On the possibility of ITER starting with full carbon

摘要

ITER is planned to start with Be as first wall armors, and W and CFC as divertor armors. Although Be is an excellent oxygen getter, its low melting point makes it unstable to use in a burning reactor. The performance of W armor in ITER-like plasma is still quite uncertain. Moreover, operating temperature window of W would be limited due to its brittleness at lower temperatures and significant grain growth at higher temperatures. Carbon is presently the most reliable plasma facing material, if we have to allow ITER plasma more flexibility and off-normal events, which should be unavoidable to obtain physics bases to attain burning plasma. Appropriate selection of divertor structure and fine tile alignment would reduce carbon deposition significantly. And if we could keep divertor surface temperature of the deposition area above 1000 K, tritium retention could be significantly reduced. We should not exclude the possibility of carbon as PFM even in a fusion reactor. Since the operating temperature of a fusion reactor is likely be above 800 K, tritium retention in carbon is not likely the problem, and erosion could be repaired by deposition of carbon layers between shots and the deposited layers must be graphitized by a succeeding shot.