Magnetization and susceptibility measurements on niobium samples for cavity production

The high design gradient of 25 MV/m in the TESLA cavities puts demanding requirements on the quality of the niobium sheets used for cavity production and also on the cavity preparation steps. Part of the quality control procedures are magnetization and AC susceptibility measurements on niobium samples which accompany the cavities in all stages of chemical etching and heat treatment. An apparatus has been built that permits magnetization hysteresis measurements at 4.2 K in a DC field of up to 0.5 T parallel to the niobium surface, using the "moving sample" technique. In the same cryostat the attenuation of a small AC magnetic field (amplitude typically 0.01-0.3 mT, frequency around 175 Hz, orientation perpendicular to the niobium surface) can be determined as a function of a DC magnetic field of up to 2 T. The first experiment yields the amount of magnetic flux pinning, mainly in the bulk niobium, while the second is a sensitive measure of the upper critical magnetic field at the RF surface of the niobium sample. Experimental results are presented on niobium samples from two manufacturers at different steps of thermal and chemical treatment.