Niobium sample results
The improvement of the cavity quality factor by thermal cycling is highly promising for the cost reduction of SRF accelerators. We think that the effect is related to a variation in the amount of trapped magnetic flux inside the superconducting niobium that depends on the cool-down conditions. Hence we perform experiments on niobium samples. The experimental setup is shown in Figure 1.So far, we found two correlations of cooling parameters and flux trapping behavior in a niobium rod sample:
Figure 1: Experimental setup cavity-tank “model” and position of instruments: A niobium rod with square cross section (8.4x8.4x300mm3) is shorted out with a titanium rod. The system is conduction cooled through the posttest and can be heated with two resistive heaters (one on each end). The temperature distribution along the rod was monitored using Cernox temperature sensors and the magnetic field inside the rod was measured with a 3D fluxgate magnetometer.
1) A temperature gradient along the material during phase transition can produce magnetic field by thermoelectric effect and thereby increase the amount of trapped flux in the superconducting material (in equilibrium). The acquired data is shown in Figure 2.
2) The duration of the (isothermal) phase transition influences the amount of expelled and trapped flux (Figure 3). The slower the material is cooled through the transition temperature, the more flux gets expelled and the less flux remains trapped in the niobium. Read more: IPAC2013