Growth of redeposited carbon and its impact on isotope retention properties on tungsten in a high flux deuterium plasma

Abstract

Experiments were performed in a linear magnetized plasma facility (PISCES-B) to simulate carbon re-deposition in the divertor of a fusion reactor such as the International Thermonuclear Experimental Reactor (ITER). The average ion energies are about 100 eV and the ion flux is 2 × 10 22 m −2 s −1. Tungsten discs and foils were exposed to deuterium plasma for a period of 45–120 min at various substrate temperatures. Carbon impurities were introduced either using graphite sample holder or downstream CD 4 puffing near the targets. In-situ XPS measurements showed a shifting of the binding energy of the carbon in the interlayer between the carbon film and the tungsten surface. Based on AES depth profile, the ratio of tungsten to carbon in the interlayer is about 1.9 : 1. Scanning electron microscopy of plasma-exposed tungsten revealed bubble and pits formation on uncontaminated surfaces. Raman measurements on deposited carbon films were also performed. Deuterium retention measurements were done using Thermal Desorption Spectrometry (TDS). The dominant factor that influences the hydrogen isotope retention is substrate temperature. Measurements indicated a transition from D 2 dominant retention at low temperature exposure to D dominant retention at high temperature exposure. Carbon-contaminated tungsten substrates also showed similar, but a moderate, transition. Total deuterium retention decreases as the exposed temperature increases. The threshold of the carbon impurity concentration in the plasma, under which carbon starts to be deposited on the tungsten surface, is about 0.75% at 850 K. Keeping the impurity concentration at 1%, the temperature threshold is about 750 K.