Beam brightness and its reduction in a 1.2-MV cold field-emission transmission electron microscope

•The probe size was experimentally shown to be proportional to the (3/8)th power of the probe current in the 1.2-MV cold FE TEM.•This is due to suppression of spherical aberrations at the gun and acceleration tube by the magnetic gun lens.•The maximum brightness was 1.62 × 1014 A/(m2sr).•Brightness reduction was observed while increasing the emission current above 18.7 µA.•Brightness reduction was most likely due to the stochastic Coulomb interactions at the emitter. In this paper we discuss probe properties in terms of probe currents, probe sizes, energy spread, virtual source sizes, and brightness in a 1.2-MV cold field-emission (cold FE) transmission electron microscope (TEM) equipped with a magnetic gun lens. The probe size increased gradually in proportion to the (3/8)th power of the probe current, very unusual behavior in cold FE guns but typical behavior in thermionic guns. This is due to the magnetic gun lens, which caused large emission angles for electron beams in the probe before being limited by aberrations at the gun and acceleration tube. The brightness reached the maximum at 1.62 × 1014 A/(m2sr) and then decreased with increasing the emission current. The energy spread of the beam, including the Boersch effect, was 0.32–0.50 eV, comparable to that of conventional cold FE guns without magnetic gun lens. Experimental analysis indicated that neither noises nor aberrations caused the probe size increase under the optimized illumination condition. The virtual source size increased from 11.1 to 24.7 nm with the increase in the emission current. To describe this behavior, we conjecture that this brightness reduction is due to increase in the virtual source size caused by trajectory displacement created by stochastic Coulomb interactions near the emitter.