Transverse emittance and current of multi-GeV trapped electrons in a plasma wakefield accelerator

Abstract

Multi-GeV trapped electron bunches in a plasma wakefield accelerator (PWFA) are observed with normalized transverse emittance divided by peak current, ${ϵ}_{N,x}/{I}_{t}$, below the level of $0.2\text{ }\text{ }\ensuremath{\mu}\mathrm{m}/\mathrm{kA}$. A theoretical model of the trapped electron emittance, developed here, indicates that emittance scales inversely with the square root of the plasma density in the nonlinear ``bubble'' regime of the PWFA. This model and simulations indicate that the observed values of ${ϵ}_{N,x}/{I}_{t}$ result from multi-GeV trapped electron bunches with emittances of a $\mathrm{\text{few}}\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ and multi-kA peak currents.