Beam Loading in a Proton Driven Plasma Wakefield Accelerator

Abstract

The Advanced Wakefield Experiment (AWAKE) at CERN is the first plasma wakefield experiment to exploit the self-modulation instability in a long proton bunch in order to accelerate electrons to high energies. The first run saw electrons accelerated from 19 MeV to 2 GeV in just 10 metres of ionised Rubidium vapour, achieving a gradient of nearly 200 MV/m.

A challenge facing such accelerator designs is the final quality of the accelerated bunch. Retaining a low spread in energy and emittance, while achieving a high accelerating gradient, are goals that to an extent are in conflict.

This thesis shows, using computer simulations, that it is possible to accelerate 30–70 pC of electrons up to 1.8–2 GeV in a 4 metre plasma stage, with an energy spread of less than 2 percent, and with no significant emittance growth.

Low energy spread is achieved by finely tuning the witness bunch size and density to fit the plasma parameters as well as the wakefields generated by the drive bunch. Low emittance growth is achieved by exploiting the wake generated by the head of the witness bunch to create a stable condition for its tail. This provides a potential experiment setup for the next stage of AWAKE.