi-analytical model to predict the injected beam length and the injection threshold in arbitrary downramps was then derived [22]. A study on the influence of the transition downramp steepness on the beam quality has then been presented in [23]. The studies in [20][21][22][23] employ different laser parameters, thus together they constitute a coherent but somewhat fragmented study of the density transition injection technique.…”

Toward a plasma-based accelerator at high beam energy with high beam charge and high beam quality

“…An example can be seen with the LPI where the down-ramp injection technique mentioned above is applied. Typically, sharper down ramps lead to more captured charges but induce a larger emittance, an effect that is accentuated by a larger density jump before and after the ramp [57]. By tuning the sharpness of the ramp and the density jump, a compromise can be obtained so that the desired emittance and charge can be reached [32].…”