Design study of JT-60SA divertor for high heat and particle controllability

In steady-state high performance plasma over 41 MW/100 s in the JT-60SA tokamak, the heat and particle flux density on the divertor targets are considerably higher than those of existing devices such as JT-60U. A divertor modeling code, SOLDOR/NEUT2D, has been applied in order to optimiz the JT-60SA divertor design in such conditions. The heat load qheat on divertor target is estimated for a conceptual divertor design as the first step. Simulation of SOL/divertor plasmas is carried out at lower single null divertor (LSN) configuration with Ip/Bt=3.5 MA/2.5 T. For the present calculation, anticipated SOL power flux of Qtotal=35 MW and particle fuelling flux of Gion=5.1021/s (ne-dege=3.1019/m) are applied. The pumping speed (Spump=50 m3/s) is specified by an albedo for neutrals in front of the cryopump set bottom of exhaust chamber. The recycling of deuterium is assumed to be 100% at the first wall. For the first simulation, the carbon contamination in SOL/divertor regions is set to 2% of electron density uniformly. Gas puff flux Gpuff=0.5.1021/s is introduced from outside midplane. We assume particle diffusion coefficient D=0.3 m2/s and thermal diffusivity of electron and ion Xe=Xi=1 m2/s. As a result, attached and detached plasma conditions are simulated on outer and inner divertor regions, respectively. The heat load around the outer strike point reaches 31 MW/m2, which largely exceeds the allowable range of 15 MW/m2 for CFC materials. Reduction of heat load must be achieved somehow. An effect of the radiation cooling is simulated to reduce such a large heat load as the second step. To enlarge the radiative cooling, we increased the gas puff flux by a factor of ten and the carbon contamination partly in the outer divertor region from 2% to 4%. It gives a favorable result that the peak heat load is reduced to 12 MW/m2 with radiation enhancement by a factor of two in the outer divertor region. Further study with wide parameter regions is being carried out with taking into account the pumping capability and with modified geometry for high heat and particle controllability. (orig.)