High Z limiter test in TEXTOR: thermal response and post-mortem analysis

In TEXTOR a high Z limiter experiment was carried out. Mo and W were used as a limiter (10 cm long, 6 cm wide and 5 cm high) and inserted up to 3 cm closer to the plasma than to the main limiter, an ALT-II graphite limiter. Both the thermal response of the limiter and the influence to the plasma performance were analyzed in detail and compared with those for the fine-grained graphite (EK98) routinely used. m power fluxes of about 20 MW m−2 for 2 s could be loaded on the Mo limiter without severe damage, and the maximum surface temperature was about 2600 °C. Consequently the limiter surface was subjected to local melting leaving the trace of molten layers. However, the evaporated Mo during the melting did not influence the main plasma very much, probably owing to redeposition of Mo within one period of gyromotion after the ionization. the experiment the surface of the Mo limiter was examined by various techniques. Although the limiter head was not seriously damaged, appreciable recrystallization and grain growth were observed, which enhanced the propagation of intergranular cracking owing to the thermal stress triggered by congruent melting with thermocouple. No deposition of impurities of B, C, O and Si which were the main impurities in TEXTOR plasma, was observed on the limiter head. The very high temperature of the limiter surface seems to promote the evaporation of once-deposited impurities.