Thermally Stable Operation of Engineering Test Reactor Tokamaks

It is desirable for the plasma operating points of future Engineering Test Reactor (ETR) tokamaks to be in parameter regimes that are inherently stable to thermal fluctuations; in other words, thermal equilibrium is maintained by properties of the power balance terms themselves without an active burn control system. Methodologies are presented for calculating thermally stable operating points and scenarios to achieve these conditions. Results are given for an ETR tokamak with major radius R₀ = 5.8 m in both the ignition and current-drive modes. Though the results are sensitive to the form of the energy confinement scaling law used, for enhancements over L-mode confinement by factors of 1.5 to 2.0, stable operating regions in (n, T) space have been identified for ignited operation with T ≥ 20 keV and for current-drive steady-state operation with T ≈ 25 keV. Burn dynamics simulations and discussion of critical issues are also presented. The analyses are general and should be applicable to a wide variety of deuterium-tritium burning tokamaks.