Appliocation of High Z materials as Plasma Facing Materials

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Properties of high Z materials as plasma facing materials, such as hydrogen recycling, thermal response to plasma heat load, high Z impurity release and behavior of their ions in plasmas, have been examined by various technique. Main conclusions are summarized as follows,(1) The W bulk limiter operated with preheating above 800K withstood plasma heat load of about〜20MW/m^2 for a few seconds except slight surface melting during the highest heat load shot. However it was severely damaged when operated at around 500K(2) The C/W twin limiter experiments gave very useful information on how low- and high-Z materials behave for the case of simultaneous utilization as plasma facing maerials (PFM) such like cross contamination process, influences of large mass difference in the hydrogen reflection and deposition, and so on.(3) About 60% of the total convection heat was absorbed in the vacuum plasma splayed-W (VPS-W) coated poloidal limiters, and the ohmic plasma with the density as high as 5×10^<13> cm^<-3> was sustained. Most of the VPS-W coated limiters tolerated to the heat load of〜20MW/m^2. However some limiters were heavily damaged. Probably their curved shape gave strong thermal stress, resulting in the exfoliation of the W layer. Operation at below DBTT would enhance the crack propagation.These series of W limiters experiments in TEXTOR have probed that W is applicable as PFM and divertor plate, if its central accumulation are avoided by NBI and/or ICRH heating. Nevertheless, some concerns are still remaining, those are, difficulty of plasma start-up, W behavior in higher temperature plasma, and materials selection.