Graphite, a material for collector probes

The applicability of graphite as a material for collector probes in plasma experiments was investigated. Four different types of graphite (vitreous, pyrolytic, fine-grain graphite, and a graphite foil) were studied, and compared with a silicon single crystal for the sake of reference. Bulk and surface impurities of the samples could be reduced by heating. As collector probes exposed to a plasma may receive high heatloads, the thermal behavior of metal layers deposited on the surface was studied. Heating of such samples was found to lead to the formation of droplets of significant size, suggesting that quantitative probe analysis may often require a technique sensitive to relatively large depths. Thus, a comparison was made between PIXE and RBS analysis of collector probes. Finally, from the material point of view the problem of converting measured amounts of deposits into actual fluxes incident on to the probe surface was discussed.