Abstract
We present numerical simulations and experimental results of the self-modulation of a long proton bunch in a plasma with linear density gradients along the beam path. Simulation results agree with the experimental results reported in arXiv:2007.14894v2: with negative gradients, the charge of the modulated bunch is lower than with positive gradients. In addition, the bunch modulation frequency varies with gradient. Simulation results show that dephasing of the wakefields with respect to the relativistic protons along the plasma is the main cause for the loss of charge. The study of the modulation frequency reveals details about the evolution of the self-modulation process along the plasma. In particular for negative gradients, the modulation frequency across time-resolved images of the bunch indicates the position along the plasma where protons leave the wakefields. Simulations and experimental results are in excellent agreement.
Highlights
A plasma wakefield accelerator (PWFA) [1] uses a relativistic particle bunch as driver
We present numerical simulations and experimental results of the self-modulation of a long proton bunch in a plasma with linear density gradients along the beam path
Simulation results agree with the experimental results reported [F
Summary
A plasma wakefield accelerator (PWFA) [1] uses a relativistic particle bunch as driver. The effect of Δv is cumulative along the bunch and more important at the back of the bunch than in its front This means that particles of the drive bunch do not necessarily remain in the focusing or defocusing phase of the wakefields all along their propagation in the plasma, which affects the process of the microbunch train formation. The effect of negative gradients is to lengthen λpe, enhancing the phase shift This is most important for the acceleration of electrons injected early along the plasma. Plasma density gradients are useful for maintaining particles of the drive bunch in phase with the wakefields, leading to trains with more microbunches [8], possibly leading to larger accelerating fields [9]. We show that a linear density gradient does not eliminate the issue of decreasing amplitude of the wakefields along the plasma past the SM saturation point
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