  We describe breakdown in 805 MHz rf accelerator cavities in terms of a number
of self consistent mechanisms. We divide the breakdown process into three
stages: 1) we model surface failure using molecular dynamics of fracture caused
by electrostatic tensile stress, 2) we model the ionization of neutrals
responsible for plasma initiation and plasma growth using a particle in cell
code, and 3) we model surface damage by assuming a process similar to unipolar
arcing. We find that the cold, dense plasma in contact with the surface
produces very small Debye lengths and very high electric fields over a large
area, consistent with unipolar arc behavior, although unipolar arcs are
strictly defined with equipotential boundaries. These high fields produce
strong erosion mechanisms, primarily self sputtering, compatible with the
crater formation that we see. We use OOPIC modeling to estimate very high
surface electric fields in the dense plasma and measure these field using
electrohydrodynamic arguments to relate the dimensions of surface damage with
the applied electric field. We also present a geometrical explanation of the
large enhancement factors of field emitters.This is consistent with the
apparent absence of whiskers on surfaces exposed to high fields. The
enhancement factors we derive, when combined with the Fowler-Nordheim analysis
produce a consistent picture of breakdown and field emission from surfaces at
local fields of 7 - 10 GV/m. We show that the plasma growth rates we obtain
from OOPIC are consistent with growth rates of the cavity shorting currents
using x ray measurements. We believe the general picture presented here for rf
breakdown arcs should be directly applicable to a larger class of vacuum arcs.
Results from the plasma simulation are included as a guide to experimental
verification of this model.
