COTSIM-based Optimization of Transport Diffusivity Models for Enhanced Predictive COTSIM Simulations of NSTX-U Discharges

An optimizer has been developed to adjust the set of coefficients associated with parameterized analytical transport models such as Bohm/Gyro-Bohm and Coppi-Tang to better represent specific NSTX-U scenarios. The parameterized transport models in the Control Oriented Transport SIMulator (COTSIM) are tuned by the optimizer in order to minimize a user-defined cost function measuring the mismatch between experimental and COTSIM-predicted plasma states based on the associated experimental inputs (total plasma current, line-average density, heating and current-drive powers, etc.). For instance, the cost function could be the integral over space of the squared difference between experimental and predicted representative plasma profiles such as the safety factor, the electron temperature, and the angular momentum. This integral could be evaluated at a specific time, at a set of predefined times, or over a time interval. Constraints on the possible range of the transport coefficients are imposed during the optimization. The resulting Nonlinear Programming problem is solved by using Sequential Quadratic Programming (SQP), which is predicated on determining a local minimizer of the original nonlinear program by iteratively solving a sequence of approximated Quadratic Programing problems.