  We tested and compared the stability and usability of three different cathode
materials and configurations in a thermionic-based ultrafast electron
microscope: (1) on-axis thermionic and photoemission from a 0.1-mm diameter
LaB6 source with graphite guard ring, (2) off-axis photoemission from the Ni
aperture surface of the Wehnelt electrode, and (3) on-axis thermionic and
photoemission from a 0.2-mm diameter polycrystalline Ta source. For each
cathode type and configuration, we illustrate how the photoelectron
beam-current stability is deleteriously impacted by simultaneous cooling of the
source following thermionic heating. Further, we demonstrate usability via
collection of parallel- and convergent-beam electron diffraction patterns and
by formation of optimum probe size. We find that usability of the off-axis Ni
Wehnelt-aperture photoemission is at least comparable to on-axis LaB6
thermionic emission, as well as to on-axis photoemission. However, the
stability and achievable beam currents for off-axis photoemission from the
Wehnelt aperture were superior to that of the other cathode types and
configurations, regardless of the electron-emission mechanism. Beam-current
stability for this configuration was found to be within 1% of the mean for 70
minutes (longest duration tested), and steady-state beam current was reached
within the sampling-time resolution used here (~1 s) for 15 pA beam currents
(i.e., 460 electrons per packet for a 200 kHz repetition rate). Repeatability
and robustness of the steady-state condition was also found to be within 1% of
the mean. We discuss the implications of these findings for UEM imaging and
diffraction experiments, for pulsed-beam damage measurements, and for practical
switching between optimum conventional TEM and UEM operation within the same
instrument.
