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Large area carbon nanotube (CNT) cathodes made from yarns, films or fibershave long been promising as next generation electron sources for high powerradio frequency (rf) and microwave vacuum electronic devices. However,experimental evidence have been pointing out spatial incoherence of theelectron beam produced by such cathodes that, in turn, impeded the progresstoward high brightness CNT electron sources and their practical applications.Indeed, typically large area CNT fibers, films or textiles emit stochasticallyacross their physical surface at large emission angles and with largetransverse spread, meaning large emittance and hence low brightness. In thiswork, using high resolution field emission microscopy, we demonstrate thatconventional electroplating of hair-thick CNT fibers followed by a femtosecondlaser cutting, producing emitter surface, solves the described incoherentemission issues extremely well. Strikingly, it was observed that the entire(within the error margin) cathode surface of a radius of approximately$75\,\mu\text{m}$ emitted uniformly (with no hot spots) in the direction of theapplied electric field. The normalized emittance on the fiber surface wasestimated of 52 nm with brightness of$>$$10^{15}\frac{\text{A}}{\text{m}^2\text{rad}^2}$ (or $>$$10^7$ Am$^{-2}$sr$^{-1}$V$^{-1}$) estimated for pulsed mode operation.

Bright Spatially Coherent Beam from Carbon Nanotube Fiber Field Emission Cathode