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.

We report on a method of photoemissive film growth that controls stoichiometry in real time. We show that stoichiometry control using a feedback loop is possible because (a) photoemissive properties exhibit a distinct dependence on the stoichiometric composition and (b) stoichiometric composition strongly depends on the ratio of the incident fluxes. The reported results were obtained on Cs3Sb but are expected to be relevant to other alkali antimonides and tellurides.

This paper reports the results of high gradient testing of the two C-band (5.712 GHz) normal conducting 𝛽𝛽 =0.5 accelerating cavities. The first cavity was made of copper and the second was made of copper-silver alloy with 0.085% silver concentration. The tests were conducted at the C-Band Engineering Research Facility of New Mexico (CERF-NM) located at Los Alamos National Laboratory. Both cavities achieved gradients more than 200 MeV/m and surface electric fields more than 300 MV/m. The breakdown rates were mapped as functions of peak surface fields. The gradients and peak surface fields observed in the copper-silver cavity were about 20% higher than those in the pure copper cavity with the same breakdown rate. It was concluded that the dominant breakdown mechanism in these cavities was not the pulse heating but the breakdown due to very high surface electric fields.

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.

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.

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.

Many applications such as compact accelerators and electron microscopy demand high brightness electron beams with small beam size and ultra-low emittance. Electric-field-assisted diamond emitters manufactured from semiconductor processes has been recognized as a leading candidate for such compact sources. The micro-scale pyramid structure of the emitter has the desirable attribute of significant electric field enhancement at the sharp interfaces (apex and edges) to facilitate electron emission. We investigate the dependence of field enhancement on the geometric shape. To account for the semiconductor charge transport in the bulk material and the tunneling through the surface, a first-principle semiclassical Monte Carlo emission model is developed and applied to the diamond pyramid. Combining the results from the Monte Carlo and the geometric field enhancement calculation, we construct a simple model to qualitatively explain the measured emission characteristics.1 The electron beam formation and dynamics in a 1D diode setup are simulated with a particle-in-cell code to obtain the macroscopic observables such as the beam energy, voltage, and divergence. The physical characteristics and parametric dependence of the emitted beam are compared with experiments and understood through the analysis of particle trajectory in a model field configuration.2 We further develop an effective mass based theoretical model accounting for the conduction band quantization in a high aspect ratio semiconductor nanostructure and the corresponding Monte Carlo implementation to describe electron transport and subsequent electron emission from the nanotip of the emitter. The effects of level quantization, electron scattering due to the nanotip diameter variation, and electron-phonon scattering on the nanotip emission properties are identified and compared with the case of a bulk slab. 3

Many applications, such as compact accelerators and electron microscopy, demand high brightness electron beams with small source size and ultralow emittance. Diamond emitters manufactured with semiconductor processes can be employed in such compact beam sources. The micrometer-scale pyramid structure of the emitter allows enhancement of the external field compared to that at the substrate, leading to electron emission with small beam size. We investigate the dependence of the field enhancement on the shape of the emitter and the resulting emission characteristics. The beam formation and dynamics are simulated with the LSP [D. Welch, D. Rose, R. Clark, T. Genoni, and T. Hughes, Comput. Phys. Commun. 164, 183 (2004)] particle-in-cell code to obtain the macroscopic observables. To account for the semiconductor charge transport in the bulk material and the tunneling through the surface, a first-principle semiclassical Monte Carlo emission model is developed and applied to the diamond pyramid. Using this Monte Carlo emission model and the result from the geometric field enhancement calculation, we construct a simple model to qualitatively explain the measured emission characteristics. A comparison between our model and experiments indicates that the beam current is mostly emitted at the apex of the emitter.

DRIVEN ACCELERATORS D.C. Nguyen , C.E. Buechler, G.E. Dale, R.L. Fleming, M.A. Holloway, J.W. Lewellen, D. Patrick Los Alamos National Laboratory J. Neilson, V. Dolgashev, E.N. Jongewaard, E.A. Nanni, A. Sy and S. Tantawi SLAC National Accelerator Laboratory Abstract Small, lightweight, few-MeV electron accelerators that can operate with low-voltage power sources, e.g., solidstate transistors running on 50 VDC, instead of highvoltage klystrons, will provide a new tool to enhance existing applications of accelerators as well as to initiate new ones. Recent advances in gallium nitride (GaN) semiconductor technologies [1] have resulted in a new class of high-power RF solid-state devices called highelectron mobility transistors (HEMTs). These HEMTs are capable of generating a few hundred watts at S-, Cand X-bands at 10% duty factor. We have characterized a number of GaN HEMTs and verified they have suitable RF characteristics to power accelerator cavities. We have measured energy gain as a function of RF power in a single low- C-band cavity. The HEMT powered RF accelerators will be compact and efficient, and they can operate off the low-voltage DC power buses or batteries. These all-solid-state accelerators are also more robust, less likely to fail, and are easier to maintain and operate. In this poster, we present the design of a low-, 5.1-GHz cavity and beam dynamics simulations showing continuous energy gain in a ten-cavity C-band prototype.

We have designed a Ka-band dielectric traveling-wave tube with high bandwidth. We are currently validating the 20-kV, 5-A electron beam gun and transport system and the structure is in fabrication. We report on the design of this structure and different experimental development elements.

The emittance of a field emission cathode can be difficult to measure close to the emitter, due to the high average current density of the beam and the potential for desorbed material from an imaging screen to contaminate the cathode. We present the design for a dual fixed-slit emittance measurement system for a field emitter cathode, implemented using electrostatic deflecting plates.

We present the results of beam divergence studies for the diamond field emitter array (DFEA) cathodes producing high-current-density electron beams. At Los Alamos National Laboratory (LANL), we fabricate and test the micrometer-scale diamond pyramids with nanometer-scale sharp tips for use as an electron beam source for a compact dielectric laser accelerator. For the beam divergence measurements, we assembled a test stand consisting of a DFEA cathode, a small mesh aperture anode, and a screen representing of a sapphire disk coated with ZnO (AZO) for beam visualization. A negative voltage of about 40 keV is applied to the cathode, and the mesh and the screen are kept at ground. We record the spot size corresponding to the size of the electron beam on the AZO screen past the mesh anode at different mesh to screen distances. We also conduct the beam dynamics simulations with General particle Tracer (GPT). In this paper, we present the results of the experimental measurements and GPT simulations, along with calculations of the beam's divergence.

Field emission from nanocrystalline diamond and especially from diamond field emitters is known to have an onset at low electric fields of a few MV/m, although the discussion on the agreement of the results with the classical Fowler-Nordheim model is still pending. While measurements of pure photoemission from flat nanocrystalline diamond agree reasonably well with the 3-step photoemission model for the wide bandgap semiconductor with low electron affinity, we are not aware of systematic studies of photoemission in the ~0.1-5 MV/ m range, where the electron emission mechanism is expected to be affected by the Schottky effect and crossover with the field emission. In order to understand applicability of field enhanced photoemission from diamond to generation of bright coherent photo-gated electron beams suitable for dielectric laser accelerators, we have designed a system to measure spectral response (quantum efficiency vs wavelength) of ~mm-sized samples with up to 5 MV/ m electric field in the anode-cathode gap. The system is based on an incoherent Xe lamp-based tunable ultaviolet light source, therefore relatively large and dense diamond field emitter arrays are required for comparative studies of arrays versus flat nanocrystalline diamond samples. We present the results of our original measurements in the range between 195 nm and 270 nm. Potential schemes of laser-triggered photoemission from a single diamond field emitter tip are discussed in view of the obtained results.

We present the design and initial test results for a simple, variable-focus solenoidal lens with integrated emittance filtering. The design was developed as a first-iteration injection optics solution for transport of a beam from a field-emitter cathode into a dielectric laser accelerator structure. The design is easy to fabricate and, while based on permanent magnets, can be readily modified to allow for remote control of the focal length. The emittance is controlled via a selection of collimating irises. The focal length can be changed by altering the spacing between the two permanent ring magnets. This allowed us to focus a 1.6 μA beam to a 10 μm spot size.