Zero-dimensional nonlinear burn control using isotopic fuel tailoring for thermal excursions

The control of plasma density and temperature are among the most fundamental problems in fusion reactors and will be critical to the success of burning plasma experiments like ITER. While stable burn conditions exist, it is possible that economic and technological constraints will require future commercial reactors to operate with low temperature, high density plasma, a burn condition that may be unstable. The instability is due to the fact that for low temperatures, the fusion heating increases as the plasma temperature rises. An active control system will be essential for stabilizing such operating points. In this work a spatially averaged (zero-dimensional) nonlinear transport model for the energy and the densities of deuterium and tritium fuel ions, as well as the alpha-particles, is used to synthesize a nonlinear feedback controller for stabilizing the burn condition of a fusion reactor. Whereas previous efforts assume an optimal 50:50 mix of deuterium and tritium fuel, this controller makes use of ITER's planned isotopic fueling capability and controls the densities of these ions separately. Also, unlike previous work which used impurity injection to mitigate thermal excursions, this design exploits the ability to modulate the DT fuel mix to control the plasma heating. By moving the isotopic mix in the plasma away from the optimal 50:50 mix, the reaction rate is slowed and the alpha-particle heating is reduced to desired levels. A zero-dimensional simulation study is presented to show the ability of the controller to bring the system back to the desired equilibrium from a given set of perturbations.