Bright spatially coherent beam from carbon-nanotube fiber field-emission cathode

Large-area carbon-nanotube (CNT) cathodes made from yarns, films, or fibers have long been promising as next-generation electron sources for high-power rf and microwave-vacuum-electronic devices. However, experimental evidence has highlighted that spatial incoherence of the electron beam produced by such cathodes impeded the progress toward high brightness CNT electron sources and their practical applications. Indeed, typically large-area CNT fibers, films, or textiles emit stochastically across their physical surface at large emission angles and with large transverse spread, meaning large emittance and hence low brightness. In this work, using high-resolution field-emission microscopy, we demonstrate that conventional electroplating of hair-thick CNT fibers followed by a femtosecond laser cutting, producing an emitter surface, solves the described incoherent emission issues extremely well. Strikingly, it was observed that the entire (within the error margin) cathode surface of a radius of approximately $75\text{μ}\text{m}$ emitted uniformly (with no hot spots) in the direction of the applied electric field. The normalized cathode emittance, i.e., on the fiber surface, was estimated as 26-$\text{nm}\text{rad}$ with brightness of $>{10}^{16}\text{A}/{\text{m}}^2{\text{rad}}^2$ (or $>{10}^7\mathrm{A}{\mathrm{m}}^{-2}{\mathrm{sr}}^{-1}{\mathrm{V}}^{-1}$) estimated for pulsed-mode operation.