Hydrogen isotope trapping on graphite collectors during an isotope exchange experiment in the Tokamak Fusion Test Reactor

Abstract

A rotatable collector probe was used to expose several graphite samples to a deuterium-to-hydrogen-to-deuterium exchange experiment in the Tokamak Fusion Test Reactor (TFTR) at the start of the 1988 operations period. This experiment proved the utility of helium conditioning discharges in accelerating the changeover process. Samples included portions of a tile taken from the inner bumper limiter (POCO AXF-5Q graphite) of TFTR during the recent machine opening, and coupons which had been conditioned in the PJasma Interactive S_urface Component Experimental Sjation (PISCES) by exposure to a helium plasma. The samples were exposed to different groups of the-100 1.4 MA discharges that comprised the experiment. They were removed and analyzed for retained deuterium and impurities by nuclear reaction analysis and Rutherford backscattering spectrometry. Codeposited carbon layers had been formed with thicknesses up to several tenths of a micron. The inferred percentages of trapped hydrogenic species were in general agreement with spectroscopic data. The computed carbon fluence per • + discharge, 1.2 x 10 17 C/cm 2 , is compared to recent measurements on limiter tiles removed from TFTR.