Fig. 2-5 Radiative divertor - a physical mechanism of the reduction of heat flux into divertor plates by enhancing radiation from high density-low temperature plasmas created near the divertor plates Fusion-produced extra-heat and particle energies are converted into light radiation and motional energy of neutral particles and dissipated through the interaction with high density-low temperature divertor plasmas leading to a reduction of heat flux to the divertor plates.

Fig. 2-6 Creation of radiative divertor plasma by neon gas puffing into divertor region About 70% of the energy influx to the divertor region was successfully dissipated as radiation by creating a radiative divertor plasma, and the heat flux onto the divertor plates was remarkably reduced as shown in the figure. This experiment in JT-60 was carried out under a maximum heating power of as high as 25 MW.

One important key technology towards a steady-state tokamak reactor is a divertor, an exhaust control system of extra-heat and impurity particles from the core plasma. The divertor is composed of a specially-shaped magnetic field at the outer edge of the plasma, divertor plates and a vacuum pumping system and works to exhaust fusion-produced extra-heat and particles ("helium ash") from the core plasma and also to prevent an influx of impurities into the plasma. Since the divertor plates serve as dumps for energetic impurity particles and high extra-heat flux from the plasma, they should have sufficient endurance for such severe engineering conditions and an intensive research and development of their materials and mechanical structures are required. It is also necessary to decrease or dissipate the heavy heat load to the divertor plates on the basis of some appropriate physical mechanisms. One most promising concept to do this is to create high density and low temperature plasma around the divertor plates and dissipate the influx of heat energy as radiation of light through various physical processes occurring in such high density and low temperature plasmas (the radiative cooling divertor), thus reducing the heat flux to the divertor plates, where the plasma around the divertor plates is working like a "cushion" for their protection. This concept is now actively studied experimentally (see Fig. 2-5). In JT-60, we have demonstrated this mechanism operates quite well to dissipate up to 70% of the energy influx to the divertor as light radiation and to reduce remarkably the heat influx to the divertor plates (see Fig. 2-6). The radiative divertor plasma was produced using neon gas puffing, a pulsed introduction of a certain amount of the gas to the divertor region of JT-60 where a rarefied plasma exists before the introduction of the neon gas. Neon gas is used because it radiates well in the plasma. We have also observed that an undesirable influence on the confinement of the main plasma can be minimized by carefully optimizing the neon gas puffing. As a result we have provided an important data base for the design of the divertor of a fusion reactor such as ITER.

Reference K. Itami et al., Improved Confinement of JT-60U Plasmas, Plasma Phys. Control. Fusion, 37, A255 (1995).

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