Studies of a-C: D film inhibition by nitrogen injection in laboratory plasmas and divertors

The injection of nitrogen, as well as other species generally considered as scavengers of the film precursors, in carbon deposition plasmas can completely inhibit the film formation under given plasma conditions, as it has been extensively shown in technical plasmas. Experiments aimed at using this technique to the suppression of tritium/carbon co-deposits in fusion devices have been initiated in several devices. To date, a moderate inhibition (∼40%) of carbon deposition efficiency has been detected in nitrogen-seeded Elmy plasmas at JET. In addition to this, the divertor plasma simulator, Pisces-B, has been used to benchmark the concept under divertor conditions, while maintaining a better control of the experimental parameters. In the present work a review of the state of the art is given. The results from the available experiments are critically analysed in view of the present understanding of the re-deposited film formation in divertors.