Paper NS-TuM5
Field Emission Electron Source Based on UltraNanoCrystalline Diamond Films for Electron Accelerators Applications

Tuesday, November 8, 2016, 9:20 am, Room 101D

Currently, commercially available electron sources for electron accelerators are photocathodes or thermionic cathodes. Both types puts limits onto the resulting duty cycle of an accelerator and adds into increasing its complexity, as they require additional accessories (lasers, pulser compressors etc.). Cold cathode field emission technology based on low work function metals and other materials is an attractive alternative to simplify the electron injector, however, field emission current stability and processing challenges associated with formation of an atomically sharp tip for these field emitters makes it difficult to adopt this technology for accelerator applications

Nitrogen incorporated ultrananocrystalline diamond (N) UNCD films developed at Argonne National Laboratory have demonstrated its remarkable field emission properties. The unique structure of atomically abrupt nitrogen-incorporated grain boundaries provides field emission sites with very high field enhancement and therefore eliminates the need to make sharp nano-tips thus drastically reducing processing steps to fabricate field emission source. More specifically, it delivers significant currents at electric gradients as low as ~10⁵ V/cm, which is far below typical breakdown thresholds in many materials (>~10⁶ V/cm), and has turn-on voltages as low as 2-5×10⁴ V/cm, and have shown excellent emission current stability for extended time periods up to 1000 hrs. Small grain size and a unique grain boundary network ensure more uniform emission properties over large areas and smaller current load per emitting site (i.e., per grain boundary). Taking advantage of these unique properties of (N)UNCD, Euclid TechLabs in collaboration with Argonne conducted a case performance study of a thin film planar (N)UNCD field emitter in an radio frequency(RF) 1.3 GHz electron gun in an electron accelerator. The field emission cathode was a 100 nm thick (N)UNCD film grown on a 20 mm cathode plug. At surface gradients 45-65 MV/m, peak currents of 1-80 mA (0.3-25 mA/cm²) were achieved. Imaging with two YAG screens confirmed emission from the planar (N)UNCD surface with beam emittance of 1.5 mm×mrad/mm-rms and longitudinal FWHM energy spread of 0.7% at 2 MeV[1]. The same technology could be adopted for industrial and scientific linear accelerators, both normal-conducting and superconducting, for isotope production for radiopharmacy; X-/gamma-ray production for medicine, non-destructive evaluation, well-logging; and materials processing.

References:

[1] S. V. Baryshev et al., Appl. Phys. Lett. 105, 203505 (2014).