On the brightness, transverse emittance, and transverse coherence of field emission beam

The highly brilliant electron beam produced by field emitters is one of the enabling factors of the high resolution electron microscope with atomic resolution. In addition to high beam brightness, field emitters are also high current, high current density, and highly coherent cathodes. These characteristics motivated the use of field emitters for high-frequency vacuum electric tubes and accelerator applications and in experiments including electron diffraction, holography, coherent diffraction imaging, to name a few. Here, we present an overview of the key characteristics of field emitters for beam applications: beam brightness, transverse emittance, and transverse coherence. We further discuss their implications on the coherent propagation of the field emission beam.