Ignition Characteristics in Deuterium-Tritium Helical Reactors

Ignition characteristics in deuterium-tritium helical (stellarator) reactors of various sizes are studied with the operation path method on the plane and the POPCON method. Based on empirical large helical device scaling, confinement must be improved by a factor > 1.5 for reaching ignition and a factor >γ_H = 2 for optimum fusion power in a helical reactor with R > 8 m, ā = 2 m, and B₀ > 6 T. The density limit and the confinement time saturation effect with respect to the density degrade the favorable density scaling of the confinement time (τ_E ∝ n^0.69) and are found to be important limiting factors for ignition characteristics. For a reactor with R = 10 m, ā = 2 m, γ_H = 2, and B₀ = 7 T and with an excess heating power P_ex = 100 MW, the minimum auxiliary heating power is ∼55 MW at an operating density 40% below the density limit, and ignition can be reached in a finite time. The ignition characteristics for larger size reactors (R = 15 and 20 m) and gyro-reduced Bohm scaling are also studied.