High-sensitivity leak testing by means of probe gas SF{sub}6 and laser gas detection

摘要

At the Institute of Technical Physics of the Research Center Karlsruhe, superconducting magnets for fusion reactors are tested in the test tank TOSKA since more than 20 years. A crucial test parameter of the magnets weighing up to 120 metric tons is the tightness at operating conditions down to 2 K temperature and max. 25 bars He-refrigerant pressure. Because the final tests at cryo-temperature are costly and time-consuming, intermediate tests at room temperature are already performed at the stages of manufacturing, certification, assembling, and before cooling-down. So far, these tightness tests by the sniffer-method have been performed with probe gas Helium employing a conventional mass spectrometer leak detector. The achievable smallest detectable leakage is limited by the He partial pressure in ambient air. Therefore, a new technique of leak measurement was investigated using the probe gas sulphur hexafluoride SF{sub}6 which does not occur in natural air and employing a laser-acoustical gas detection. The experimental results reveal substantial advantages of the new method with respect to detection sensitivity, testing expenditure, and costs. The results can be transferred to tightness tests in other fields of technology.