  Active plasma lensing is a compact technology for strong focusing of charged
particle beams, which has gained considerable interest for use in novel
accelerator schemes. While providing kT/m focusing gradients, active plasma
lenses can have aberrations caused by a radially nonuniform plasma temperature
profile, leading to degradation of the beam quality. We present the first
direct measurement of this aberration, consistent with theory, and show that it
can be fully suppressed by changing from a light gas species (helium) to a
heavier gas species (argon). Based on this result, we demonstrate emittance
preservation for an electron beam focused by an argon-filled active plasma
lens.
