Follow the Power – Pathways to Steady State Reactors*

Diagramming pathways of normalized power is a potent method for extrapolating between operating points in current tokamaks and future burning plasma devices. The current drive power, loss power from transport and L-H threshold power can be expressed in dimensionally correct form using the normalized power Pa^3/4 (where a is the minor radius), with the relative gyroradius (ρ_*) dependence ranging from gyroBohm-like for transport (Pa^3/4~ρ_*^-1.5), Bohm-like for current drive (Pa^3/4~ρ_*^-2.5) and worse than Bohm-like for L-H transition (Pa^3/4~ρ_*^-3). The D-T fusion power cannot be normalized in the same fashion since it is governed by nuclear physics, but at fixed B_T it scales like Pa^3/4~ρ_*^-4.5. Diagramming these normalized powers vs. ρ_* for fully non-inductive scenarios shows a specific pathway corridor between DIII-D and ITER, capped off at small ρ_* by the Greenwald density limit, with the more favorable pathways residing at high β and B_T. Using a steady-state hybrid with β_N<4 from DIII-D as the starting point, reactors with ITER’s B_T=5.3 T achieve high fusion gain (Q~20) only at small ρ_*, whereas reactors with B_T=10 T have a pathway to Q~20 even at large ρ_* (e.g., a JET-sized device).