The pursuit to operate photocathodes at high accelerating gradients to increase brightness of electron beams is gaining interests within the accelerator community, particularly for applications such as free electron lasers (FEL) and compact accelerators. 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 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.

Understanding performance and limitation of CsTe photocathodes under high field gradients in a radio-frequency gun requires adequate theoretical models for material properties, photoemission and surface morphology. We are developing a suite of models based on Density Functional Theory (DFT), moment and Monte-Carlo (MC) photoemission models, and meso-scale material surface model informed by DFT and Molecular Dynamic (MD) simulations. Our DFT calculations provide detailed structural, elastic, electronic, optical, and transport properties of CsTe for photoemission applications. Temperature, density of states, and thin film optical effects have recently been incorporated in a moment-based photoemission model, while the high field effects for electron transport and emission are being modeled in the MC model. Our meso-scale surface model addresses surface morphology under high field stress and surface heating. Machine-learning technique has also been used to enhance the DFT and MD calculations for CsTe. This poster will present an overview of these theoretical models and their results with applications to the LANL CARIE project and other relevant experiments.

This talk will report on the status of commissioning of the Cathodes And Radio-frequency Interactions in Extremes (CARIE) C-band high gradient photoinjector test facility and other high-gradient C-band research activities at Los Alamos National Laboratory (LANL). The construction of CARIE began in October of 2022. CARIE is powered by a 50 MW 5.712 GHz Canon klystron and will house a high gradient copper RF photoinjector with a high quantum-efficiency cathode and produce an ultra-bright 250 pC electron beam accelerated to the energy of 7 MeV. The klystron was received, installed, and conditioned in 2024. The output of the klystron is connected to a circulator that was conditioned to operate for up to 12 MW of power. The WR187 waveguide line brings the power from the circulator into a concrete vault. The test RF injector is made of copper and does not have cathode plugs. It will be commissioned to validate operation of the CARIE facility in Spring of 2025. The second injector that will accommodate cathode plugs is in fabrication. The designs of the photoinjector and the beamline, and status of the high-power testing of the injector and other C-band components will be presented.

We present updated simulation results on the maximum brightness achievable by a 1.6-cell cold copper C-band photoinjector, designed for testing and commissioning as part of the Cathodes and RF in Extremes project at Los Alamos National Laboratory. Previous simulations highlighted the high brightness attainable with a 250 pC bunch charge, attributed to the high accelerating gradients and the benefits of a radially symmetrized photoinjector design. However, these earlier simulations relied on idealized temporal beam profiles, overlooked the temporal evolution of the gradients, and did not account for the influence of cathode plug geometry on the gradients—factors that significantly affect the maximum achievable brightness. In this work, we report the results of Multi-Objective Genetic Optimizations that incorporate more realistic temporal beam profiles and gradients, accounting for both the cathode plug geometry and the effects of time-dependent gradient evolution.

Emittance preservation in the main linac is the driving consideration for developing the alignment and vibration tolerances of any future linear collider. Here we present the first beam dynamics analysis of the main linac for the Cool Copper Collider (C-3), a novel concept for a linear collider utilizing a cryogenically-cooled copper linac with a distributed coupling architecture. The C(3)main linacs bring the beam energy from 10 GeV to 125 GeV for 250 GeV center-of-mass collisions. This concept allows for the use of standing-wave accelerating structures with integrated damping and detuning for higher-order mode suppression. The use of standing wave accelerating structures allows for optimization of the magnetic FODO lattice period to improve transport (maximize alignment tolerances) without impacting the rf to beam efficiency. In this work, beam transport is analyzed including both the short and long-range wakefields. Alignment and vibration tolerances are determined to provide emittance preservation sufficient to meet target luminosity metrics. Alignment tolerances are studied under the assumption of beam based alignment to achieve dispersion free steering. Closely spaced main linac components must be aligned to 10 mu m within a cryomodule. At longer ranges, up to hundred micron scale alignment is sufficient between components separated by 100s m to a km. In order to keep emittance growth at similar to 10%, it was determined that vibration tolerances, or random motion of components at frequencies >30 Hz, for rf structures are 10 mu m. For quadrupoles, vibration tolerances are 10 nm.

The occurrence of radio-frequency (rf) breakdown limits operational electromagnetic gradients in accelerator structures. Experimental evidence often suggests that breakdown events are associated with temperature and dark current spikes on the surface of rf devices. In the past decade, there has been increased interest in unveiling the mechanism behind breakdown initiation in metal copper and copper alloys. However, effort regarding breakdown phenomenon in photocathode relevant semiconductors have been more limited. In this work, we explore field-emission-assisted heating via Nottingham and Joule processes, as a possible candidate for breakdown initiation. For this, field emission from intrinsic Cs2Te ultra-thin film coated on a copper substrate was modeled within the Stratton/Baskin/Lvov/Fursey (SBLF) formalism, describing the processes and effects in the bulk and on the surface of a photocathode exposed to high radio-frequency electromagnetic fields. It is shown that field emission characteristic deviates significantly from the classical Fowler-Nordheim (FN) theory, whereby predicting that dark current is orders of magnitude lower than one expected by FN law. Conventional pulsed heating was also found to impose negligible heating to the photo-cathode. Both conclusions suggest that Cs2Te photocathode coated on a metal substrate would be insensitive to catastrophic thermal-material runaway breakdown, unlike what is observed for metal surfaces. Finally, a few unconventional breakdown candidate scenarios are identified and discussed including thermo-elastic deformation and avalanche breakdown.

The Linear Collider Vision calls for a Linear Collider Facility with a physics reach from a Higgs Factory to the TeV-scale with e^+e^- collisions. One of the technologies under consideration for the accelerator is a cold-copper distributed-coupling linac capable of achieving high gradient. This technology is being pursued by the C^3 collaboration to understand its applicability to future colliders and broader scientific applications. In this input we share the baseline parameters for a C^3 Higgs-factory and the energy reach of up to 3 TeV in the 33 km tunnel foreseen under the Linear Collider Vision. Recent results, near-term plans and future R&D needs are highlighted.

The phenomenon of electric breakdown poses serious challenge to the design of devices that operate under high gradient environments. Experimental evidence often points towards breakdown events that are accompanied by elevated temperatures and dark current spikes, presumably due to high-asperity nano-structure formation that enhances the local electric field and triggers a runaway process. However, the exact mechanistic origin of such nano-structures under typical macroscopic operational conditions of electric gradient and magnetic-field-mediated heating remains poorly understood. In this work, a model is presented that describes the evolution of a typical copper surface under the combined action of the electric fields and elevated temperatures. Using a mesoscale curvature-driven growth model, we show how the copper surface can undergo a type of dynamical instability that naturally leads to the formation of sharp asperities in realistic experimental conditions. Exploring the combined effect of fields and temperature rise, we identify critical regimes that allow for the formation of breakdown precursors. These regimes strongly resonate with previous experimental findings on breakdown of copper electrodes, hence suggesting surface diffusion to be a crucial breakdown precursor mechanism.

Recently, considerable work has been directed at the development of an ultracompact X-ray free-electron laser (UCXFEL) based on emerging techniques in high-field cryogenic acceleration, with attendant dramatic improvements in electron beam brightness and state-of-the-art concepts in beam dynamics, magnetic undulators, and X-ray optics. A full conceptual design of a 1 nm (1.24 keV) UCXFEL with a length and cost over an order of magnitude below current X-ray free-electron lasers (XFELs) has resulted from this effort. This instrument has been developed with an emphasis on permitting exploratory scientific research in a wide variety of fields in a university setting. Concurrently, compact FELs are being vigorously developed for use as instruments to enable next-generation chip manufacturing through use as a high-flux, few nm lithography source. This new role suggests consideration of XFELs to urgently address emerging demands in the semiconductor device sector, as identified by recent national need studies, for new radiation sources aimed at chip manufacturing. Indeed, it has been shown that one may use coherent X-rays to perform 10–20 nm class resolution surveys of macroscopic, cm scale structures such as chips, using ptychographic laminography techniques. As the XFEL is a very promising candidate for realizing such methods, we present here an analysis of the issues and likely solutions associated with extending the UCXFEL to harder X-rays (above 7 keV), much higher fluxes, and increased levels of coherence, as well as methods of applying such a source for ptychographic laminography to microelectronic device measurements. We discuss the development path to move the concept to rapid realization of a transformative XFEL-based application, outlining both FEL and metrology system challenges.

Nitrogen-incorporated ultrananocrystalline diamond [(N)UNCD] pyramid tip cathode has been considered as a next-generation high peak current electron source for dielectric laser accelerators as well as other high peak current particle accelerator applications. In this work, we study non-linear photoemission from an (N)UNCD pyramid tip cathode using an ultrafast laser with the pulse length of 150 fs with the central wavelength of 800 nm in the peak intensity range of 109–1010W/cm2. We demonstrated that as the incident laser intensity increases, the current emitted from the nano-tip first increases as a power function with an exponent of about 5 and then starts to roll over to an exponent of 3. This roll over is attributed to the Coulomb interaction between electrons emitted from the tip also known as the space charge. We also measured the photoemission electron energy spectra that show electrons with energies as high as ∼10 eV. Based on the shape of the electron energy spectra, we conclude that the high-energy electrons are thermally emitted electrons due to ultrafast laser heating at the tip of the (N)UNCD pyramid tip cathode.

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.

This paper reports the design, fabrication, and results of the high-gradient conditioning and testing for a two-cell, pi-mode, standing wave normal-conducting prototype booster cavity for the proposed 3 GeV proton linac upgrade at Los Alamos Neutron Science Center. Increasing the energy of proton beam from the existing 800 MeV to 3 GeV will improve resolution of the proton radiography by up to 10 times. The proposed energy boost can be achieved with a compact normal-conducting high-gradient radio-frequency (rf) linac section. The C-band section of the booster linac was designed with optimized-shaped copper accelerator cavities with distributed rf coupling. A short two-cell test prototype structure was designed for the frequency of 5.712 GHz, fabricated, and tested at the C-band Engineering Research Test Facility in New Mexico (CERF-NM) at Los Alamos National Laboratory. The maximum klystron power coupled into the test structure was 8.3 MW with 1 mu s pulse length and 100 Hz repetition rate. The breakdown probabilities were recorded as functions of the accelerating gradient and peak surface fields. Operation of the test cavity at accelerating gradients of up to 100 MV/m was demonstrated.

Understanding the underlying factors responsible for higher-than-anticipated local field enhancements required to trigger vacuum breakdown on pristine metal surfaces is crucial for the development of devices capable of withstanding intense operational fields. In this study, we investigate the behavior of nominally flat copper electrode surfaces exposed to electric fields of hundreds of MV/m. Our novel approach considers curvature-driven diffusion processes to elucidate the formation of sharp breakdown precursors. To do so, we develop a mesoscale finite element model that accounts for driving forces arising from both electrostatic and surface-tension-induced contributions to the free energy. Our findings reveal a dual influence: surface tension tends to mitigate local curvature, while the electric field drives mass transport toward regions of high local field density. This phenomenon can trigger the growth of sharper protrusions, ultimately leading to a rapid enhancement of local fields and, consequently, to a runaway growth instability. We delineate supercritical and subcritical regimes across a range of initial surface roughness. Our numerical results are in qualitative agreement with experimentally reported data, indicating the potential practical relevance of field-driven diffusion in the formation of breakdown precursors.

Many applications for miniaturized millimeter/THz-wave tubes and electron accelerators demand compact size of source, high emission currents, and small emittance. We propose that diamond field emitter arrays (DFEA) can be used as an electron beam source of those tubes and accelerators. DEFA cathodes have 10–20 nanometer sharp tips on top of the micrometer scale of pyramids. Those cathodes have high thermal conductivity and can operate in poor vacuum $(< 10^{5} \text{Torr})$ that matches well for compact millimeter/THz-wave sources. Using mold-transfer method and electron beam lithography (EBL) skills, we improve the current fabrication techniques [1] to establish precise and uniform emission tip morphology. With 2 nm resolution from the EBL, we are capable of fabricating 10 nm single tip on each pyramid in the array of $6 \mathrm{x}100$ . Also, with enhanced high quality of the synthetic nitrogen-doped ultrananocrystalline diamond from the Michigan State University, each emitter will provide higher pet-tip current ( $> 15\mu \mathrm{A}$ per-tip), and total emission current will be about 9 mA. We will present the details of the fabrication process using EBL skills.

C 3 is an opportunity to realize an e + e - collider for the study of the Higgs boson at √ s = 250 GeV, with a well defined upgrade path to 550 GeV while staying on the same short facility footprint [2,3]. C 3 is based on a fundamentally new approach to normal conducting linear accelerators that achieves both high gradient and high efficiency at relatively low cost. Given the advanced state of linear collider designs, the key system that requires technical maturation for C 3 is the main linac. This paper presents the staged approach towards a facility to demonstrate C 3 technology with both Direct (source and main linac) and Parallel (beam delivery, damping ring, ancillary component) R&D. The primary goal of the C 3 Demonstration R&D Plan is to reduce technical and cost risk by building and operating the key components of C 3 at an adequate scale. This R&D plan starts with the engineering design, and demonstration of one cryomodule and will culminate in the construction of a 3 cryomodule linac with pre-production prototypes. This R&D program would also demonstrate the linac rf fundamentals including achievable gradient and gradient stability over a full electron bunch train and breakdown rates. It will also investigate beam dynamics including energy spread, wakefields, and emittance growth. This work will be critical to confirm the suitability of the C 3 beam parameters for the physics reach and detector performance in preparation for a Conceptual Design Report (CDR), as well as for follow-on technology development and industrialization. The C 3 Demonstration R&D Plan will open up significant new scientific and technical opportunities based on development of high-gradient and high-efficiency accelerator technology. It will push this technology to operate both at the GeV scale and mature the technology to be reliable and provide high-brightness electron beams. The timeline for progressing with C 3 technology development will be governed by practical limitations on both the technical progress and resource availability. It consists of four stages: Stage 0) Ongoing fundamental R&D on structure prototypes, damping and vibrations. Stage 1) Advancing the engineering maturity of the design and developing start-to-end simulations including space-charge and wakefield effects. This stage will include testing of strucutres operating at cryogenic temperatures. Beam tests would be performed with high beam current to test full beam loading. Stage 2) Production and testing of the first cryomodule at cryogenic temperatures. This would provide sufficient experimental data to compile a CDR and it is anticipated for Stage 2 to last 3 years and to culminate with the transport of photo-electrons through the first cryomodule. Stage 3) Updates to the engineering design of the cryomodules, production of the second and third cryomodule and their installation. Lower charge and lower emittance beams will be used to investigate emittance growth. The successful full demonstration of the 3 cryomodules to deliver up to a 3 GeV beam and achieve the C 3 five gradient will allow a comprehensive and robust evaluation of the technical design of C 3 as well as mitigate technical, schedule, and cost risks required to proceed with a Technical Design Report (TDR).

processing buildings at four Technical Areas (TAs) accumulates in sumps where particulate HE settles out and barium is precipitated. Wastewater is then released from the sumps to the environment at 15 permitted outfalls without treatment. The released water may contain suspended and dissolved contaminants, such as HE and solvents. This Environmental Assessment (EA) analyzes two alternatives, the Proposed Action and the Alternative Action, that would meet the purpose and need for agency action. Both alternatives would treat all HE process wastewater using sand filters to remove HE particulates and activated carbon to adsorb organic solvents and dissolved HE. Under either alternative, LANL would burn solvents and flash dried HE particulates and spent carbon following well-established procedures. Burning would produce secondary waste that would be stored, treated, and disposed of at TA-54, Area J. This report contains the Environmental Assessment, as well as the Finding of No Significant Impact and Floodplain Statement of Findings for the High Explosives Wastewater Treatment Facility.

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.

weight is placed on the inner shell by giving the shell material an appropriate density and applying a global acceleration. In addition to the weight loading, there will also be a pressure loading applied to both end plates as a result of preloading the calorimeter plates compressively. This pressure is estimated to be 20 pSi, and was represented in the model as a uniform pressure applied across each end plate. The large axial force produced by this pressure precludes the possibility of attaching the inner shell to both end plates. Such attachments would be under unreasonably high stress as the plates were preloaded, and the inner shell would be under a state of tension in trying to resist the axial force. In the real structure, the inner shell will be attached to at most one of the end plates. The axial force is then developed solely in the outer shell, which has a considerable area of attachment. To emulate this in the finite model, nodal coupling was used to couple the shell laterally to both end plates and all intermediate discs to ensure weight transfer, but axially the shell was only coupled to one of the end plates. The materials used were assumed to be SS 3011 with a Young's modulus of 28.3 (10{sup 6}) psi. Stresses were evaluated according to the limits and claSSifications of the ASME Boiler and Pressure Vessel Code, Section VIII, Division 2, Appendix 11 assuming a maximum allowable stress intensity of 20000 psi for primary membrane stress.

Construction of an e(+)e(-) Higgs factory has been identified as a major goal for particle physics. Such a collider will offer precise measurements of the Higgs bosons couplings to other particles. A Higgs factory extendable in energy can also establish the Higgs self-coupling, measure the Higgs coupling to the top quark, and expand the reach to probe new phenomena. We propose a strategy for an energy-extendable Higgs factory based on a new linear accelerator technology. This strategy offers a compact and cost-effective design that could be realized as an accelerator project in the US. The core technologies to be developed have broad applications to accelerators for medicine and for X-ray science. The challenge of realizing these technologies will offer unique and exciting opportunities to young scientists.