A time resolved study of the mitigation of graphite chemical erosion in deuterium plasmas containing beryllium

The interaction of the graphite strike plate material in ITER, with a divertor plasma containing beryllium, is simulated experimentally in PISCES-B. Graphite targets are exposed at 600 K to deuterium plasmas with fixed beryllium fractions (f(Be) = n(Be+)/n(e)) in the range 0.0003 <= f(Be) <= 0.011. From the onset of beryllium injection, the intensity of plasma CD band optical emission caused by target chemical erosion, diminishes exponentially at a rate found to depend on f(Be). The rate of chemical erosion decay, 1/tau(CBe), is found to increase with f(Be) over the range of beryllium fractions explored and can be described by a simple power law expression, 1/tau(fBe) = alpha f(Be)(beta), where alpha = 785 +/- 161 s(-1) and beta = 2.07 +/- 0.10. Evidence of chemical binding between the surface carbon and beryllium is found in XPS analysis of targets following exposure. A reduction in available surface carbon bonds, due to the presence of beryllium, is believed to be responsible for the observed mitigation of chemical erosion.