Proton Bunch Self-Modulation in Plasma with Density Gradient

TL;DR

This study experimentally investigates how linear plasma density gradients affect the self-modulation of high-energy proton bunches, revealing that gradients influence micro-bunch formation, charge distribution, and modulation frequency, consistent with theoretical predictions.

Contribution

It provides new experimental insights into how plasma density gradients modify proton bunch self-modulation, confirming theoretical models with real-world data.

Findings

Positive gradients increase the number of micro-bunches

Negative gradients decrease the micro-bunch count

Density gradients alter the modulation frequency and introduce multiple frequencies

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.