Underdense Passive Plasma Lens for Focusing Relativistic Electron Beams*

The utility and application of an accelerated particle beam is limited significantly by its emittance. When exposed to the strong transverse focusing forces of a plasma wakefield accelerator over large distances, the projected emittance of a beam can grow significantly due to chromatic phase spreading. This emittance growth can be mitigated by if the beam is initially matched to the plasma, which generally requires it to have a very small spot size. Conventional quadrupole electromagnet focusing devices struggle to focus the beam to a sufficiently small spot size at the entrance of a plasma wakefield accelerator. A plasma lens may be more appropriate for this task, as it can focus an electron beam with hundreds-to-thousands of times greater strength than magnetic focusing devices. When an electron beam travels through the plasma lens, an attractive force is exerted radially inward, which focuses the electrons of the beam in the same way that a converging lens focuses light. If the plasma is less dense than the electron beam, a nonlinear blowout wake will form, which exerts a linear focusing force whose strength scales with the plasma density. We call this the underdense, passive plasma lens.

The plasma lens offers several advantages over conventional magnetic focusing devices: it is significantly stronger, ultra-compact, and it is axisymmetric, focusing in both the vertical and horizontal dimensions simultaneously. Through simulation, this project examines the interaction between an accelerated electron beam and a passive, underdense plasma lens using realistic parameters for upcoming experiments planned at SLAC’s FACET-II facility.