Surface Resistance Minimization in SRF Cavities by Reduction of Thermocurrents and Trapped Flux

Trapped magnetic flux is known to be a major cause of radio frequency RF dissipation in superconducting RF cavities for particle accelerators. Especially in many new machines, which operate at high field in the continuous wave mode, these additional losses can be unacceptably high, both from an operational and economic point of view. Recent measurements demonstrated that the procedure with which SRF cavities are cooled to the superconducting state dramatically impacts the niobium surface resistance which in turn governs the RF power dissipation. We found a direct correlation between the temperature difference during cooldown and the surface resistance. We believe that thermocurrents generated during the cooldown at the niobium cavity and the titanium tank, which holds the helium, generate a magnetic field. This field is subsequently trapped when the cavity transitions to the superconducting state. To determine the extent of thermocurrents, the thermopowers of niobium and titanium were measured in the temperature range from 10K to 100K. Numerical simulations of the cavity system were performed based on these results. The obtained current distribution was used to estimate the magnetic field at the RF surface of the cavity, which critically depends on the temperature profile of the cavity. Direct measurements of the trapped flux confirmed the simulations and were consistent with the observed increase in surface resistance. The extent to which the magnetic flux is actually trapped also depends on the cooldown conditions. Recent experimental findings, including those of other groups and a theoretical description, were compiled. Two selected topics were addressed by additional measurements. For one, we studied the flux expulsion in a conduction cooled cavity and found that it is favored by a homogenous temperature profile during the superconducting transition. Secondly, we used magneto optical studies to visualize the different shapes of the superconducting phase front during either cooldown or during field penetration. The results provide important starting points for further investigations of flux expulsio