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...

A Compact Linear Collider prototype traveling-wave accelerator structure fabricated at Tsinghua University was recently high-gradient tested at the High Energy Accelerator Research Organization (KEK). This X-band structure showed good high-gradient performance of up to 100 MV=m and obtained a breakdown rate of 1.27 × 10 −8 per pulse per meter at a pulse length of 250 ns. This performance was similar to that of previous structures tested at KEK and the test facility at the European Organization for Nuclear Research (CERN), thereby validating the assembly and bonding of the fabricated structure. Phenomena related to vacuum breakdown were investigated and are discussed in the present study. Evaluation of the breakdown timing revealed a special type of breakdown occurring in the immediately succeeding pulse after a usual breakdown. These breakdowns tended to occur at the beginning of the rf pulse, whereas usual breakdowns were uniformly distributed in the rf pulse. The high-gradient test was conducted under the international collaboration research program among Tsinghua University, CERN, and KEK.

We present the first demonstration of high-power, reversed Cherenkov wakefield radiation by electron bunches passing through a metamaterial structure. The structure supports a fundamental TM-like mode with a negative group velocity leading to reversed Cherenkov radiation, which was clearly verified in the experiments. Single 45 nC electron bunches of 65 MeV traversing the structure generated up to 25 MW in 2 ns pulses at 11.4 GHz, in excellent agreement with theory. Two bunches of 85 nC with appropriate temporal spacing generated up to 80 MW by coherent wakefield superposition, the highest RF power that metamaterial structures ever experienced without damage. These results demonstrate the unique features of metamaterial structures that are very attractive for future high-gradient, wakefield accelerators, including two-beam and collinear accelerators. Advantages include the high shunt impedance for high power generation and high gradient acceleration; the simple and rugged structure; and a large parameter space for optimization.

Undesirable electron field emission (a.k.a. dark current) in high gradient RF photocathode guns deteriorates the quality of photoemission current and limits the operational gradient. To improve the understanding of dark current emission, a high-resolution (∼100 µm) dark current imaging experiment has been performed in an L-band photocathode gun operating at ∼100 MV/m of surface gradient. Dark current from the cathode has been observed to be dominated by several separated strong emitters. The field enhancement factor, β, of selected regions on the cathode has been measured. The post scanning electron microscopy (SEM) and white light interferometer (WLI) surface examinations reveal the origins of ∼75% strong emitters overlap with the spots where rf breakdown have occurred.Electrons can tunnel through a surface barrier modified by the presence of an electric field, resulting in a field emission (FE) current [1][2][3][4]. While the existence of this physical phenomenon allows the operation of field emission electron sources [5][6][7][8], it has a negative (parasitic) impact on the performance of vacuum resonator-based dc and rf systems such as traveling wave tubes, photocathode guns, and particle accelerators [3,[9][10][11][12]. The troublesome field emission current is referred to as dark current. It is an incoherent source of electrons that impacts the energy budget of a device, and is a source of undesired secondary electrons and ions [9,13,14]. Historically, dark current has been considered to be a trigger of breakdown in vacuum devices which may interrupt the normal operation of the device and even jeopardize the entire facility [3].To date many questions surrounding FE still remain, especially in the rf case which limit the improvement of electron sources and high gradient accelerators for TeVscale linear colliders [15] and compact X-ray electron sources [16,17]. For example, a large discrepancy exists between emitter properties obtained through direct observation using advanced surface analysis tools and those indirectly obtained from fitting the experimental data to the Fowler-Nordheim (F-N) equation [3,18]; the temporal evolution of the FE area under high electromagnetic fields is mostly unknown [19]; and empirical methods and procedures to suppress or enhance dark current lack theoretical support. All these questions result from the lack of a means for in situ high-resolution FE observation. In earlier FE studies under a dc field, emitter mapping with better than 1 µm resolution has been achieved by scanning an anode along the cathode [20][21][22]. However, imaging the field emitters at high resolution while they are emitting under an rf field is extremely challenging due to the wide emitting phase (the timing with respect to the applied rf field) and energy spread range of the dark current [13,[23][24][25]. In this Letter, we present observations of in situ dark current emission in a high gradient photocathode gun using a dedicated dark current imaging beamline. The study was conducted at Argonne Wakefie...