On the double-plane plasma lensing
  Plasma lensing is the refraction of low-frequency electromagnetic rays due to
cold free electrons in the universe. For sources at a cosmological distance,
there is observational evidence of elongated, complex plasma structures along
the line of sight requiring a multi-lens-plane description. To investigate the
limits of single-plane plasma lensing, we set up a double-plane lens with a
projected Gaussian electron density in each lens plane. We compare double-plane
scenarios with corresponding effective single-plane configurations. Our results
show how double-plane lenses can be distinguished from single-plane lenses by
observables, i.e. resolved multiple image positions, relative magnifications,
time delays, and pulse shapes. For plasma lensing of fast radio bursts, the
observed pulse shape may be dominated by the lensing effect, allowing us to
neglect the intrinsic source pulse shape to distinguish different lensing
configurations. The time-domain observables turn out to be the most salient
features to tell multi- and single-plane lenses apart.
