  In light optics, beams with orbital angular momentum (OAM) can be produced by
employing a properly-tuned two-cylinder-lens arrangement, also called $\pi$/2
mode converter. It is not possible to convey this concept directly to the beam
in an electron microscope due to the non-existence of cylinder lenses in
commercial transmission electron microscope (TEM). A viable work-around are
readily-available electron optical elements in the form of quadrupole lenses.
In a proof-of-principle experiment in 2012, it has been shown that a single
quadrupole in combination with a Hilbert phase plate produces a
spatially-confined, transient vortex mode.
  Here, an analogue to an optical $\pi$/2 mode converter is realized by
repurposing a CEOS DCOR probe corrector in an aberration corrected TEM in a way
that it resembles a dual cylinder lens using two quadrupoles. In order to
verify the presence of OAM in the output beam, a fork dislocation grating is
used as an OAM analyser. The possibility to use magnetic quadrupole fields
instead of, e.g., prefabricated fork dislocation gratings to produce electron
beams carrying OAM enhances the beam brightness by almost an order of magnitude
and delivers switchable high-mode purity vortex beams without unwanted
side-bands.
