Experimental Observation of Plasma Wakefield Growth Driven by the Seeded Self-Modulation of a Proton Bunch

Abstract

We observe for the first time an effect on the driver caused by the motion of ions in a plasma wakefield accelerator. The effect manifests itself as a beam tail, which only occurs when sufficient motion of ions suppresses wakefields. By changing the plasma ions (helium, argon, xenon) in the experiment, we show that the effect depends inversely on the ion mass, as predicted from theory and simulations. Wakefields are driven resonantly by multiple bunches, and simulation results indicate that the ponderomotive force causes the motion of ions. In this case, the effect is also expected to depend on the amplitude of the wakefields, as also observed by varying the bunch charge.

Experimental study of wakefields driven by a self-modulating proton bunch in plasma

“…To design experiments based on this acceleration scheme-with potential applications for high-energy physics [10]understanding the development of the SM of a charged particle bunch along the plasma is important. The SM process grows from seed wakefields and reaches saturation [11]. Ideally this process would be directly studied by changing the plasma length, as is for example done with free electron lasers by adjusting the effective undulator length [12].…”

“…The evolution of SM can be deduced from proton defocusing caused by transverse wakefields (W r ) [11]. We measure the time-integrated, transverse distribution of the self-modulated proton bunch downstream from the plasma exit [15].…”

Electron beam characterization with beam loss monitors

“…AWAKE [1], the Advanced WAKEfield experiment at CERN, recently demonstrated acceleration of externally injected electrons in plasma wakefields resonantly excited by a self-modulated [2] [3] relativistic proton bunch [4].…”

Accelerating field enhancement due to ion motion in plasma wakefield accelerators

“…The effect of wakefield enhancement was first noticed in numerical simulations of the AWAKE experiment. In AWAKE [18][19][20], a long proton bunch undergoes seeded self-modulation in the plasma [21][22][23] and transforms into a train of short micro-binches that resonantly drive the plasma wave [24,25]. The number of micro-bunches depends on the plasma density and is typically about one hundred, so the plasma wave exists sufficiently long to move the ions.…”