AbstractVarious 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 τE.Because of the fuel dilution and of the bulk radiation, optimal performance does not always correspond to modes with the longest τE. We confirm that tokamak research should, at present, proceed to the discharge mode with the highest fusion product rather than that with the longest τE. 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.