Experimental observation of proton bunch modulation in a plasma, at varying plasma densities

TL;DR

This paper provides direct experimental evidence of proton bunch self-modulation in plasma, demonstrating how the modulation period depends on plasma density and is seeded by a laser-induced ionization front.

Contribution

First experimental observation of full transverse self-modulation of a relativistic proton bunch in plasma with controlled seeding and density variation.

Findings

Proton bunch exits plasma with density modulation due to radial wakefields.

Modulation period scales with plasma density as expected.

Seeding by laser pulse effectively initiates the modulation.

Abstract

We give direct experimental evidence for the observation of the full transverse self-modulation of a relativistic proton bunch propagating through a dense plasma. The bunch exits the plasma with a density modulation resulting from radial wakefield effects with a period reciprocal to the plasma frequency. We show that the modulation is seeded by using an intense laser pulse co-propagating with the proton bunch which creates a relativistic ionization front within the bunch. We show by varying the plasma density over one order of magnitude that the modulation period scales with the expected dependence on the plasma density.