• J. Norem
  ANL, Argonne, Illinois
• A. Bross, A. Moretti, Z. Qian
  Fermilab, Batavia, Illinois
• D. Huang, Y. Torun
  IIT, Chicago, Illinois
• D. Li, M. S. Zisman
  LBNL, Berkeley, California
• R. A. Rimmer
  Jefferson Lab, Newport News, Virginia

The MuCool Experiment has been continuing to take data with 805 and 201 MHz cavities in the MuCool Test Area. The system uses rf power sources from the Fermilab Linac. Although the experimental program is primarily aimed at the Muon Ionization Cooling Experiment (MICE), we have been studying the dependence of rf limits on frequency, cavity material, high magnetic fields, gas pressure, coatings, etc. with the general aim of understanding the basic mechanisms involved. The 201 MHz cavity, essentially a prototype for the MICE experiment, was made using cleaning techniques similar to those employed for superconducting cavities and operates at its design field with very little conditioning.

• 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.