Geometric and electromagnetic characterization of electron beams produced by nanodiamond photocathodes

In this work, the geometric and electromagnetic characteristics of electron beams generated by three photocathodes (PCs), two based on nanodiamond (ND) layers and one based on Cu (generally used as reference) were investigated. Specifically, the active layers of the ND-based PCs consisted of untreated and hydrogenated (H-ND) nanoparticles (250 nm in size) deposited by pulsed spray technique on p-doped silicon substrates as uniform coating. Photoemission measurements carried out by a KrF nanosecond excimer laser (lambda = 248 nm) in a vacuum chamber at 10(-6) mbar and the emittance evaluation, performed by the pepper pot method, are reported and discussed. For the last, radio-chromic films (HD-810 Gafchromic) were used as sensible screen for electrons. The study of the emittance was performed by varying the laser spot onto the PC surface and the accelerating voltage (5, 10 and 15 kV). From emittance values, the normalized brightness was also estimated for all the cathodes. The obtained results showed quantum efficiency values of the ND-based photocathodes higher than that of the reference Cu one, but, at the same time, higher emittances and therefore worse performancers as the result of the enlarged beam divergence. Despite this, H-ND resulted to be the best PC between those investigated for the highest normalized beam brightness, thanks to its high electron current and low normalized emittance.