Ehab Hassan

Poloidal field (PF) and central solenoid (CS) coils play a crucial role in sustaining the equilibrium and preserving the shape of highly confined tokamak plasmas. Ensuring that PF coil current and mechanical stress stay within superconducting and structural limitations is an important check in the design assessment. Minimizing the PF coil currents and mechanical stresses influences reliability, cost, and performance. A free-boundary MHD equilibrium code—FreeGS is employed within the fusion reactor design and assessment (FREDA) whole facility modeling (WFM) framework to construct the plasma equilibrium based on the configuration and currents in the PF coils. Here, we present the capability of the FreeGS code to minimize the currents, forces, and electromagnetic stresses on the PF coils by optimizing their number, sizes, structures, and locations while maintaining an MHD stable plasma configuration with a large confinement factor. The workflow is initialized with a configuration of plasma parameters and coils’ locations from the 0-D tokamak build systems code in the FREDA framework. Then, FreeGS is called to calculate the initial equilibrium at the minimum total current in PF coils. Thereafter, FreeGS’s internal optimizer minimizes the currents and hoop and central forces on the PF coils while maintaining the reference equilibrium. Finally, the input configuration is updated with the optimized parameters for equilibria over the ramp-up phase of a burning-plasma operation. FREDA’s whole facility optimization capability, which includes all magnetic field coil systems, blanket, vacuum vessel (VV), first wall, divertor, etc., is under development and out of the scope for this study.

The objective of this work is to develop a three-dimensional multiphysics modeling approach to study the effects of plasma shape on the structural performance of in-vessel components under transient electromagnetic loads. The multiphysics modeling is developed by the sequential coupling of free boundary plasma equilibrium physics, electromagnetics and solid mechanics. A midplane disruption in a tokamak based fusion reactor is modeled using an exponential decay of the plasma current, combined with the free boundary solutions for the construction of equilibria for plasma geometries with desired triangularity from the FreeGS code are fed as inputs into the ANSYS Maxwell code. The Maxwell equations posed within the T-Ω formulation are employed to solve for the magnetic field (B) and the induced current (J) on the in-vessel conductors. The JxB forces are fed into a transient solid mechanics module for the computation of stresses and displacements produced from the plasmas of different triangularity. It was shown that increasing the negative plasma triangularity would produce higher forces and consequently higher stresses on the vacuum vessel (VV) indicating that additional precautions should be taken to design negative plasma experiments in a tokamak fusion reactor that is designed for positive triangularity plasmas. Computations showed that the blanket significantly reduces the electromagnetic loads on the VV, thereby protecting it. The developed multiphysics model improves understanding of the effects of current quenches in triangular plasmas, showing how variations in plasma shape can result in significantly different forces, stresses, and deformations affecting large in-vessel fusion components. The proposed methodology supports rapid force assessment and clearer analysis of stress wave propagation, helping to shorten design iteration cycles.