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

Area of Science:

• Plasma Physics
• Particle Acceleration
• Accelerator Science

Background:

• The self-modulation instability (SMI) of charged particle beams in plasma is a key process for plasma wakefield acceleration.
• Controlling the phase of plasma waves is essential for precise particle injection and acceleration.

Purpose of the Study:

• To investigate the effect of initial transverse wakefield amplitude on the phase reproducibility of proton bunch self-modulation in plasma.
• To identify the threshold amplitude for reproducible modulation phase.

Main Methods:

• Utilizing a relativistic ionization front to generate controlled initial transverse wakefield amplitudes.
• Experimentally probing the self-modulation of a long proton bunch across a range of initial wakefield amplitudes.
• Analyzing the phase variations along the proton bunch for different initial conditions.

Main Results:

• Demonstrated that initial transverse wakefield amplitudes greater than or equal to (4.1±0.4) MV/m lead to reproducible modulation phase along the bunch.
• Observed significant phase variations (3%-7% rms of 2π) along the bunch when the initial amplitude was insufficient.
• Characterized the transition regime between reproducible and non-reproducible phase behavior.

Conclusions:

• Sufficient initial transverse wakefield amplitude is critical for achieving phase-locked self-modulation of proton bunches in plasma.
• Phase reproducibility is experimentally confirmed to be dependent on the initial wakefield amplitude.
• This work provides essential insights for deterministic external injection schemes in plasma-based accelerators.