A case performance study of a planar field emission cathode (FEC) based on nitrogen-incorporated ultrananocrystalline diamond, (N)UNCD, was carried out in an RF 1.3 GHz electron gun. The FEC was a 100 nm (N)UNCD film grown on a 20 mm diameter stainless steel disk with a Mo buffer layer. At surface gradients 45-65 MV/m, peak currents of 1-80 mA (equivalent to 0.3-25 mA/cm 2 ) were achieved. Imaging with two YAG screens confirmed emission from the (N)UNCD surface with (1) the beam emittance of 1.5 mm×mrad/mm-rms, and (2) longitudinal FWHM and rms energy spread of 0.7% and 11% at an electron energy of 2 MeV. Current stability was tested over the course of 36×10 3 RF pulses (equivalent to 288×10 6 GHz oscillations).Despite being successfully deployed for RF microelectronics for decades 1 and considered conceptually for accelerator applications, 2, 3 it is only now that field emitters are making inroads into the applied area of scientific and industrial accelerators driven by RF guns. 4-6 Field emitters are generally shaped in the form of a wire/cone/pyramid with a sharp tip of a few tens of nm in order to locally enhance electric fields to the GV/m range under modest applied macroscopic fields of the order of 1-10 MV/m. When directly subjected to an electric RF field in the injector, the FEC may significantly simplify the architecture of RF electron guns. In a straightforward picture, electron bunches are generated and phased by the RF electric field itself every time a positive electric field peaks on the FEC's surface, and high repetition rates equal to the RF frequency are supported automatically. A simplified injector on an FEC platform would greatly benefit superconducting RF (SRF) technology by leading to fully cryogenic compact SRF linacs with 50-60% wall-plug efficiency for medical and industry applications. SRF examples include electron-linac factories for Mo-99 production for nuclear medicine to rule out weapons grade uranium from the production cycle or compact bright inverse Compton sources for basic science and semiconductor lithography. These and many other applications (like cargo inspection) require normal conducting or SRF systems delivering beam power of 10 to 100 kW at electron energy 10 to 50 MeV. Thus, average currents of 1 to 10 mA are needed. Operation of high-duty-cycle (10 -3 to CW) accelerators with FEC as a source would compel the use of advanced materials for its design so that the FEC performs stably under high repetition rate or CW conditions inducing high electric and/or thermal loads. Synthetic diamond thin films and structures are among the most promising materials for high-frequency field emission applications which include both vacuum microelectronics 7 and accelerators. 5 In Ref.5, currents of ≥10 mA were produced by directly subjecting a diamond tip array to an RF field in an L-band injector. With RF gradients of about 25 MV/m, the local actuating electric field on a tip was as high as ~1 GV/m. This example with an electron source of a simple design indeed suggests that this route...
By applying different symmetric boundary conditions, we found that the transverse wakefields generated by an electron bunch traveling through a partially loaded rectangular dielectric structure at an off center position can be decomposed into corresponding orthogonal longitudinal section electric (LSE) and longitudinal section magnetic (LSM) modes for guided waves as in the case of longitudinal wakefields treated previously. The wakefields are characterized using the normalized shunt impedance R/Q, a function of the geometry of the accelerating structure, for both LSE and LSM modes. A numerical example is given for an X-band waveguide structure and detailed results are given for the several leading transverse wakefield terms. The analytic results obtained are in agreement with the results from the time domain simulation tool MAFIA.
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.
We demonstrate, theoretically and experimentally, that a traveling electric charge passing from one photonic crystal into another generates edge waves -electromagnetic modes with frequencies inside the common photonic bandgap localized at the interface -via a process of transition edgewave radiation (TER). A simple and intuitive expression for the TER spectral density is derived and then applied to a specific structure: two interfacing photonic topological insulators with opposite spin-Chern indices. We show that TER breaks the time-reversal symmetry and enables valley-and spin-polarized generation of topologically protected edge waves propagating in one or both directions along the interface. Experimental measurements at the Argonne Wakefield Accelerator Facility are consistent with the excitation and localization of the edge waves. The concept of TER paves the way for novel particle accelerators and detectors. arXiv:1901.05640v3 [physics.optics]
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