Measurement of the Saturation Length of the Self-Modulation Instability

TL;DR

This paper experimentally and numerically determines the saturation length of the self-modulation instability in plasma, revealing its dependence on plasma density and initial field amplitude, crucial for plasma wakefield accelerator design.

Contribution

First measurement of the saturation length of self-modulation instability using combined experimental and numerical methods, advancing understanding of plasma wakefield acceleration.

Findings

Saturation length decreases with plasma density.

Saturation length reduces with higher initial field amplitude.

Results inform design of plasma-based accelerators.

Abstract

The self-modulation (SM) instability transforms a long charged particle bunch traveling in plasma into a train of microbunches that resonantly drives large-amplitude wakefields. We present the first determination of the saturation length of SM using experimental and numerical results. The saturation length is the distance over which wakefields reach their maximum amplitude along the plasma. By varying the plasma length and measuring the radius of the transverse distribution of the bunch, we find that the saturation length of SM decreases with plasma density and initial field amplitude, e.g., when seeding. The saturation length is a fundamental parameter of the instability, and these results are key for understanding SM and designing plasma wakefield accelerators driven by long bunches, such as AWAKE, or by long laser pulses for radiation production.