Transition between Instability and Seeded Self-Modulation of a Relativistic Particle Bunch in Plasma

TL;DR

This study demonstrates how initial transverse wakefield amplitudes influence the reproducibility of self-modulation phase in a proton bunch in plasma, highlighting a transition crucial for controlled particle acceleration.

Contribution

It experimentally investigates the transition between instability and seeded self-modulation in plasma wakefield acceleration using a relativistic ionization front.

Findings

Reproducible phase occurs at initial amplitudes ≥ 4.1 MV/m.

Phase variations are 3-7% of 2π in the reproducible regime.

Transition observed when initial amplitude drops below threshold.

Abstract

We use a relativistic ionization front to provide various initial transverse wakefield amplitudes for the self-modulation of a long proton bunch in plasma. We show experimentally that, with sufficient initial amplitude ($\ge(4.1\pm0.4)$ MV/m), the phase of the modulation along the bunch is reproducible from event to event, with 3 to 7% (of 2$\pi$) rms variations all along the bunch. The phase is not reproducible for lower initial amplitudes. We observe the transition between these two regimes. Phase reproducibility is essential for deterministic external injection of particles to be accelerated.