Controlled Growth of the Self-Modulation of a Relativistic Proton Bunch in Plasma

Abstract

Self-modulation is a beam-plasma instability that is useful to drive large-amplitude wakefields with bunches much longer than the plasma skin depth. We present experimental results showing that, when increasing the ratio between the initial transverse size of the bunch and the plasma skin depth, the instability occurs later along the bunch, or not at all, over a fixed plasma length, because the amplitude of the initial wakefields decreases. We show cases for which self-modulation does not develop and we introduce a simple model discussing the conditions for which it would not occur after any plasma length. Changing bunch size and plasma electron density also changes the growth rate of the instability. We discuss the impact of these results on the design of a particle accelerator based on the self-modulation instability seeded by a relativistic ionization front, such as the future upgrade of the AWAKE experiment.

Focusing of a Long Relativistic Proton Bunch in Underdense Plasma

“…After propagation in preformed plasma, the transverse size of the bunch front, measured at a screen downstream of the plasma exit, is smaller than in the case without plasma, while in the back of the bunch the overall transverse size is larger due to the occurrence of SMI [6]. The focusing effect associated with the smaller size of the bunch front was also observed for much lower values of n pe [7], when the occurrence of SMI and the wakefield excitation are suppressed.…”

“…When increasing the density (r p c/ω pe ), the measured transverse size of the beam decreases, as an increasing number of plasma electrons are available in the plasma. Since the bunch does not drive wakefields, each longitudinal slice evolves according to its own charge density and does not affect the evolution of the following ones [6]. When increasing n pe further, the focusing effect reaches a maximum, and the transverse size measured at the screen remains constant until SMI becomes the dominant effect [6].…”

“…Since the bunch does not drive wakefields, each longitudinal slice evolves according to its own charge density and does not affect the evolution of the following ones [6]. When increasing n pe further, the focusing effect reaches a maximum, and the transverse size measured at the screen remains constant until SMI becomes the dominant effect [6]. We will present these experimental results in a dedicated publication [18].…”

Characteristics of betatron radiation in AWAKE Run 2 experiment

“…Parameters are generally similar to that used in the preliminary electron seeding experiment (Verra et al. 2022). The electron beam has an initial energy of 18.5 MeV, charge of 250 pC, radial size of and duration of .…”

Controlling Instabilities in a Plasma Particle Accelerator

“…An alternative technique that can get particles up to speed much more quickly is proton-driven plasma wakefield acceleration, but this method is prone to the self-modulation instability (SMI), which breaks the particle bunch into a train of microbunches. Now, researchers at the Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) at CERN have found a way to control how the SMI develops, suggesting the possibility of exploiting the effect in future particle accelerators [1].…”