We investigated the surface resistance of fully dressed TESLA-type SRF cavities that were cooled down to the operating temperature (1.8 K) under different conditions, in particular different temperature distributions along the cavity axis during superconducting phase transition. All tests were performed in the HoBiCaT horizontal test facility setup with double magnetic shielding (one cold and one warm) as shown in Figure 1.
A TTF3-coupler (coupling values between 1 and 2) was employed. The temperature of the cavity was monitored by three Cernox sensors. Figure 2 indicates the positions at which the sensors were located.
Figure 3 shows the obtained surface resistance of one TESLA cavity that was cooled through the transition temperature under different conditions (different temperature gradients). We believe that not only do the cooldown conditions impact the level to which external magnetic flux is trapped in the cavity but also that thermoelectric currents are generated between the cavity that is fabricated from niobium and the helium tank that is fabricated from titanium which in turn create additional flux that can be trapped.
Figure 4 shows the correlation of temperature difference along the cavity at the instance of phase transition and the obtained residual surface resistance. These measurements are a strong indication for the existence of a thermoelectric contribution to trapped flux in SRF cavities.
Therefore, we investigated the generation of flux and the dynamics of flux trapping and release in a simple model niobium-titanium system that mimics an SRF cavity in its helium tank. We indeed found that thermal gradients along the system during the superconducting transition can generate a thermoelectric current and magnetic flux that subsequently can be trapped. Furthermore, increased flux mobility in the superconducting state could be demonstrated in the temperature range between 9.08K up to Tc which impacts the level of trapped flux. These effects may explain the observed variation of the cavity’s Rres with cooldown conditions. “Read more here.”