In-vessel tritium measurements using beta decay in the Tokamak Fusion TestReactor

Tritium on the inside walls of the Tokamak Fusion Test Reactor was detected by configuring the vacuum vessel as an ionization chamber and measuring th e secondary electron current from the tritium beta decay. The vessel was ty pically filled with approximate to 5 Torr of dry nitrogen and the secondary electron current was collected by an internal electrode biased to about +1 5 V with respect to the vessel wall. The measured variations of the collect ed current with gas pressure, bias voltage, and applied magnetic fields are described, as well as an in situ calibration made by injecting a known amo unt of tritium gas. Improved versions of this diagnostic may be useful to t rack the in-vessel content of surface tritium in future fusion devices. (C) 1999 American Institute of Physics. [S003-46748(99)56301-6].