Research and development on selecting first wall material of JT-60.

Abstract

Research and development (R&D) on the selection of molybdenum first wall during FY 19751976 are described. The JT-60 machine parameters are plasma current of 2.7 MA, toroidal magnetic field of 4.5 T, duration time of 5 to 10s and additional heating power of 20 to 30 MW. From the viewpoint of first wall design, these parameters are more stringent in JT-60 than in medium size tokamaks, Therefore, R&D on selection of materi-al and structure of the JT-60 first wall was carried out. Initially, comparison between candidate materials were made regarding material, thermal, mechanical and vacuum properties. Molybdenum, pyrolytic graphite (PyG) and CVD-SiC coated graphite (SiC/C) were primary candidate materials. Of these three materials, full-sized trial productions of the first wall were made. High heat load tests with electron beam were carried out to compare thermal shock and thermal cycle properties. Test conditions were heat fluxes of 350 to 1, 000 W/cm2, duration of 10s and cycle numbers from 10 to 320. From the test results, many cracks and crater-like damage were observed on the surfaces of PyG and SiC/C, but no damage was observed on the Mo surface. Following evaluation of all properties including these results, Mo was selected as primary first wall material for JT-60. Moreover, a trial production of Mo honeycomb structure was done. However, the honeycomb structure was not applied because of the expensive fabrication cost. After the operation of JT-60, the first wall materials (limiter, armor plates and magnetic limiter plate) were changed to graphite in FY1987 in order to reduce severe plasma contamination.