Depth profiling a near surface carbon contaminant in implanted first wall materials

Current attempts to generate stable plasmas in CTR devices are encountering severe material problems. Plasma species, interacting with the surface of the containment vessel, can desorb surface species which can contaminate the plasma or, interacting with surface impurities, can change the properties of the near surface region. Although methods to minimize these synergistic effects have been suggested, there exists only minimal information related to the fundamental surface and near surface (< 100 Å) processes involved. One process of interest, the interaction of energetic plasma species with adsorbed hydrocarbon contaminants has been examined using Imaging, Field-Desorption Mass Spectrometry. Angstrom resolved depth profiles of a carbon contaminant produced by ion-beam decomposition of an adsorbed surface hydrocarbon layer have been measured for tungsten, molybdenum and stainless steel specimens. The observed, 30 Angstrom penetration of carbon into the near surface region suggests that conventional first-wall cleaning techniques may be ineffective in completely removing this impurity.