Subignited, ITER-like designs - a question of confinement margin

If the next-step International Thermonuclear Experimental Reactor (ITER) is designed to operate at finite energy multiplication (Q to approximately 10 to 20), as opposed to ignition (Q to approximately ∞), appreciable reductions in size and cost will result. Ignition will be attainable in such a `high-Q targeted' device under slightly enhanced confinement conditions. For example, with the nominal design guidelines from the ITER Conceptual Design Activity (CDA), designing for Q = 15 instead of ignition results in approximately 20% savings in size and cost. Ignition would still be achievable in such a reduced-size device if the L-mode energy confinement enhancement factor (i.e., H factor) is approximately 15% higher than the assumed nominal value of 2.0. This size/cost impact is large compared to other sensitivities, and the range of H-factor improvement needed to recoup ignition is small compared to the uncertainty in the confinement scalings themselves.