Erosion behavior and surface characterization of beryllium under high-flux deuterium plasma bombardment

Abstract

The erosion of beryllium subject to a high-flux deuterium plasma is studied, including post-bombardment surface characterization. Simulating divertor plasma conditions of the ITER tokamak, the PISCES-B steady-state plasma facility at U.C., San Diego is used to bombard beryllium specimens. The measured sputtering yield is compared with theoretical Monte Carlo calculations using the TRIM.SP code, and with experimental values obtained by others using an ion beam facility. It is found that at elevated specimen temperatures (250° C ≤ T ≤ 720° C), low atomic number impurities of the plasma, such as carbon, nitrogen, and oxygen, form an impurity layer on the beryllium surface. This layer is not eroded away by extended plasma exposure but appears to be continuously regenerated. The presence of this film reduces the apparent sputtering yield of beryllium by up to two orders of magnitude. An in-situ emission spectroscopy technique is used to confirm the effect of deposited contaminants. Depth-profile Auger electron spectroscopy (AES) data shows that relative concentration of carbon, nitrogen, and oxygen in the thin film is each in the range, 10 to 20%. X-ray photoelectron spectroscopy (XPS) data indicates the formation of BeO and a small amount of Be 2C on the surface. X-ray diffraction (XRD) patterns obtained with a small angle of incidence show no evidence for crystalline Be 2C and suggest that the impurity layer may have a disordered or amorphous structure. Importantly, no impurity deposition is observed when the specimen is maintained at room temperature (∼ 40° C). Application of energy dispersive X-ray spectroscopy (EDX) confirms that the low temperature surface is free of impurities except for a trace amount of oxygen. The sputtering yield measured at room temperature agrees with theoretical values within a factor of two.