Helium leak detection on fusion devices such as JET is hampered by the presence of a high deuterium partial pressure emanating mainly from the loaded tiles of the first wall. At JET selective pumping methods have been developed to enable the detection of very small leaks. A novel reference leak system has also been designed to provide for the calibration of the leak detection system. Leak detection is performed by backing a 3500 l/s turbomolecular pump with several pumping systems placed in series: E ΒΈN cooled trap -to pump all condensable gases. E Molecular drag pump -to enhance gas flow through the getter pump. E JEΒΉ designed non-evaporable getter pump -to pump hydrogen and its isotopes. E Helium mass spectrometer leak detector.
A key to the success of the system is the unique design of the JET getter pump which, with small volume, low conductance and operated at elevated pressure (&10 mbar) ensures maximum absorption of the unwanted hydrogen species, in particular Deuterium. The design of the novel leak detection components is outlined. Details of results and experience are given, which show that leak detection sensitivity in the 10\ mbar l s\ range can now be achieved in the presence of high deuterium partial pressure.