Growth of carbon impurity density and adequate divertor wall temperature for fusion experimental reactors

Sudden increase of carbon impurity called carbon bloom has terminated the energy breakeven condition in the present large tokamak. In order to lengthen the burning plasma state in the next device, carbon bloom has to be well suppressed. The temporal evolution of carbon impurity density is analytically examined by using a simple one-point kinetic or zero-dimensional model including the effects of graphite erosions due to oxygen and ion, and gettering for oxygen due to boron or beryllium. The growth of carbon bloom due to radiation-enhanced sublimation is discussed based on the effective self-sputtering of carbon. Even when the self-sputtering yield is less than unity, carbon density is observed to continuously increase with the discharge time if the oxygen gettering action is not perfectly conducted. From the present analysis and data on the erosion of carbon materials, and the evaporation of gettering materials, it is suggested that the divertor wall temperature has to be kept less than approximately 900–1000°C to avoid the continuous growth of the carbon density.