Diamond field emitter array field emission cathodes (DFEA FECs) are attractive for the next generation of injectors due to their ability to produce transversely shaped beams without the need for complex masking or laser schemes. However, reliability of this cathode technology remains a challenging issue as principal mechanisms guiding and allowing for output beam shaping remained poorly understood. This paper reports the results of testing two DFEA FECs with the same pattern and emitter tip geometry. Although both cathodes were able to sustain gradients of 44 MV/m and produce maximum output integral charge of 0.5 nC per radio frequency pulse, their emission patterns were different. One cathode did not produce a shaped beam, while the other one did. This difference was explained by the extent of the local variations of the diamond material across the arrays as discovered by spatially resolved Raman spectroscopy. The main practical takeaways were (i) tip sharpness was not a prerequisite for producing a shaped beam and instead (ii) material characteristics resulting in different cathode ballast resistance affected emission spatial uniformity across the array and hence the beam shaping.

FEgen is a comprehensive user interface designed to create initial particle distributions for field emission sources operated in rf guns (additional capability exists for dc pulsed power systems).

We report the results of high gradient testing of two single cell off axis coupled standing wave accelerating structures. Two brazed standing wave side coupled structures with the same geometry were tested one made of pure copper Cu and one made of a copper silver CuAg alloy with silver concentration of 0.08 percent. A peak surface electric field of 450 MV per m was achieved in the CuAg structure for a klystron input power of 14.5 MW and a 1 mirco s pulse length which was 25 percent higher than the peak surface electric field achieved in the Cu structure. The superb high gradient performance was achieved because of the two major optimizations in the cavity geometry 1 the shunt impedance of the cavity was maximized for a peak surface electric field to accelerating gradient ratio of 2 for a fully relativistic particle 2 the peak magnetic field enhancement due to the input coupler was minimized to limit pulse heating. These tests allow us to conclude that C band accelerating structures can operate at peak fields similar to those at higher frequencies while providing a larger beam iris for improved beam transport.

This Snowmass21 Contributed Paper encourages the Particle Physics community in fostering R&D in Superconducting Nb 3 Sn coated Copper RF Cavities instead of costly bulk Niobium. It describes the pressing need to devote effort in this direction, which would deliver higher gradient and higher temperature of operation and reduce the overall capital and operational costs of any future collider. It is unlikely that an ILC will be built in the next ten years with Nb as one of the main cost drivers of SRFs. This paper provides strong arguments on the benefits of using this time for R&D on producing Nb 3 Sn on inexpensive and thermally efficient metals such as Cu or bronze, while pursuing in parallel the novel U.S. concept of parallel-feed RF accelerator structures. A technology that synergistically uses both of these advanced tools would make an ILC or equivalent machines more affordable and more likely to be built. Such a successful enterprise would readily apply to other HEP accelerators, for instance a Muon Collider, and to accelerators beyond HEP. We present and assess current efforts in the U.S. on the novel concept of parallel-feed RF accelerator structures, and in the U.S. and abroad in producing Nb 3 Sn films on either Cu or bronze despite minimal funding.

This paper describes the current status of the newly commissioned C-band Engineering Test Facility in New Mexico (CERF-NM) at Los Alamos National Laboratory (LANL). At LANL, we designed and assembled a new test stand with the goal to conduct high gradient breakdown studies at C-band frequencies (5.712 GHz). The test stand is powered by a 50 MW, 5.712 GHz Canon klystron. It is capable of conditioning single cell accelerating cavities for operation at surface electric fields in excess of 300 MV/m. The first two C-band cavities have just undergone the high power testing and operation at high gradients was demonstrated. Probabilities of breakdown were measured as functions of the peak surface fields. Currently the high gradient cavities are operated at room temperature, but we consider adding a capability for cryogenic operation with a cryo-cooler. The test stand is operational and is open to collaborators.

Field emission cathodes (FECs) are attractive for the next generation of injectors due to their ability to provide high current density bright beams with low intrinsic emittance. One application of FECs worthy of special attention is to provide transversely shaped electron beams for emittance exchange that translates a transverse electron beam pattern into a longitudinal pattern. FECs can be fabricated in a desired pattern and produce transversely shaped beams without the need for complex masking or laser schemes. However, reliable and consistent production of transversely shaped beams is affected by material properties of the FEC. This paper reports the results of testing two diamond field emitter array (DFEA) FECs with the same lithography pattern and emitter geometry but different material and tip characteristics. Although both cathodes were able to sustain gradients of 44 MV/m and produce maximum output integral charge of 0.5 nC per radiofrequency (rf) pulse, their emission patterns were quite different. One cathode did not produce a patterned beam while the other one did. Differences in field emission characteristics and patterned beam production were explained by the differences in the tip geometry and the cathode material properties. The main practical takeaway was found to be that the tip sharpness was not a prerequisite for good patterned beam production. Instead, other material characteristics, such as the ballast resistance, determined cathode performance.

This paper reports the initial results of high gradient testing of two proton β=0.5 C-band accelerating cavities. The cavities for proton acceleration were fabricated at SLAC National Accelerator Laboratory (SLAC) and are in the process of being tested at the high gradient C-band accelerator test stand at Los Alamos National Laboratory (LANL). One cavity was made of copper, and the second was made of a copper-silver alloy. LANL test stand was constructed around a 50 MW, 5.712 GHz Canon klystron and is capable of providing power for conditioning single cell accelerating cavities for operation at surface electric fields up to 300 MV/m. These β =0.5 C-band cavities are the first two cavities to be tested at LANL’s C-band test stand. This presentation reports achieved gradients, breakdown probabilities, and other characteristics measured during the high power operation.

High peak power, tunable, narrowband terahertz emitters are becoming sought after given their portability, efficiency, and ability to be deployed in the field for industrial, medical, and military applications. The use of accelerator systems producing THz frequencies via Cherenkov radiation, generated by passing an electron beam through a slow-wave wakefield structure, is a promising method to meet future THz requirements. To date, efforts have been dedicated to analysis and design of sources utilizing laser seeded bunched electron beam drivers with relativistic energies beyond 5 MeV. Presented here is a wakefield THz generation scheme based on passing a long quasi-dc nonrelativistic beam (200 keV) through a dielectric loaded traveling wave structure. Reduced energy allows for compactness and portability of the accelerator as the size and weight of the dielectric slow-wave structure is vanishingly small compared to the accelerator unit. The presented scheme can serve as a tunable high peak power THz source operated between 0.4 and 1.6 THz and produces power gain by a factor of five with an average efficiency of 6.8%.

Planar polycrystalline synthetic diamond with nitrogendoping/incorporation was found to be a remarkable field emitter. It is capable of generating a high charge beam and handling moderate vacuum conditions. Integrating it with an efficient RF cavity could therefore provide a compact electron source for RF injectors. Understanding the performance metrics of the emitter in RF fields is essential toward developing such a device. We investigated a test setup of the field emitter at the X-band frequency. The setup included an X-band cavity operating at the TM02 mode. The field emitter material will be plated on the tip of an insertion rod on the cavity back plate. Part of the back plate and the emitter rod are demountable, allowing for exchange of the field emitters. The TM02 mode was chosen such that the design of the demountable back plate does not induce field enhancement at the installation gap. The cavity was optimized to achieve a high surface field at the emitter tip and a maximum energy gain of the emitted electrons at a given input power. We will present the RF and mechanical design of such a TM02 X-band cavity for field emitter testing. INTRODUCTION Nitrogen-incorporated ultrananocrystalline diamond (N-UNCD) films have been shown to be an excellent field emitter with a low turn-on surface field and a stable emission current [1-4]. The planar (N)UNCD cathodes have been tested and shown to perform in both normal and superconducting environments, and in DC and RF fields. Hence, it may be of use in both normal and SRF linac systems. Unlike legacy field emission electron sources consist of Spindt-type arrays of nanometer-size high aspect-ratio tips, the N-UNCD can be a thin film synthesized on a substrate material without needing of complicated processes such as lithography. Moreover, the N-UNCD can be grown directly onto any refractory metal. So that the field emitter based on the N-UNCD can be made into a variety of shapes and sizes. It provides flexibilities in the emitter design to optimize the emission current and beam characteristics. It potentially capable of generating a high charge beam, ~1-10 pC per RF cycle, and handling moderate vacuum conditions and can be refurbished [5]. Integrating it with an efficient RF cavity could therefore provide a compact electron source for RF injectors. Such electron sources, if validated, could significantly benefit the system design of a wide range of accelerator applications. For example, this would eliminate the high voltage elements in a compact accelerator application thus greatly reduce the size and weight of the system; the source produces bunched electrons thus eliminates the bunching elements and associated beam loss heating. The N-UNCD emitter has the potential to reach ultralow emittance and energy spread due to intrinsic properties [6] and via various gating methods [7, 8]. Experiments have shown emission at field levels of 1 MV/m and up. The full operational range, from turn-on up to the breakdown field, must be explored systematically before N-UNCD can be considered for accelerator applications. Understanding the performance metrics of the emitter in RF fields in miniature high frequency environment is essential. Maintaining high field emission cathode performance over long-term operation remains an important goal to realize high-performance field emission electron sourcebased injectors. Toward this goal, we investigated a test setup of the field emitter at the X-band frequency. The setup included an X-band cavity operating at the TM02 mode. The field emitter material will be plated onto the tip of an insertion rod on the cavity back plate. Part of the back-plate and the emitter rod are demountable, allowing for exchange of the field emitters. Mechanically, the cavity body and the demountable back plate are brazed onto a standard 2.75’’ flange. An insertion hole is machined out on the flange that is brazed on the cavity. A large cylinder piece is brazed onto the other side of the flange and is inserted into the hole to form part of the cavity back wall. This insertion may leave with a coaxial gap on the back wall of the cavity, by design or by the tolerance of a tight fit. This can induce field enhancement and potentially causing multipacting in the gap. To mitigate these issues, the TM02 mode was chosen such that the design of the demountable back-plate does not induce field enhancement at the insertion gap and the field in the gap is supressed by design to eliminate the multipacting. The cavity was optimized to achieve a high surface field at the emitter tip and a maximum energy gain of the emitted electrons at a given input power. This paper presents the RF and mechanical design of such a TM02 X-band cavity. CAVITY RF AND MECHANICAL DESIGN The cavity geometry is required to accommodate the flexibility of a demountable back-plate so that different emitters can be tested or swapped at the end of their lifetime with ease. The field emission material is plated onto the tip of a cylinder rod. The rod is intruded into the cavity to produce a high electric field enhancement at the tip. The tip is rounded to obtain a uniform field distribution in a large portion of the hemispheric surface. The emitter rod is __________________________________ *Work at SLAC was supported by DOE under contract No. DE-AC0276SF00515. Work at MSU was supported by DOE under Award No. DESC0020429 and under Cooperative Agreement Award No. DESC0018362. #lizh@slac.stanford.edu 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW Publishing ISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-WEPAB148 MC3: Novel Particle Sources and Acceleration Techniques T02 Electron Sources WEPAB148 2961 C on te nt fr om th is w or k m ay be us ed un de rt he te rm s of th e C C B Y 3. 0 lic en ce (© 20 21 ). A ny di st ri bu tio n of th is w or k m us tm ai nt ai n at tr ib ut io n to th e au th or (s ), tit le of th e w or k, pu bl is he r, an d D O I

C-Band structures research is of increasing interest to the accelerator community. The RF frequency range of 4-6 GHz gives the opportunity to achieve significant increase in the accelerating gradient, and having the wake fields at the manageable levels, while keeping the geometric dimensions of the structure technologically convenient. Strong team of scientists, including theorists researching properties of metals under stressful thermal conditions and high electromagnetic fields, metallurgists working with copper as well as alloys of interest, and accelerator scientists developing new structure designs, is formed at LANL to develop a CERF-NM facility. A 50 MW, 5.712 GHz Canon klystron, was purchased in 2019, and laid the basis for this facility. As of Jan-21, the construction of the Test Stand has been finished and the high gradient processing of the waveguide components has been started. Future plans include high gradient testing of various accelerating structures, including benchmark C-band accelerating cavity, a proton β=0.5 cavity, and cavities made from different alloys. An upgrade to the facility is planned to allow for testing accelerator cavities at cryogenic temperatures.

FEbeam is an all-in-one field emission data processing interface with the capability to analyze the field emission cathode performance in an rf injector by extracting the field enhancement factor, local field, and effective emission area from the Fowler-Nordheim equations. It also has the capability of processing beam imaging micrographs using its sister software, FEpic. The current version of FEbeam was designed for the Argonne Cathode Test-stand of the Argonne Wakefield Accelerator facility switch yard. With slight modifications, FEbeam could work for many rf field emission injectors. This software is open-source and can be found at GitHub.

A coupon tester cavity allows a material in the form of a simple removable “coupon,” or plate, to be exposed to high-strength radiofrequency (RF) fields, without the need to fabricate a complete cavity from each material to be explored. As we are interested in the breakdown performance of materials intended for high-gradient, normal-conducting structures, we must apply both electric and magnetic fields to the coupon. We present the design criteria for our coupon tester cavity, nominal operating parameters, and our structure concepts. The cavity design will be finalized over the next several months, and is intended to be constructed and inservice near the start of 2022.

High-current bright sources are needed to power the next generation of compact rf and microwave systems. A major requirement is that such sources 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, 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 was demonstrated (300 nC per rf pulse corresponding to an rf pulse current of 120 mA). This operating condition revealed a two-dimensional space charge limited emission where the one-dimensional Child-Langmuir limit was surpassed. An injector brightness of 10(14) A=(rad m)(2) was estimated for the given operating conditions.

This work is a part of an ongoing research to develop a high gradient test stand called the C-band engineering research facility New Mexico (CERF-NM) at Los Alamos National laboratory (LANL). Our team has developed the software called FEbeak (a part of the FEmaster series) which allows to analyse breakdown in real time. This software will be able to provide high accuracy breakdown analysis while coupling it to the field emission dark current effects and breakdown in situ imaging software diagnostics. FEbreak has shown a 97% efficiency for pulse acquisition and analysis when processing1 μs long pulses at 100 Hz repetition rate, which is a standard setting for testing many normal conducting cavities for high gradient.

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 $\sim$42~MV/m in a well-controlled manner. It reached a maximum output charge of 15~nC corresponding to an average current of 6~mA during an emission period of 2.5~$\mu$s. Imaging of emission current revealed a large number of isolated emitters (density over 100/cm$^{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 effective Debye temperature () of the surface of a single crystal of U0.71Th0.79O2 alloy prepared by hydrothermal synthesis is obtained from temperature‐dependent X‐ray photoemission in the temperature range of 300–623 K. The effective average Debye temperature is determined to be 217 ± 24 K. X‐ray fluorescence (XRF) spectroscopy confirms the crystal's composition as U0.71Th0.79O2. Photoemission spectroscopy reveals both thorium and uranium 4f states, which includes uranium and thorium in 4+ oxidation states. The Debye temperature of the single crystal U0.71Th0.79O2 alloy, as measured via photoemission, is lower than urania or thoria literature values, indicating either higher defect scattering or the presence of added vibrational modes within the alloyed sample.