Vacuum RF breakdown in normal-conducting structures

6 problems

• Origin of the steep power-law scaling of breakdown rate

  Open

  The chance of a spark per pulse grows roughly as the 30th power of the field and the 5th power of the pulse length; this empirical rule has no derivation.

• Why conditioning progresses with number of RF pulses, not breakdowns

  Open

  New structures reach high fields only after a slow initial period of operation called conditioning. Data suggest the improvement tracks how many pulses were applied, not how many sparks occurred, and the reason is unclear.

• Why cryogenic copper structures sustain much higher accelerating gradients

  Open

  Copper structures cooled to about 45 K have held fields roughly twice as strong as room-temperature ones, and the reason is not settled.

• Microscopic mechanism that initiates vacuum RF breakdown on copper

  Open

  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.

• Which local field quantity determines the breakdown-limited gradient

  Open

  Engineers judge whether a new structure will spark using rules of thumb based on surface field, surface heating or energy flow; it is unknown whether one quantity works for all designs.

• Breakdown limit of RF cavities inside strong solenoid fields

  Partially resolved

  A muon collider needs accelerating cavities placed inside very strong magnets, and such cavities spark at lower fields than without the magnet.