Underdense, Passive Plasma Lens for Electron Beams

The concept of using plasma lenses to focus high energy electron beams is old, yet it has seen little use in practice. This is in part because the many varieties that have been considered are often limited by fundamental constraints that can lead to unmanageable aberrations in the focal quality. Here we present a largely unexplored and underutilized incarnation of the plasma lens: the underdense plasma lens (UPL). Unlike most plasma lenses, the UPL operates passively in a regime where the plasma density is lower than the driving electron beam density, resulting in the nonlinear blowout of plasma electrons and the formation of a wake behind the head of the electron beam. The UPL can produce focusing strengths orders of magnitude stronger than conventional quadrupole magnets found in accelerators while being orders of magnitude more compact. In addition, the focusing is linear and axisymmetric, allowing for aberration-free focusing. This type of electron beam optic is an attractive solution to the problem of matching beams to plasma wakefield accelerators. It may also find use as a final focus device for applications such as a future lepton collider or in high energy energy density physics research. Experimental plans to study the UPL at SLAC’s FACET-II facility will be presented.