The high average current polarized electron source for future electron ion collider (EIC) requires extremely high vacuum. Currently, we have constructed a DC gun based on principle of beams funneling and this gun is under commissioning. Superlattice GaAs cathodes will be used as our electron source. The lifetime of these cathodes is dependent on ion back bombardment which caused by the residual gas pressure. Multiple GaAs cathodes with almost same lifetime are extremely crucial for the operation of this gun. Therefore, produce a uniform extremely high vacuum environment is essential in gun design. We studied residual molecules distribution in funneling gun by Molflow+ and a python based Monte-Carlo simulator. Twenty cathodes testing is under planning. Cathodes lifetime changes will give us indication on vacuum distribution inside the gun. This articles describes our coding and modeling for the gun vacuum, analyzes the residual gas distribution on the gun and discusses on multiple cathodes lifetime measurement test. We also discusses the XHV achievement in our gun test.

Semiconductors obtain new functionality by the incorporation of dilute concentrations of impurities. This principle has been utilized to tailor the properties of narrow gap semiconductors, such as Si or GaAs, but tailoring the properties of wide band gap materials for optical or high power applications has been extremely challenging. As a result, there has been slow progress in identifying new material combinations (bulk material + dilute dopants) to enable novel functionality. To help overcome these obstacles, we have developed and implemented a point defects database in which we store formation, ionization, and optical transition energies determined by hybrid exchange-correlation density functional theory methods. These predictions from first principles are then strongly coupled with synthesis and characterization efforts to identify troublesome point defects and also suggest routes to realizing desired properties if particular defect cannot be removed. The stored data is also used to solve mass balance equations to determine the number of carriers and compensating defects. In this talk, I will present our recent efforts in applying these tools to determine solutions for unwanted optical absorption in AlN grown by PVT. With our data we have demonstrated that substitutional carbon on the nitrogen site is a deep acceptor. When ionized it is the source of unwanted optical absorption. This was confirmed by PL, SIMS, and HVPE. Solution of mass balance equations reveals that the compensating defect is a singly ionized nitrogen vacancy for relevant growth conditions. The presence of this defect has been confirmed by an optical emission predicted to originate from a donor acceptor pair recombination and measured by PLE spectroscopy. When carbon cannot be removed from the growth process, we have used our database to determine mechanisms important to removing unwanted optical absorption in high carbon samples. In total, this approach has accelerated the design of new materials and also led to deeper understanding of the important mechanisms, which will impact future efforts to tailor properties of AlN and its alloys.

The meso-scale materials modeling community, over the last two decades, has developed vast capabilities in microstructurally-based modeling of complex ceramic and metals. While several modeling techniques have been developed, the Potts kinetic Monte Carlo model and the phase-field model form the foundation of most materials modeling efforts for a variety of microstructural evolution processes experienced by ceramics and metals during fabrication and engineering service. Harnessing these modeling capabilities and applying them to design materials to tailor their microstructure for optimal engineering performance properties and designing fabrication processes to obtain the desired microstructure can greatly accelerate development and optimizing of materials for a large number of technologies. This presentation will give a general overview of the core capabilities of the microstructural evolution modeling capabilities by reviewing the two most commonly used methods, Potts and phase-field. The former is a discrete, statistical-mechanical model that has been successfully used to simulate many microstructural evolution processes, such as grain growth, sintering, coarsening in the presence of mobile and immobile pinning phase, and recrystallization. The phase-field model is a continuum, thermodynamic model that has been used to successful simulation solidification, phase transformation and coarsening processes. The capabilities and limitation of each model will be reviewed and appropriate application of the models to different materials microstructural evolution processes will be discussed. The presentation will also demonstrate how these two models can be applied to understand and predict materials processes including coarsening, sintering and phase transformations. Specific examples of microstructural modeling and their application to design materials microstructure for optimal performance will be presented and discussed. Examples will included simulation of microstructural evolution during sintering of complex powder compacts; the generation, transport and release of fission gases from nuclear fuels during service in a reactor; and development of grain shapes and sizes during welding processes. Finally, the current trends in microstructure model development will be discussed.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

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