  Active plasma lenses have the potential to enable broad-ranging applications
of plasma-based accelerators owing to their compact design and radially
symmetric kT/m-level focusing fields, facilitating beam-quality preservation
and compact beam transport. We report on the direct measurement of magnetic
field gradients in active plasma lenses and demonstrate their impact on the
emittance of a charged particle beam. This is made possible by the use of a
well-characterized electron beam with 1.4 mm mrad normalized emittance from a
conventional accelerator. Field gradients of up to 823 T/m are investigated.
The observed emittance evolution is supported by numerical simulations, which
suggest the potential for conservation of the core beam emittance in such a
plasma lens setup.
