Speaker
Description
Dephasing between the driver and witness is one of the main mechanisms limiting witness energy gain in plasma acceleration. We show time-resolved, picosecond-scale measurements of the self-modulating proton driver in AWAKE (Advanced WAKefield Experiment at CERN) as a function of plasma length, revealing the evolution of this drive bunch caused by transverse wakefield development and dephasing. Comparison with numerical simulations allows us to establish a correspondence between the modulated bunch distribution and the wakefields, which cannot be directly measured. Analysis of the bunch train confirms that the wakefield phase velocity is slower than the proton velocity, in agreement with simulations. Analysis of the bunch train evolution confirms that the wakefield phase velocity during self modulation in uniform plasma is lower than the proton velocity, as previously predicted. We further show that the wakefield phase velocity can be controlled through the plasma density profile, with negative and positive density gradients leading to a decrease and increase in phase velocity respectively.
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