Experimental studies of radiation resistance of boron nitride, C2C ceramics Al_2O_3 and carbon-fiber composites using a PF-1000 plasma-focus device

摘要

This paper reports on experiments undertaken to compare the radiation resistance of two types of ceramics, boron nitride (BN) and pure alumina (Al_2O_3), which are used in a TAEA antenna coil installed in the MAST spherical tokamak. Samples of the investigated materials (bulk BN and a 20 μm film of Al_2O_3 on Al substrate) were exposed on the axis of the plasma-focus PF-1000 device, which can emit intense streams of hot plasma (v≈10~7 cm s~(-1) and N_(pl)≈ 10~(18) cm~(-3)) and fast deuteron beams (E_i≈100 keV). The most powerful plasma-ion pulse lasted 0.2-1.0 μs and its intensity decayed in about 100 μs. The irradiation process was diagnosed using fast optical cameras, laser interferometry and optical spectrometry. Experiments were performed at power flux densities equal to 10~9-10~(10) W cm~(-2) or 10~8-10~9 W cm~(-2) during the most powerful stage of the interaction process. The irradiated specimens were investigated by means of optical microscopy and x-ray structure analysis (XRSA). It was shown that at 1010 W cm~(-2) pulses the Al _2O_3 coating was completely evaporated, whereas a surface of the BN sample became smoother than in the virgin one. A direct comparison of both samples after the action of 10~8 W cm~(-2) pulses demonstrated a wave-like structure (more distinct on Al_2O _3). Weighing of these samples showed, however, that the evaporation of BN was about two times stronger than that of Al_2O_3 in spite of the lower irradiation flux; the XRSA showed no evidence of cracking of Al_2O_3 after these pulses. The insulation properties of Al_2O_3 did not decline, and the Al_2O_3 coating may be potentially more beneficial, provided that it is kept below its melting point. Characteristic features of damages of a material based on the carbon-fiber composite with additions of silicium carbide (SiC; 8-40% volumetric) were also investigated. It was found that at q=10~9 W cm~(-2), the surface erosion is associated with sputtering and evaporation. The degree of this erosion depends on the fibers' orientation in relation to the direction of the plasma-ion streams, and on the percentage of the SiC admixture.