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The pursuit to operate photocathodes at high accelerating gradients to increase brightness of electron beams is gaining interests within the accelerator community. Cesium telluride (Cs2Te) is a widely used photocathode material and it is presumed to offer resilience to higher gradients because of its wider band gap compared to other semiconductors. Despite its advantages, crucial material properties of Cs2Te remain largely unknown both in theory and experiments. In this study, we employ first-principles calculations to provide detailed structural, elastic, electronic and transport properties of Cs2Te. It is found that Cs2Te has an intrinsic mobility of 20 cm2/Vs for electrons and 2.0 cm2/Vs for holes at room temperature. The low mobility is primarily limited by the strong polar optical phonon scattering. Cs2Te also exhibits ultralow lattice thermal conductivity of 0.2 W/(m*K) at room temperature. Based on the energy gain/loss balance under external field and electron-phonon scattering, we predict that Cs2Te has a dielectric breakdown field in the range from ~60 MV/m to ~132 MV/m at room temperature dependent on the doping level of Cs2Te. Our results are crucial to advance the understanding of applicability of Cs2Te photocathodes for high-gradient operation.

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.

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 Snowmass21 Contributed Paper encourages the Particle Physics community in fostering R&D in Superconducting Nb3Sn 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 Nb3Sn 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 Nb3Sn films on either Cu or bronze despite minimal funding., Comment: Contribution to Snowmass 2021

Particle acceleration in microstructures driven by ultrafast solid state lasers is a rapidly evolving area of advanced accelerator research, leading to a variety of concepts based on planar-symmetric dielectric gratings, hollow core fibers, photonic crystals, and plasmonic meta-surfaces. This approach leverages well-established industrial fabrication capabilities and the commercial availability of tabletop lasers to reduce cost, with demonstrated axial accelerating fields in the GV/m range. Wide-ranging international efforts have significantly improved understanding of gradient limits, structure design, particle focusing and transport, staging, and development of compatible low-emittance electron sources. With a near-term focus on low-current MeV-scale applications for compact scientific and medical instruments, as well as novel diagnostics capabilities, structure-based laser-driven accelerators have several key benefits that warrant consideration for future high-energy physics machines, including low beamstrahlung energy loss, modest power requirements, stability, and readiness of supporting technologies., Comment: arXiv admin note: substantial text overlap with arXiv:2203.03811, arXiv:1901.10370

In this paper, we present the results of experimental observation of strong-field photoemission from a diamond field-emitter array (DFEA) illuminated by a focused laser beam with 1035 nm wavelength. Having the advantage of high emission current and low beam emittance, DFEAs can emit ultra-short high charge electron bunches required for multiple accelerator applications. We have performed strong-field photoemission experiment at a newly commissioned test stand at Los Alamos National Laboratory (LANL). We triggered a diamond tip with a typical apex of 10-20 nm with 300 femtosecond (fs) laser pulses to produce electron beam emission. The profile of the emitted beam was determined by the analysis of an image formed on the fluorescent screen after a microchannel plate (MCP) intensifier; beamlets corresponding to emission from the pyramid sides and nano-tip were identified. Image analysis of the beamlet associated with emission from the nano-tip was conducted to understand dependencies of intensities of the emission on polarization of the laser's light and its peak intensity., Comment: Proceedings of the Advanced Accelerator Concepts Seminar Series 2020

We demonstrate a novel scheme for imposing and removing large energy chirp on relativistic electron bunches relying on the transverse-to-longitudinal mixing accomplished with a set of transverse deflecting cavities. The working principles of the new concept are explained using the first order matrix formalism in the approximation of the linear single-particle dynamics. The scheme demonstrates versatility regarding the average beam energy, and hence, positioning along the accelerator beamline. The performance of the scheme is numerically investigated with the elegant particle tracking code. It is shown that the associated nonlinear effects, causing emittance deterioration for the extreme quantities of the total energy spread can be effectively compensated by optimization of the Twiss parameters while relying on the eigen-emittance analysis. The impact of the longitudinal space charge effects on the beam dynamics in the proposed scheme is also investigated., Comment: submitted to Phys. Rev. ST-AB