BULK RETENTION OF DEUTERIUM IN GRAPHITES EXPOSED TO DEUTERIUM PLASMA AT HIGH-TEMPERATURE

A highly ionized deuterium plasma with a low residual gas pressure and a high intensity D-2(+)-ion beam were used for the study of deuterium retention in RG-Ti-91 and POCO AXF-5Q graphites. Deuterium retention in the samples was estimated by TDS during heating to 2000 K. Mechanic al removal of a surface layer 100 mu m thick was used to distinguish b ulk and surface fractions of retained deuterium. The samples of RG-Ti and POCO graphites were exposed to a plasma with an ion flux of 3 x 10 (17) D/cm(2) . s in the 'Lenta' plasma device for 10 to 10(4) s at res idual deuterium pressure of 0.04 Pa at 1400 K. Under plasma exposure d euterium capture in RG-Ti graphite reached the saturation level at a f luence of 4 x 10(20) D/cm(2) while the bulk inventory was negligible. As for POCO graphite, deuterium retention increased with fluence and w as equal to 18 appm in the bulk for a fluence of 7 x 10(21) D/cm(2). T he same amount of deuterium in the bulk was obtained after gas exposur e of POCO at an effective pressure of 0.8 Pa (1400 K, 6 h). With this result, the tritium concentration in the plasma-facing graphite materi als can reach 1500 appm or 380 grams of tritium per ton of graphite. T o understand the role of ion flux in generation of effective pressure, POCO was irradiated with 16 keV D-2(+)-ions at 1400 K for 4 h to 8 x 10(20) D/cm(2) (ion flux was 6 x 10(16) D/cm(2) . s, residual deuteriu m pressure was 0.004 Pa). The results are discussed on the basis of st ructural differences for POCO and RG-Ti graphites.