High heat load test of molybdenum

Abstract Three different types of molybdenum, powder metallurgical polycrystalline (PM-Mo), and as-forged polycrystalline and single crystalline of highly purified electron-beam-melted Mo (AFEB-Mo and SCEB-Mo), have been subjected to high heat load test with neutral beam injection (NBI) stands at Japan Atomic Energy Research Institute (JAERI) and National Institute for Fusion Science (NIFS). These materials have also been tested as a movable limiter in a reversed field pinch machine (RFP: TPE-1RM15) in Electrotechnical Laboratory (ETL). The results are summarized as follows. The SCEB-Mo shows the least damage with slight local melting after a very high heat load of 260 MW/m 2 for 250 ms with NBI, while for the PM-Mo the whole irradiated area melt with many crater due to impurity gas evaporation under less heat load (200 ms). All movable limiter heads of the RFP are severely damaged with partial melting. The appearance of the SCEB-Mo limiter after melting is not good and shows the crystalline cleavage. However, SEM observation of the microstructure opposes the surface appearance. In the SCEB-Mo, appreciable recrystallization is not observed and hence no crack is seen to go into the bulk except the crystalline cleavage. In the PM-Mo, on the other hand, the resolidification to columnar grains as well as the recrystallization is apparent, and the cracks not only go along the columnar grains but also separate the recrystallized region from the matrix. In the AFEB-Mo, a slight grain growth occurs and several cracks enter deep along the grain boundaries. Thus the SCEB-Mo is a very nice plasma-facing material if used under the critical heat load for melting.