Microscopic mechanism that initiates vacuum RF breakdown on copper
In plain words
Breakdowns start at small spots on a polished copper surface, but which process lights the spark (electrons pulled out by the field, crystal defects moving to the surface, local melting) is not agreed.
Precise statement
In Cu accelerating structures at 3-30 GHz, breakdown occurs at surface fields $eE_{s} \sim 2-3\,\mathrm{MeV}/\mathrm{cm}$, and Fowler-Nordheim fits of dark current give field-enhancement factors $\beta \sim 30-100$ although no protrusions of matching height are observed. Identify the sequence of processes (dislocation-driven surface roughening, field-emission heating of emitters, neutral evaporation, plasma ignition) that turns an intact surface into an arc, with a model that predicts the threshold field and its dependence on hardness and crystal structure of the metal.
What would settle it
In-situ imaging of breakdown sites before and after the event together with a model that reproduces measured thresholds for at least two metals of different crystal structure.
Related problems
- More general than Origin of the steep power-law scaling of breakdown rate
- More general than Why cryogenic copper structures sustain much higher accelerating gradients