Building new experimental facilities to house experiments is an expensive and time-consuming activity. Although usually less expensive, repurposing old experimental facilities to accommodate new ones has its own set of challenges with regard to obsolete equipment, adequacy of electrical power, radioactive shielding and cooling capacity. At Los Alamos National Laboratory (LANL), one such facility was previously used to provide a platform for Free Electron Laser (FEL) experiments that were completed 20 years ago. This paper explores the techniques and process to repurpose an existing experimental facility to accommodate the CARIE compact accelerator and the choices made to select and size equipment for success. Radio Frequency (RF) energy waveguide layout with vacuum calculation methods will be included as well as electrical power and radiation shielding requirements.

High-quality electron beams are critical for generation of intense X-ray pulses from free electron lasers. It was proposed that complex thin films and heterostructures with semiconductor photoemissive layers may be used in photocathodes to produce electron beams with better quality. New developments in material science allow designing alkali-antimonide photocathodes with specific coatings that could significantly increase their lifetime while not markedly degrading their quantum efficiency (QE). Moreover, results from recent experiments demonstrated that QE can be increased by optical interference absorption effects in layered materials. Modeling of these complex photocathode material designs is needed to predict and optimize their electron emission properties. We apply recently developed extended moments and thin film models to evaluate quantum efficiency and intrinsic emittance from thin film cesium-telluride and alkali-antimonide semiconductor photocathodes grown on different substrates. We will discuss simulation results and suggest possible ways to optimize electron emission properties from these thin films photocathodes.

Semiconductor photocathodes, particularly those produced with thin films and heterostructures, are promising candidates of high brightness electron sources. It is also well-known that electron beam brightness increases with the photocathode gun’s operating gradient. Combining both heterostructure semiconductor photocathode and cyro-cooled high-gradient photocathode gun may improve electron sources for many applications. However, effects of the high field gradient on the semiconductor photocathode need to be understood in order to preserve and optimize the quality of the emitted photo-electron beams, which can be done with from detailed simulation study and theoretical analysis. In this work, we apply Monte-Carlo method to study high field transport and emission from semiconductor photocathodes such as Cs2Te. The results will be used to inform a theoretical transport model based on the moments method and the cathode development for the CARIE project at LANL.

Unprecedented electron beam brightness can potentially be achieved by exploiting high gradients in cryo-cooled RF cavities functionalized with high QE semiconductor photocathode materials. However, strong electric fields and thermal stresses from associated emission currents could potentially affect material stability, leading to breakdown events which may shorten device lifetime or degrade performance. To ensure robust performance and long operational lifetimes, the underlying processes leading to microstructural evolution in such semiconductor photocathodes needs to be explored under strong fields. Here, we present a suite of multiscale modeling tools specially tailored to probe the electro-thermo-mechanical behavior of semiconductor photocathodes. We first parameterize a machine learning interatomic potential suitable for classical charge equilibration molecular dynamics (MD) using density functional theory (DFT) calculations of CsTe. DFT and MD informed material properties are further incorporated into a meso-scale finite element (FE) model to predict morphological evolution of cathode surfaces under fields and thermal stresses due to emission currents.

Several concepts for future linear colliders are dependent on very high gradient normal conducting RF cavities achieved by operation at cryogenic temperatures in order to reduce breakdown rates (BDR). These maximum fields are intended to be in excess of 200 MV/m. The concepts include the ultra compact Xray free electron laser and the C$^3$ collider. The theory involved with the complex physics of breakdown is a diverse and rich field of study. Most results are empirical so continued understanding of the phenomena becomes necessary. One contributing factor to the reduced BDR is the increased hardness at cryogenic temperatures of the copper. in order to test that assumption we can consider obtaining hardness improvements from the alloying of copper with silver. We will here present a preliminary theory of this alloy based improvement especially with respect to an improved understanding of the surface resistivity using our previously established theory improvements which go beyond the usual Reuter and Sondheimer explanation. We will compare this to quality factors measured in Cband pillbox cavities as a function of temperature.

Cryo-cooled C-band (5.7 GHz) copper distributed-coupling cavities are a new approach to the structure-based accelerators for the future multi-TeV energy range linear collider. It provides numerous degrees of freedom to optimize the cavity geometry to achieve high gradient and high-power in the linear collider. In this study, we analyze the dipole modes of C-band 20-cells cavity and calculate the wall loss Q-factors, shunt impedance, and the impact of transverse wakefields in the frequency range up to 40 GHz by using ACE3P code (Omega3P and ACD tools). Next, we equip each cavity with four waveguide manifolds with damping loads to suppress undesirable higher-order-modes (HOM). The results of ACE3P simulations are compared with the CST microwave studio simulations.

This talk will report on the status C-band high gradient research program at Los Alamos National Laboratory (LANL). The program is being built around two test facilities: C-band Engineering Research Facility in New Mexico (CERF-NM), and Cathodes And Radio-frequency Interactions in Extremes (CARIE). Modern applications require accelerators with optimized cost of construction and operation, naturally calling for high-gradient acceleration. At LANL we commissioned a high gradient test stand powered by a 50 MW, 5.712 GHz Canon klystron. The test stand is capable of conditioning accelerating cavities for operation at surface electric fields higher than 300 MV/m. CERF-NM is the first high gradient C-band test facility in the US. CERF-NM was fully commissioned in 2021. In the last several years, multiple C-band high gradient cavities and components were tested at CERF-NM. Currently we work to implement several updates to the test stand including the ability to autonomously operate at high gradient for the round-the-clock high gradient conditioning. Adding capability to operate at cryogenic temperatures is considered. The construction of CARIE began in October of 2022. CARIE will house a cryo-cooled copper RF photoinjector with a high quantum-efficiency cathode and produce an ultra-bright 250 pC electron beam accelerated to the energy of 10 MeV. The status of the facility, the designs of the photoinjector and the beamline, and plans for photocathode testing will be presented.

We present our 1.6-cell radiofrequency cavity design for a photoinjector under development for producing intense electron bunches with 250-pC beam charge and normalized emittance below 100 nm rad for cryogenic temperature operation. The cavity cell profile was designed by SLAC and UCLA, optimized for maximal shunt impedance and minimal peak magnitude of the electric and magnetic field. The pi-mode accelerating fields are established in the cells with power coupled into each cell individually through the slot on the sidewall, and the peak electric field magnitude has been tuned to be equal in the two cells. The coupling waveguide network was designed to achieve critical coupling into the port of the input power waveguide and to achieve the desired power distribution. The cavity design has been completed for initial high-gradient test at room temperature.

Nanostructured electron sources exhibiting simultaneous spatio-temporal confinement to nanometer and femtosecond level along with a low emittance can be used for developing future ordered electron sources to generate unprecedented electron beam brightness and can revolutionize stroboscopic ultrafast electron scattering and steady-state electron microscopy applications. In addition, high current density electron beams generated from nanostructured electron sources can be used for applications that include nanoelectronics and dielectric laser accelerators. In this work, we report our efforts to develop and characterize two kinds of nanostructured electron sources: (i) nitrogen incorporated ultrananocrystalline diamond [(N)UNCD] tips and (ii) plasmonic Archimedean spiral focusing lens. We demonstrate the ability to fabricate these cathodes and characterize them using a photoemission electron microscope under femtosecond laser illumination thereby demonstrating the ability of these structures to be used for next generation electron sources.

Increasing energy of proton beam at the Los Alamos Neutron Science Center (LANSCE) from 800 MeV to 3 GeV will improve radiography resolution ten-fold. This energy boost can be achieved with a compact cost-effective linac based on normal conducting high-gradient (HG) RF accelerating structures. Such an unusual booster is feasible for proton radiography (pRad), which operates with short beam pulses at very low duty. The pRad booster starts with a short L-band section to capture and compress the 800-MeV proton beam from the existing linac. The main HG linac is based on S- and C-band cavities. An L-band de-buncher at the booster end reduces the beam energy spread at 3 GeV three times below that at the exit of the existing 800-MeV linac. We continue developing proton HG standing-wave structures with distributed RF coupling for the booster. Results of measurements for a two-cell test cavity at the LANL C-band RF Test Stand and a comparison with conventional traveling-wave structures are presented.

The Los Alamos Neutron Science Center (LANSCE) is a MW-class H-/H+ 800-MeV proton linear accelerator and storage ring that serves five distinct user facilities in support of LANL’s national security mission and DOE’s Office of Science medical isotope program. We will describe future directions of LANSCE over the next two decades, which includes revitalization and modernization of existing subsystems and upgrades with significantly increased operational capabilities. We will also be describing ongoing and future R&D activities will enable these enhancements. Some of this R&D is truly cross-cutting, leading to foundational technologies that broadly support multiple LANSCE directions, such as high-gradient normal-conducting RF, artificial intelligence/machine learning, and high-brightness, robust cathodes. Other R&D is more specific to particular applications, and include such topics as narrow bandwidth inverse-Compton scattering, short-range wakefield studies, and novel X-ray free-electron laser architectures.

A simple acceleration of a high charge, needle-shaped electron bunch from a cathode is affected by strong correlated emittance growth due to current-dependent transverse space-charge forces. It was shown that such emittance growth could be reversed by focusing the bunch soon after it emerges from the cathode, and that one can expect to retrieve the emittance the beam was born with – the intrinsic emittance. We present a space charge emittance compensation study for a 250 pC radiofrequency photoinjector based on a 100 pC design developed by the UCLA team. We expect that a bright electron beam with an order of magnitude improvement over currently operating photoinjectors can be achieved with 250 pC electron bunches that maintain their emittance below 100 nm-rad.