Speaker
Description
Plasma wakefield accelerators can sustain very high accelerating gradients (1-100GeV/m), but energy gain is often limited by energy depletion of the drive bunch. The AWAKE experiment addresses this limitation by using a 400GeV proton bunch from the SPS. The bunch is much longer than the plasma wavelength and must first transform into a train of microbunches to excite large-amplitude wakefields. This train forms through the transverse self-modulation (SM) instability. High-gradient acceleration of a witness bunch can only occur once the train is fully formed, i.e., after saturation of SM, and thus measuring the saturation length ($L_{sat}$) of SM is crucial for the design of an accelerator with this scheme.
We present the first determination of $L_{sat}$ using experimental and numerical simulation results. By measuring the transverse distribution of the bunch after the plasma as a function of plasma length, we determine $L_{sat}$ and find that it decreases when increasing the plasma density, and that seeding SM makes $L_{sat}$ shorter. For all parameters relevant to AWAKE, saturation of SM occurs well within the 10m foreseen for the length of the self-modulator in future acceleration experiments.
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