The erosion of metallic materials exposed to a high heat flux pulsed plasma is under investigation at NCSU. Component erosion under these conditions is a concern in tokamaks and electric launch devices. A tokamak disruption may impart 10 to 100 MJ/m² or higher to the first wall in 0.1 to 1 ms. Electrically driven launchers can impart between 10 to 12 MJ/m² in 0.1 to 5 ms. These fluxes are sufficient to cause simultaneous vaporization and liquefaction of the exposed surface. Erosion occurs through the loss of this material, which leads to a limitation of component lifetime.

Loss of the melt-layer will significantly enhance the net erosion. Losses occur through many mechanisms. Two-fluid hydrodynamic instabilities, such as Rayleigh-Taylor and Kelvin-Helmholtz, may exist between the melt-layer and the impinging plasma. Lorentz forces may act upon the melt layer as well. For longer pulse lengths (> 1 ms), gravitational runoff may occur as well. These forces will act to remove the melt-layer. Experiments are conducted to measure the thickness of the resolidified melt-layer following pulsed plasma exposure. These results will be used to investigate the role of the melt-layer on erosion damage to metallic plasma facing components.

The electro-thermal gun PIPE is used for the experiments. PIPE produces a high density (1025 to 1026 1/m³) low temperature (1 to 3 eV) plasma for pulse lengths greater than 100 ms. The samples are cylinders, 4.8 mm in diameter and 12.7 mm in length. Materials currently under investigation are 306 SS, OFHC, and Al. The samples are placed 6.35 mm from the end of the ET barrel. In this configuration the sample diameter is smaller than the plasma diameter. The samples are instrumented with two thermocouples, which are used to measure the energy absorbed by the sample.

Following exposure, the samples are bisected axially, polished, and etched for microscopy. The resolidified material has a different grain structure than the unaffected material. This visual difference allows the thickness of the resolidified material to be measured. Preliminary results show a variation in the resolidified thickness and melt-front depth across the diameter of the sample. Resolidified thickness varies from less than 10 mm to greater than 100 mm.

Work supported by the US Army Research Office contract DAAH04-95-1-0214.