Magnetization and susceptibility measurements on niobium samples for cavity production

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