• J. Norem, Z. Insepov, Th. Proslier
  ANL, Argonne
• S. Mahalingam, S.A. Veitzer
  Tech-X, Boulder, Colorado

We are developing a model of arcing to explain breakdown phenomena in high-gradient rf systems used for particle accelerators. This model assumes that arcs develop as a result of mechanical failure of the surface due to electric tensile stress, ionization of fragments by field emission, and the development of a small, dense plasma that interacts with the surface primarily through self sputtering and terminates as a unipolar arc capable of producing field emitters with high enhancement factors. We are modeling these mechanisms using Molecular Dynamics (mechanical failure, Coulomb explosions, self sputtering), Particle-In-Cell (PIC) codes (plasma evolution), mesoscale surface thermodynamics (surface evolution), and finite element electrostatic modeling (field enhancements). We believe this model may be more widely applicable and we are trying to constrain the physical mechanisms using data from tokamak edge plasmas.

• J. Norem, M. Kharitonov, J. Klug, M.J. Pellin, Th. Proslier
  ANL, Argonne
• N. Becker, J. Zasadzinski
  IIT, Chicago, Illinois
• G. Ciovati
  JLAB, Newport News, Virginia
• A.V. Gurevich
  NHMFL, Tallahassee, Florida

An effort, centered at Argonne, has started to explore the use of Atomic Layer Deposition (ALD) to study and improve the performance of superconducting rf (SRF) accelerating structures. This effort has a number of parts: a survey the properties of ALD deposited films, a study of loss mechanisms of SRF structures, and a program of coating single cell cavities, to begin to optimize the performance of complete systems. Early results have included improving the performance of individual structures and, identification of magnetic oxides as a loss mechanism in SRF. We describe the program and summarize recent progress.