Field and loss limits of superconducting RF cavities

6 problems

• Origin of the anti-Q-slope in nitrogen-doped niobium

  Contested

  When niobium is lightly doped with nitrogen, its energy loss falls as the field rises, the opposite of the usual behavior.

• Mechanism of the high-field Q-slope and its cure by baking

  Contested

  Above a certain field, niobium cavities start wasting far more energy; a two-day bake at 120 C removes the effect, and why it works is still debated.

• Why Nb3Sn cavities quench far below their predicted field

  Open

  Nb3Sn should hold about twice the field of niobium, but real cavities fail at roughly half of niobium's best.

• What sets the residual surface resistance floor of niobium cavities

  Open

  Even far below the superconducting transition, cavities keep a small leftover energy loss whose sources are only partly identified.

• Can multilayer coatings raise the RF field above niobium's limit

  Open

  A superconductor can carry an RF magnetic field only up to a maximum (the superheating field); thin layered coatings might push past niobium's value, but this has not been shown in a working cavity.

• Origin of the medium-field Q-slope in niobium-on-copper film cavities

  Contested

  Cavities made by coating copper with a thin niobium film are cheaper and better cooled than solid niobium, but their energy loss rises steeply with field, for reasons still argued.