  An electron plasma lens is a cost-effective, compact, strong-focusing element
that can ensure efficient capture of low-energy proton and ion beams from
laser-driven sources. A Gabor lens prototype was built for high electron
density operation at Imperial College London. The parameters of the stable
operation regime of the lens and its performance during a beam test with 1.4
MeV protons are reported here. Narrow pencil beams were imaged on a
scintillator screen 67 cm downstream of the lens. The lens converted the pencil
beams into rings that show position-dependent shape and intensity modulation
that are dependent on the settings of the lens. Characterisation of the
focusing effect suggests that the plasma column exhibited an off-axis rotation
similar to the $m=1$ diocotron instability. The association of the instability
with the cause of the rings was investigated using particle tracking
simulations.
