Deuterium retention in carbon flakes and tungsten–carbon mixed flakes produced by deuterium arc discharge

Abstract

In order to estimate the in-vessel tritium inventory of carbon flakes or dust, deuterium gas absorption and deuterium ion irradiation experiments were conducted on carbon flakes prepared by using electron beam evaporation of graphite. The retained amount of deuterium after the deuterium gas absorption was very low; D/C = 10−3 atomic ratio. The retained amount of deuterium after the deuterium ion irradiation was very similar when compared with that for graphite. The deuterium concentration of the carbon flakes resulting from ion irradiation becomes close to zero if the wall temperature is higher than 1000 K. Co-deposited carbon flakes were prepared by a deuterium arc discharge with carbon electrodes at different gas pressures and substrate temperatures. The co-deposited carbon flakes had a high deuterium concentration. Under such conditions with gas pressure of 1 Pa and wall temperature of 573 K, the deuterium concentration becomes approximately D/C = 0.2. In a DT fusion device like ITER, we therefore, expect a T/C ratio of 0.1. This value is presumed to be an upper limit of carbon flakes in ITER since the wall temperature may be higher than 573 K. Tungsten–carbon mixed flakes were also produced by the deuterium arc discharge with carbon and tungsten electrodes. The deuterium was trapped mainly in the carbon atoms and the structure of carbon in the tungsten–carbon mixed flakes was more amorphous compared with that of the co-deposited carbon flakes. Then, the deuterium concentration was higher than that of the carbon flakes, and increased with the increase in the concentration of tungsten. The higher concentration in the tungsten–carbon mixed flakes may not affect the in-vessel tritium inventory if the tungsten concentration is low.