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 complete design, fabrication, and performance evaluation of a compact, single cell, X-band (∼9 GHz) electron injector based on a field emission cathode (FEC) are presented. A pulsed electron beam is generated by a 10's of kW radiofrequency (RF) magnetron signal from a plug-in thin film nitrogen-incorporated ultrananocrystalline diamond (N)UNCD FEC cartridge. Testing of the Xband injector with the (N)UNCD FEC was conducted in a beamline equipped with a solenoid, Faraday cup and imaging screen. The results show that typically the (N)UNCD FEC cartridge produces 1 mA/cm 2 at a surface electric field of 28 MV/m. The diameter of the output beam generated from the 4.4 mm diameter (N)UNCD cartridge can be as small as 1 mm. In terms of its practical applications, the demonstrated X-band electron injector with the (N)UNCD plug-in FEC can serve as a source for X-ray generation, materials processing, travelling-wave tubes (including GHz and THz backward wave oscillators), or can be used to drive slow-wave accelerating structures. The results presented also suggest that this field emitter technology based on planar (N)UNCD thin films, which are simply grown on the surface of optically polished stainless steel, can enable a vast number of device configurations that are efficient, flexible in design, and can be packaged with ease.
Nitrogen incorporated ultrananocrystalline diamond ((N)UNCD) could be an enabling material platform for photocathode applications due to its high emissivity. While the quantum efficiency (QE) of UNCD was reported by many groups, no experimental measurements of the intrinsic emittance/mean transverse energy (MTE) have been reported. Here, MTE measurement results for an (N)UNCD photocathode in the photon energy range of 4.41 to 5.26 eV are described. The MTE demonstrates no noticeable dependence on the photon energy, with an average value of 266 meV. This spectral behavior is shown to not to be dependent upon physical or chemical surface roughness and inconsistent with low electron effective mass emission from graphitic grain boundaries, but may be associated with emission from spatially-confined states in the graphite regions between the diamond grains. The combined effect of fast-growing QE and constant MTE with respect to the excess laser energy may pave the way to bright UNCD photocathodes.Photocathode-based RF and pulsed DC guns are bright electron injectors for free electron lasers and advanced time resolved microscopes 1 . Further progress of electron laser and microscopy facilities (improved sensitivity, spatiotemporal resolution, high throughput) largely depends on development and understanding of materials with the potential to be utilized as photocathodes. Photocathode development challenges include achieving simultaneously (i) high QE, (ii) high transverse coherence (meaning low intrinsic emittance/low MTE), (iii) rapid response time.The ratio of the charge to the MTE determines the photocathode brightness, which in many applications is the most critical figure of merit. For a classical metal photocathode such as copper, the Fowler-Dubridge law 2 predicts that the emitted charge is a fast-growing function of excess energy (a power law), where excess energy ∆E is the difference between the laser primary incident photon energy ω and the work function φ defined as ∆E = ω−φ. Dowell and Schmerge 3 have found that the transverse momentum for metals also grows with excess energy as ∼ √ ω − φ. For the latter reason, to attain the highest quality (low divergence) electron beam metal photocathodes are often operated in the near threshold region (having the smallest ∆E, with the primary photon energy nearly matching the work function), although brightness increases with excess energy.A great number of metal and thin film alkali antimonide photocathodes obey the Dowell-Schmerge (DS) model 3-5 . However, some semiconductor photocathodes, e.g. GaAs and PbTe, show various MTE versus excess energy trends that are different from those specific to metals. Negative electron affinity (NEA) GaAs photocathodes 6 , for instance, demonstrate ∼1,000-fold QE increase as the excess energy increases from 0 to about 1 eV while the MTE remains low and nearly constant with the same ∆E range (within measurement precision).(N)UNCD is another example of a NEA photocathode that has high electron conductivity through the bulk of a semi-me...
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