Operation of the JET Neutral Beam Injectors with tritium is described. Supplying the tritium feed via the special electrically grounded gas feed compromised the performance of the up-graded high current triode Positive Ion Neutral Injectors (PINI) due to gas starvation of the source and the methods adopted to ameliorate this effect are described. A total of 362 PINI beam pulses were requested, circulating a total of 4.73g tritium through the PINI’s, of which 9.3mg was injected into the torus. Safety considerations required a continuous, cumulative total to be maintained of the mass of tritium adsorbed onto the cryo- pumping panel: a daily limit of 0.5g was adopted for the Trace Tritium Experiment (TTE). A subsequent clean up phase using 115keV deuterium beams completed the isotopic exchange of components in the beamline. I. THE JET NEUTRAL BEAM INJECTION SYSTEM The JET NBI system consists of two Neutral Injection Boxes (NIBs) each equipped with up to eight Positive Ion Neutral Injectors (PINIs) [1]. The PINIs on the NIB located at machine octant 4 operate at 80kV/56A in deuterium, whilst the PINIs located on octant 8 (NIB 8) were recently upgraded to operate at 130kV/60A in deuterium [2, 3] compared with 140kV/30A previously. An illustration of a JET NIB is shown in Fig.1; the eight PINIs are mounted in two vertical banks of four. Each set of four is further divided into two pairs, each pair sharing a common deflection magnet. The ion beam extracted from the source is passed through a gas neutraliser and the subsequent mixed beam of ions and neutral particles passes through a deflection magnet. The magnet removes the unwanted ionic beam component and injection into the neutral beam either passes into the torus via the duct or is collected on the calorimeter. For the JET Trace Tritium Experiment (TTE) [4] only two PINIs on NIB 8 were required to operate in tritium, the remaining six PINIs operating simultaneously in deuterium. The tritium gas feed is supplied to each pair of PINIs from the central Active Gas Handling System (AGHS) [5] by the Tritium-Deuterium Gas Introduction System (TDGIS) [6]. The gas is then delivered to an individual PINI via a single inlet in the ground-potential accelerating grid holder, avoiding the use of the frangible glass insulating break in the conventional gas feed. The TDGIS, its transfer lines and the AGHS share a common secondary containment envelope and thus the deuterium feed to the remaining six NIB8 PINIs is also supplied via the TDGIS. The single gas feed to each PINI must supply gas both to the ion source and the neutraliser and the consequent reduction in source pressure created additional operational problems as discussed in Section IIA. The TDGIS was also used to maintain a continuous log of the amount of tritium delivered to the NIB, from which the mass of tritium condensed on the cryopump panels was obtained as described in Section III. Safety considerations imposed a daily maximum tritium inventory of 0.5g on the cryo-panel [4] and this too had consequences for the operation and commissioning of the tritium PINIs as described in Section II. II. OPERATIONAL EXPERIENCE II.A. Commissioning The PINIs can be commissioned independently of JET operations in the asynchronous (Async) mode. In this case the two gates of the calorimeter (C in Fig.1) are opened to NEUTRAL BEAM INJECTION IN THE JET TRACE TRITIUM EXPERIMENT E. Surrey, D. Ciric, S.J. Cox, L. Hackett, D. Homfray, I. Jenkins, T.T. C. Jones, D. Keeling, R. King, A. Whitehead and D. Young EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, Oxfordshire OX14 3DB, UK Figure 1: Illustration of a JET Neutral Injection Box Cryopumps SF6 Tower NIB JG04.411/1c B D C M Tritium gas introduction lines B : Box Scrapers D : Ion Dump C : Calorimeter M : Deflection Magnets PINI Ion Source / Accelerator