Diagnostic Experience during Deuterium-Tritium Experiments in JET, Techniques and Measurements

During 1997 JET was operated for an extensive period using a D-T mixture (DTE1). The use of tritium in the machine required a number of changes in the design and operation of diagnostic systems. All diagnostics with a direct vacuum interface with the main vessel have been fitted with tritium compatible pumps and their exhaust gases have been re-routed to the Active Gas Handling Plant. All items on the torus which could lead to a significant leak in the event of failure, were required to have double containment. Therefore, all windows, and a majority of bellows and feedthroughs, were designed and installed with a double barrier, making a total of some 330 interspaces. Extra shielding was installed on a number of diagnostics to deal with the higher neutron fluxes during DTE1. A radiation-hardened video camera was installed to monitor the plasma during the DTE1 discharges. A total of 15 heated fibre hoses were installed to transmit the plasma light beyond the biological shield for spectroscopic purposes. Blind fibres and fibre loops were also installed to study the effects of higher neutron fluxes on these fibres and to compensate for luminescence effects. During the DTE1 experiments the tritium fraction was measured at the edge and in the core using several diagnostic methods. High resolution Balmer α line spectroscopy gave a measurement typical of the plasma edge region. In the JET sub-divertor volume the tritium concentration of the neutral gas was measured using Balmer α spectroscopy of a Penning discharge. Using Neutral Particle Analysis by electrostatic deflection and time of flight, the tritium concentration was measured typically in a zone 20 to 40cm from the plasma edge. Local measurements of the tritium fraction have been made using active Balmer α charge exchange spectroscopy. The error on this local measurement is, however, large, ~30%. After the discharge the tritium fraction of the exhaust was measured using the exhaust monitoring system. Using short deuterium neutral injection pulses allowed neutron rate measurements of the tritium concentration in the core region. A further technique used the measured neutron rate and calculated neutron rate from other plasma parameters to determine the tritium concentration.