Apparatus for measuring RRR

The evaluation of purity and thermal conductivity at liquid Helium temperatures of the Niobium to be used in the fabrication of superconducting RF cavities is normally done by measuring the Residual Resistivity Ratio (RRR) of a sample of the material. The relationship between the thermal and the electrical conductivity (Wiedemann-Franz Law) simplifies the task by leading to the measurement of electrical instead of thermal resistance. The RRR is the ratio between the resistances of the sample at room temperature and at the operating temperature of the cavity. A more precise definition is discussed later. The conductivity at low temperatures depends on lattice defects and impurities. Impurities are also important for cavities in a direct way as affecting the RF properties of its surface when exposed by chemical etching. The following describes the experimental apparatus for RRR measurements developed at Fermilab's Beams Division. Part 2 contains a description of the sample-holder and measurement hardware. Part 3 contains a discussion on definition, measurements and errors. Part 4 gives a step-by-step description of the measurement procedure. Finally, Part 5 gives an example of results obtained recently on a Niobium sample for CKM cavities.