I n s i t u spectroscopic measurements of erosion behavior of Tokamak Fusion Test Reactor redeposited carbon materials under high-flux plasma bombardment in Plasma Interactive Surface Component Experimental Station-A

Abstract

The chemical erosion behavior of graphite materials preexposed in the Tokamak Fusion Test Reactor (TFTR) as the bumper limiter has been investigated spectroscopically under deuterium plasma bombardment in the Plasma Interactive Surface Component Experimental Station-A (PISCES-A) facility. The deuterium plasma bombardment conditions are ion bombarding energy of 300 eV, ion flux of 1.7×1018 ions s−1 cm−2, plasma density of 1.4×1012 cm−3, electron temperature of 11 eV, and neutral pressure of 3×10−4 Torr. The chemical erosion yield is measured with calibrated CD-band spectroscopy during the temperature ramp from 100 to 900 °C at an average rate of ∼5 K/s. The materials used include virgin POCO graphite and graphite tile pieces from the redeposited and eroded areas of the bumper limiter of TFTR. The deuterocarbon formation rate from TFTR redeposits maximizes at ∼500 °C. Essentially the same chemical erosion behavior is observed for TFTR-eroded and virgin graphites and is characterized by the compound peak, indicative of two erosion yield maxima at around 575 and 700 °C. The maximum erosion yield for TFTR redeposits is found to be ∼15% higher than those for TFTR-eroded and virgin POCO graphites, which is attributed to deuterium incorporated in the redeposited material. In addition, the removal behavior of redeposits by helium plasma bombardment has been studied. The removal rate is evaluated to be similar to the physical sputtering yield of carbon by helium. The surface morphology and surface composition have been analyzed with scanning electron microscopy and electron microprobe analysis along with these erosion yield measurements.