Planar nitrogen-incorporated ultrananocrystalline diamond, (N)UNCD, has emerged as a unique field emission source attractive for accelerator applications because of its capability to generate high charge beam and handle moderate vacuum conditions. Most importantly, (N)UNCD sources are simple to produce: conventional high aspect ratio isolated emitters are not required to be formed on the surface, and the actual emitter surface roughness is on the order of only 100 nm. Careful reliability assessment of (N)UNCD is required before it may find routine application in accelerator systems. In the present study using an L-band normal conducting single-cell rf gun, a (N)UNCD cathode has been conditioned to ∼42 MV/m in a well-controlled manner. It reached a maximum output charge of 15 nC corresponding to an average current of 6 mA during an emission period of 2.5 µs. Imaging of emission current revealed a large number of isolated emitters (density over 100/cm 2 ) distributed on the cathode, which is consistent with previous tests in dc environments. The performance metrics, the emission imaging, and the systematic study of emission properties during rf conditioning in a wide gradient range assert (N)UNCD as an enabling electron source for rf injector designs serving industrial and scientific applications. These studies also improve the fundamental knowledge of the practical conditioning procedure via better understanding of emission mechanisms.
The present letter extends the prior findings on self-induced heating of solid state field emission devices. It was found that a vacuum diode (base pressure ∼ 10 −9 Torr), that makes use of graphiterich polycrystalline diamond as cathode material, can switch from diode regime to resistor regime, to glow discharge plasma regime without any external perturbation, i.e. all transitions are self-induced. Combined results of in situ field emission microscopy and ex situ electron microscopy and Raman spectroscopy suggested that the nanodiamond cathode of the diode heated to about 3000 K which caused self-induced material evaporation, ionization and eventually micro-plasma formation. Our results confirm that field emission, commonly called cold emission, is a very complex phenomenon that can cause severe thermal load. Thermal load and material runaway could be the major factors causing vacuum diode deterioration, i.e. progressive increase in turn-on field, decrease in field enhancement factor, and eventual failure. arXiv:1811.04186v1 [cond-mat.mtrl-sci]
Detailed structural and electrical properties of ultra-nano-crystalline diamond (UNCD) films grown in H2/CH4/N2 plasma were systematically studied as a function of deposition temperature (Td) and nitrogen content (% N2) to thoroughly evaluate their effects on resistivity. It was found that even the films grown with no nitrogen in the synthetic gas mixture could be made as conductive as 10−2 Ω cm. The overall resistivity of all the films was tunable over 4 orders of magnitude through varying growth parameters. On a set of 27 samples, Raman spectroscopy and scanning electron microscopy show a progressive and highly reproducible material phase transformation from ultra-nano-crystalline diamond to nano-crystalline graphite as deposition temperature increases. The rate of this transformation is heavily dependent on the N2 content estimated by secondary ion mass spectroscopy. The addition of nitrogen greatly increases the amount of sp2 bonded carbon in the films, thus enhancing the physical connectivity in the grain boundary (GB) network that has high electronic density of states. However, the addition of nitrogen greatly slows down crystallization of the sp2 phase in the GBs compromising electron transport. Therefore, the proper balance between GB connectivity and crystallinity is the key in resistivity engineering of UNCD.
High current bright sources are needed to power the next generation of compact rf and microwave systems. A major requirement is that such a source could be sustainably operated at high frequencies, well above 1 GHz, and high gradients, well above 100 MV/m. Field emission sources offer simplicity and scalability in a high frequency era of the injector design, but the output rf cycle charge and high gradient operation remain a great and largely unaddressed challenge. Here, a field emission cathode based on ultra-nano-crystalline diamond or UNCD, an efficient planar field emission material, was tested at 100 MV/m in an L-band injector. A very high charge of 38 pC per rf cycle (300 nC per rf pulse corresponding to rf pulse current of 120 mA) was demonstrated. This operating condition revealed a space charge dominated emission from the cathode and revealed a condition under which the 1D Child Langmuir limit was surpassed. Finally, a beam brightness of ~10 14 -10 15 A/m 2 rad 2 was estimated.
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