Effects of wall boron coating on FTU plasma operations

To achieve good performance on FTU in a large density range (0.3–6.0×1020 m−3), boronization with a mixture of He (90%) and B2H6 (10%) (diborane) as the feeding gas has been tested with thermal loads on the limiter surface up to 2.5 MW/m2. With boronized limiter (TZM alloy with 98% of Mo) and walls (SS AISI 304), the total radiated power drastically drops from 70–90% down to 35–45% and the Zeff decreases from 6.0 to 2.2 at 0.3–0.4×1020 m−3 related to a strong reduction of heavy-metal concentration and to the getter effect of boron on oxygen (<0.5%). During this phase the action of the boron film as particle reservoir and its quick saturation due to the low temperature of FTU walls makes it difficult to obtain reproducible plasmas. Another consequence of boronization is the large dilution of the plasma with the hydrogen particles released from the B film. All these effects decrease after about 60 discharges when boron is eroded by the limiter but it is still present on the chamber walls. During this phase which lasts for more than 500 discharges, oxygen concentration does not increase at all and metal influx is lower than before boronization because the physical sputtering by oxygen ions and atoms is strongly reduced.