Improved oxygen diffusion model to explain the effect of low-temperature baking on high field losses in niobium superconducting cavities

Radio-frequency (rf) superconducting cavities made of high purity niobium are widely used to accelerate charged particle beams in particle accelerators. The major limitation to achieve rf field values approaching the theoretical limit for niobium is represented by “anomalous” losses which degrade the quality factor of the cavities starting at peak surface magnetic fields of about 100mT, in the absence of field emission. These high field losses are often referred to as Q drop. It has been observed that the Q drop is drastically reduced by baking the cavities at 120°C for about 48h under ultrahigh vacuum. An improved oxygen diffusion model for the niobium-oxide system is proposed to explain the benefit of the low-temperature baking on the Q drop in niobium superconducting rf cavities. The model shows that baking at 120°C for 48h allows oxygen to diffuse away from the surface, and therefore increasing the lower critical field towards the value for pure niobium.