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 expulsion.