A plasma-based accelerator operating in the blowout regime exerts a strong, transverse focusing force on the electron beam being accelerated. If the beam divergence is not matched to the focusing force, the beam will undergo chromatic emittance growth. A ramped plasma density profile at the entrance to the plasma source can focus the beam down to the matched size, limiting emittance growth during acceleration. When the beam exits the plasma source, the focusing force disappears and the beam rapidly diverges. The divergence can be controlled by using a ramped plasma density profile at the exit of the plasma. We present an analytic theory describing the transverse beam dynamics within the ramps. Our theory can be used to predict the emittance growth within a given ramp or to predict the ramp lengths required for coupling a given beam into and out of an accelerator with negligible computational cost.
|Working group||Theory and computation|