Reactor compatibility of the H-mode

H-mode operation presents challenges for ITER and for reactor application in the areas of performance, accessibility, sustainability and divertor compatibility. Some issues are more severe for ITER, while others are more problematic for an economic fusion reactor producing 3 MW m^-2 of neutron flux to its walls. The problems that are more severe for ITER include the power requirements for the H-mode, and confinement uncertainties. Problems which are more severe for a reactor include capability for high β* operation, the role of the Greenwald density limit, and divertor compatibility. Within the present ITER concept, it would be very helpful to have a well defined upgrade path for the auxiliary heating systems, including options for strong profile control. The world fusion research programme should maintain a major focus on improving the reactor compatibility of the H-mode, with an eye to the special advantages of H-mode plasmas with strong shaping.