Improved operating modes and the evaluation of confinement improvement

Various improved confinement modes of tokamaks are surveyed and their characteristics reviewed. In search of optimal plasma performance for a fusion reactor, the core performance has been examined. The fusion product near ignition, where Q is large, is used for the evaluation. The dilution of the fuel ions and the profile effect are examined in combination with the effect of increased τ_E. Because of the fuel dilution and of the bulk radiation, optimal performance does not always correspond to modes with the longest τ_E. We confirm that tokamak research should, at present, proceed to the discharge mode with the highest fusion product rather than that with the longest τ_E. If the time evolution is taken into account, the problem becomes more severe. Control of the fuel purity is surely needed to ensure long pulse operation,which certainly requires further extensive research.