  We argue that the physics of unipolar arcs and surface cracks can help
understand rf breakdown, and vacuum arc data. We outline a model of the basic
mechanisms involved in breakdown and explore how the physics of unipolar arcs
and cracks can simplify the picture of breakdown and gradient limits in
accelerators, tokamaks as well as laser ablation, micrometeorites and other
applications. Cracks are commonly seen in SEM images of arc damage and they are
produced as the liquid metal cools, they produce the required field
enhancements to explain field emission data data and can produce fractures that
would trigger breakdown events. Unipolar arcs can produce currents sufficient
to short out rf structures, should cause the sort of damage seen in SEM images,
should be unstable and possibly self-quenching as seen in optical fluctuations
and surface damage.
