AbstractThe importance of improvements of plasma behaviour (its energy confinement time, purity, etc) to fusion research is investigated for the ITER-grade plasmas, and the influence of such improvements on the experimental reactor, intended for physics experiments on burning plasma and nuclear testing, is analysed. Improved energy confinement times allow the given objectives to be attained in a smaller device and thus at a lower cost; the required physics extrapolation is less, so that more accurate prediction is possible; and the technological requirements are less demanding, which may be essential for the feasibility and realization of the device. Constraints are chosen for continuous operation with noninductive current drive of the full plasma current. By introducing a factor h representing the enhancement of the energy confinement time over that of the L-mode scaling, the impact made, in terms of the plasma size, power amplification factor Q, fusion power generation, and divetor heat load, can be quantified as a function of h. Also evaluated is the h-dependence of the divergence of present predictions on the necessary plasma size of next-step devices. Quantification of the effects of improving plasma purity and current-drive efficiency is also discussed. It is shown that improved confinement has a strong and favourable influence on these aspects.