Asymmetric surface recombination of hydrogen on palladium exposed to plasma

Recombination coefficient of deuterium on either side of a palladium membrane was experimentally studied. Under conditions that one side of the membrane was continuously exposed to a deuterium plasma and the permeation flux was monitored on the other side, two experiments were conducted. One was to observe the transient behavior of the permeation when the incident flux from the plasma was quickly changed and the other was to observe the deuterium concentration by the nuclear reaction analysis. The permeation was limited by the second-order kinetics with respect to deuterium concentration, and the recombination coefficient ku on the plasma-facing side was expressed by ku=1.5×10−27exp(−0.48 eV/kT) m4 s−1, which is explained by the Pick’s model. The recombination coefficient kd on the downstream side was the same as ku at lower temperatures but showed different temperature dependence above 400 K. This can also be explained by the model but another process of the second-order kinetics may be present.