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. We further show that a negative linear plasma density gradient reduces the phase velocity further. Based on these results, we discuss mitigation strategies, including positive density gradients or steps.
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