  The MuCool Test Area (MTA) at Fermilab is a facility to develop the
technology required for ionization cooling for a future Muon Collider and/or
Neutrino Factory. As part of this research program, we have tested two 805 MHz
vacuum RF cavities in a multi-Tesla magnetic field to study the effects of the
static magnetic field on the cavity operation. This study gives useful
information on field emitters in the cavity, dark current, surface
conditioning, breakdown mechanisms and material properties of the cavity. All
these factors determine the maximum accelerating gradient in the cavity. This
paper discusses the image processing technique for quantitative estimation of
spark damage spot distribution on cavity interior surfaces. The distribution is
compared with the electric field distribution predicted by a computer code
calculation. The local spark density is proportional to probability of surface
breakdown and shows a power law dependence on the maximum electric field (E).
This E dependence is consistent with the dark current calculated from the
Fowler-Nordheim equation.
