Hello, A new project on the LCRC cluster has been requested. Please forward the information on to the LCRC Allocation sub-committee. Applicant's name: Denise Ford Applicant's institution: ANL Applicant's division: MSD Project Name: SRF-materials Project title: Modeling of Impurity Structures in Niobium Superconducting Radio-Frequency Cavities Associated funding: DOE-HEP Other Systems: ANL CNM - Carbon, FNAL - Wilson Cluster Science: This objective of this project is to model the effects of chemical impurities on the niobium used to construct superconducting radio-frequency (SRF) cavities. Niobium SRF cavities are a key technology for high-performance linear particle accelerators, and will be used in high energy physics applications such as a proposed 8 GeV proton source at Fermilab, and potentially in accelerators for waste management from nuclear power sources, for synchrotron light sources, and for neutron sources. SRF cavity performance is characterized by the accelerating gradient and the quality factor (a measure of the stored energy versus the dissipated power), both of which are highly sensitive to the structure and composition of the top ~100 nm of a cavity’s surface. Absorption of chemical impurities during processing can affect both the maximum accelerating gradient and the quality factor. Two examples, which will be the focus of this project, are (i) the formation of a surface oxide film, approximately 20 nm thick, which is composed of layers of Nb2O5, NbO2, NbO, and a gradient of dissolved oxygen in the niobium bulk; and (ii) the formation of hydride precipitates. The niobium oxides can accommodate substantial off-stoichiometry, which results in some cases in local magnetic moments. This is detrimental for superconductivity because magnetic impurities create normal conducting states within the superconducting energy gap. Hydride precipitation is detrimental because the hydrides with an ordered structure are not superconducting above 2 K. On the other hand, the presence of interstitial oxygen, nitrogen, and carbon may be beneficial to SRF cavity performance at low concentrations. A steep drop-off of the quality factor at the onset of the RF field can be observed in cavities made from high purity niobium, but not typically in cavities from lower purity reactor grade niobium. This is because oxygen, nitrogen, and possibly carbon can trap hydrogen atoms, which prevents their diffusion and precipitate formation. The tetragonal and cubic niobium nitrides also have superconducting transition temperatures that are higher than that of pure niobium, so formation of these phases may potentially benefit niobium SRF cavities. This project will utilize VASP to model the interactions of chemical impurities in niobium and the formation of the phases described above to shed light on processes that may positively or negatively affect the performance of niobium SRF cavities. Project description: VASP will be used to perform density functional theory calculations of defective niobium and niobium oxide structures to assess the effects of impurities on the material. We will model the niobium oxides ranging from NbO - Nb2O5, including ideal structures and plausible defective structures, to determine the magnitude of magnetic impurities that the can be accommodated in the oxide surface layers. We will also model interstitial H, O, C, and N in niobium, with focus on the ability of C and N to trap H and prevent the formation of niobium hydride precipitates. We will also assess the electronic, magnetic, and structural properties of these impurities in niobium in consideration of the side-effects that they may have on SRF cavity performance. The models for these studies will involve several hundred atoms to accurately describe the structures and minimize undesired interactions between periodic images. The models for the oxide studies will require some parameter determination, such as selection of appropriate functionals, where as the models for the hydride studies have been previously established. Peter Zapol will be an additional project member. Project URL: Requested allocation: 200000 Q1: 50000 Q2: 50000 Q3: 50000 Q4: 50000 Justification: The requester has used undetermined amount hours of their initial startup project. In addition to approving an initial amount, please specify a Category and Subcategory for this project. For a list of the current selection of approved categories, please see: https://wiki.lcrc.anl.gov/wiki/Processes/Categories Once the Allocation committee has approved the project, please go to the Project Management page to create it: https://accounts.lcrc.anl.gov/projects.php Thank You, The LCRC Accounts System