• M. BastaniNejad, A. A. Elmustafa
  Old Dominion University, Norfolk, Virginia
• M. Alsharo'a, P. M. Hanlet, R. P. Johnson, M. Kuchnir, D. J. Newsham
  Muons, Inc, Batavia
• C. M. Ankenbrandt, A. Moretti, M. Popovic, K. Yonehara
  Fermilab, Batavia, Illinois
• D. M. Kaplan
  Illinois Institute of Technology, Chicago, Illinois

Microscopic images of the surfaces of metallic electrodes used in high-pressure gas-filled 800 MHz RF cavity experiments are used to investigate the mechanism of RF breakdown. The images show evidence for melting and boiling in small regions of ~10 micron diameter on tungsten, molybdenum, and beryllium electrode surfaces. In these experiments, the dense hydrogen gas in the cavity prevents electrons or ions from being accelerated to high enough energy to participate in the breakdown process so that the only important variables are the fields and the metallic surfaces. The distributions of breakdown remnants on the electrode surfaces are compared to the maximum surface gradient E predicted by an ANSYS model of the cavity. The surface local density of spark remnants, presumably the probability of breakdown, shows a power law dependence on the maximum gradient, with E¹⁰ for tungsten and molybdenum and E⁷ for beryllium. This is reminiscent of Fowler-Nordheim behavior of electron emission from a cold cathode, which is explained by the quantum-mechanical penetration of a barrier that is characterized by the work function of the metal.

• T. J. Roberts
  Muons, Inc, Batavia
• D. M. Kaplan
  Illinois Institute of Technology, Chicago, Illinois

G4beamline is a single-particle simulation program optimized for the design and evaluation of beam lines. It is based on the Geant4 toolkit, and can implement accurate and realistic simulations of particle transport in both EM fields and in matter. This makes it particularly well suited for studies of muon collider and neutrino factory design concepts. G4beamline includes a rich repertoire of beamline elements and is intended to be used directly without C++ programming by accelerator physicists. The program has been enhanced to handle a larger class of beamline and detector systems, and to run on Linux, Windows, and Macintosh platforms.