TEST OF THE BORON CONTAINING COATING OF THE GRAPHITE LIMITER IN THE T-10 TOKAMAK

Test results of different boron containing coatings on different graphite substrates as operation elements of the rail limiter in the T-10 are reported. This rail limiter consisted of some elements made from MPG-8 graphite, uglesitall USB-15 and pyrolytic graphite. Some MPG-8 and pyrolytic graphite elements were covered with B-C films of 200 mm thickness. The limiter was exposed to about 2500 operation pulses with plasma parameters: n(e) = 0.5 X 10(12) cm-3, T(e) = 20-50 eV, plasma flux F = 5 X 10(18)-2 X 10(19) cm-2 s-1, power load up to 5 MW m-2 s-1 in ohmic discharges and up to 20 MW m-2 s-1 in ECR heating discharges. The comparison of the behavior of boron-carbon coatings on different graphites in T-10 boundary plasma condition was a purpose of these experiments. The test revealed that the erosion of the boron carbide coating is considerably less than that for MPC graphite and USB-15 (difference about 50-100%). But the total sputtering both for the graphite and for the B-C film is much higher than one can conclude from the model experiments. The melting of the B4C coating was observed on the MPG substrate but not on the substrate from the pyrolytic graphite due to its high thermal conductivity. No cracks or flakes were observed on the B4C coating which are typical for graphite material under such thermal loads.