EROSION BEHAVIOR AND SURFACE CHARACTERIZATION OF BERYLLIUM UNDER HIGH-FLUX DEUTERIUM PLASMA BOMBARDMENT

The erosion of beryllium subject to a high-flux deuterium plasma is st udied, including post-bombardment surface characterization. Simulating divertor plasma conditions of the ITER tokamak, the PISCES-B steady-s tate plasma facility at U.C., San Diego is used to bombard beryllium s pecimens. The measured sputtering yield is compared with theoretical M onte 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 degrees C less than or equal t o T less than or equal to 720 degrees 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 exte nded plasma exposure but appears to be continuously regenerated, The p resence of this film reduces the apparent sputtering yield of berylliu m by up to two orders of magnitude. An in-situ emission spectroscopy t echnique is used to confirm the effect of deposited contaminants. Dept h-profile Auger electron spectroscopy (AES) data shows that relative c oncentration of carbon, nitrogen, and oxygen in the thin film is each in the range, 10 to 20%, X-ray photoelectron spectroscopy (XPS) data i ndicates the formation of BeO and a small amount of Be2C on the surfac e. X-ray diffraction (XRD) patterns obtained with a small angle of inc idence show no evidence for crystalline Be2C and suggest that the impu rity layer may have a disordered or amorphous structure. Importantly, no impurity deposition is observed when the specimen is maintained at room temperature (similar to 40 degrees C). Application of energy disp ersive X-ray spectroscopy (EDX) confirms that the low temperature surf ace is free of impurities except for a trace amount of oxygen. The spu ttering yield measured at room temperature agrees with theoretical val ues within a factor of two.