HIGH-Z LIMITER TEST IN TEXTOR - THERMAL RESPONSE AND POSTMORTEM ANALYSIS

In TEXTOR a high Z limiter experiment was carried out. Mo and W were u sed 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 grap hite limiter. Both the thermal response of the limiter and the influen ce to the plasma performance were analyzed in detail and compared with those for the fine-grained graphite (EK98) routinely used. Maximum po wer fluxes of about 20 MW m(-2) for 2 s could be loaded on the Mo limi ter without severe damage, and the maximum surface temperature was abo ut 2600 degrees C. Consequently the limiter surface was subjected to l ocal melting leaving the trace of molten layers. However, the evaporat ed Mo during the melting did not influence the main plasma very much, probably owing to redeposition of Mo within one period of gyromotion a fter the ionization. After the experiment the surface of the Mo limite r was examined by various techniques. Although the limiter head was no t seriously damaged, appreciable recrystallization and grain growth we re observed, which enhanced the propagation of intergranular cracking owing to the thermal stress triggered by congruent melting with thermo couple. No deposition of impurities of B, C, O and Si which were the m ain impurities in TEXTOR plasma, was observed on the limiter head. The very high temperature of the limiter surface seems to promote the eva poration of once-deposited impurities.