Can surface cracks and unipolar arcs explain breakdown and gradient   limits?
  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. 
Jim Norem, Zeke Insepov
physics.acc-ph