Surface analysis of field-ion samples exposed to the plasma of the impurities studies experiment (ISX-A) tokamak

Abstract

Measurements of the modifications to the surface of field-emitter samples (tips) exposed to plasma discharges in the impurities studies experiment (ISX-A) tokamak have been carried out for the first time in order to help further identify the physical processes occurring at the tokamak wall during machine operation. The specimen tips were exposed to 40 high-power hydrogen discharges in ISX-A at a position 5.0 cm back from the plasma edge defined by carbon and molybdenum limiters. In contrast to results obtained on silicon samples exposed at a similar location in ISX-A using Rutherford ion backscattering analysis, field-ion images of tungsten tips taken before and after exposure indicate that no structural rearrangement of the specimen near-surface region occurred, even on an atomic scale. Contaminant films 60±10 Å thick, deposited on the specimen surfaces during exposure, were observed in the transmission electron microscope. Control tips, placed in the tokamak during the discharges but shielded from direct plasma exposure, remained free of such deposits. Imaging atom-probe depth profiles into the exposed sample substrates revealed that neither plasma nor impurity species were implanted beyond the deposited surface films. The results from this study are compared with those from earlier analyses of samples exposed in the Princeton large torus (PLT) using similar techniques and also with the results of analyses of macroscopic samples exposed at the same ISX-A location using other surface-sensitive techniques.