  Focusing petawatt-level laser beams to a variety of spot sizes for different
applications is expensive in cost, labor and space. In this paper, we propose a
plasma lens to flexibly resize the laser beam by utilizing the laser
self-focusing effect. Using a fixed conventional focusing system to focus the
laser a short distance in front of the plasma, we can adjust the effective
laser beam waist within a certain range, as if a variety of focusing systems
were used with the plasma lens acting as an adjustable eyepiece in a telescope.
Such a setup is a powerful tool for laser wakefield accelerator experiments in
state-of-art petawatt laser projects and allows for scanning focal spot
parameters.
