Proton Bunch Self-Modulation in Plasma with Density Gradient

Abstract

We study experimentally the effect of linear plasma density gradients on the self-modulation of a 400 GeV proton bunch. Results show that a positive/negative gradient in/decreases the number of micro-bunches and the relative charge per micro-bunch observed after 10 m of plasma. The measured modulation frequency also in/decreases. With the largest positive gradient we observe two frequencies in the modulation power spectrum. Results are consistent with changes in wakefields' phase velocity due to plasma density gradient adding to the slow wakefields' phase velocity during self-modulation growth predicted by linear theory.

Wakefield decay in a radially bounded plasma due to formation of electron halo

“…The effect was noticed while simulating the AWAKE experiment at CERN [26][27][28]. In AWAKE, a 400 GeV proton beam self-modulates in the plasma [29][30][31] and excites the wakefield [32], which then accelerates the externally injected witness electrons [33,34]. Agreement between simulations and measurements is only achieved for wide simulation windows that correctly accounts for the halo electrons [35,36].…”