Magnetically confined fusion plasmas are being designed within the International Tokamak Experimental Reactor (ITER) and other projects that will operate in physics regimes never achieved through experiment. Accordingly, modeling and simulation activities that require exascale computational resources are required to design and optimize these new facilities. The WDMApp project is developing a whole device modeling approach that will provide predictive numerical simulations of the physics required for magnetically confined fusion plasmas to enable design optimization and fill in the experimental gaps for ITER and future fusion devices.
Nuclear fusion offers a potential long-term energy source that uses abundant fuel supplies and does not produce greenhouse gases or long-lived radioactive waste, but achieving stable fusion conditions requires larger and more complex reactors and experimental facilities than humans have ever built. ITER, a 35-nation, multi-billion dollar collaboration, is developing the largest-ever experimental fusion reactor.
Nuclear fusion reactors cannot generate energy without efficiently forming stable plasmas under magnetic confinement, a complex process which is not fully understood. Forming stable fusion plasma in facilities the size of ITER poses new design challenges due to changes in plasma dynamics within larger reactors. These challenges, along with the high cost of designing and building a functional nuclear fusion facility, can be significantly mitigated with the support of high-performance computation. Next-generation supercomputers will guide the design of fusion plasma containment devices—which impact the construction of entire facilities—to accelerate the achievement of stable and energy-positive nuclear fusion.
The Exascale Computing Project’s Whole Device Model Application (WDMApp) provides predictive numerical simulations of plasma physics for fusion research facilities of all sizes. These simulations reveal a large and realistic set of dynamics for magnetically contained fusion plasma, including the causes and effects of micro instabilities in containment, which strongly impact whether power is generated or expended during fusion. WDMApp simulations will ultimately be used to create a comprehensive model of the physics within a magnetically confined fusion reactor, which will improve the stability and efficiency of current and future reactor designs.
Previous simulations were unable to realistically model plasma dynamics in entire devices, largely because of their inability to integrate differences in plasma behavior at the core and the edge of the magnetic containment field. This integration requires coupling of two distinct physical models, which are each highly computationally intensive, and cannot be achieved without the application of exascale-class computing. Inaccurate modeling of dynamic interactions at the core and edge of containment greatly limits predictive power, especially as simulated devices deviate farther from existing ones in design and in size and power specifications.
The WDMApp code suite is the first model created with integrated core and edge plasma physics. The code suite is composed of two coupled gyrokinetic codes for modeling combined plasma in unprecedented detail, allowing for accurate simulation of a much wider range of plasma behaviors including complex scenarios such as the generation of micro instabilities and high-confinement states—which are necessary for achieving meaningful energy gain. The WDMApp team has also optimized the application to easily accept updated physics parameters, allowing for expanded simulations including new reactor designs and whole-facility models.
Using WDMApp, researchers and engineers can create and follow an experimental blueprint through which to understand nuclear fusion plasma dynamics and ultimately to create sustainable and affordable energy through nuclear fusion power plants. These simulations are a critical component in maximizing the return of US investments in the ITER international facility partnership and in optimizing the design of future fusion facilities.
The Whole Device Model Application (WDMApp) project aims to develop a high-fidelity model of magnetically confined fusion plasmas, which is urgently needed to plan experiments on ITER and optimize the design of future next-step fusion facilities. These devices will operate in high- fusion-gain physics regimes not achieved by any current or past experiments, making advanced and predictive numerical simulation the best tool for the task. WDMApp is focused on building the main driver and coupling framework for the more complete Whole Device Model (WDM), with the ultimate goal of completing a comprehensive computational suite that includes all the physics components required to simulate a magnetically confined fusion reactor. The main driver for the WDM will be the coupling of two advanced gyrokinetic codes, one in the edge (XGC) and the other in the core (GENE or GEM). XGC is a particle-in-cell (PIC) code optimized for treating the edge plasma. While GENE is a continuum code, GEM is a PIC code optimized for the core plasma. WDMApp takes advantage of the complementary nature of these two applications in the core to build the most advanced and efficient whole device kinetic transport kernel for the WDM, and also to mitigate risk.
A major project thrust is the coupling framework EFFIS 2.0 (End-to-end Framework for Fusion Integrated Simulation 2.0), which will be further developed for operations exascale and optimized for coupling most of the physics modules that will be incorporated in the WDM. The current MPI+X implemented in the main GENE and XGC applications is to be enhanced with communication-avoiding methods, task-based parallelism, in situ analysis with resources for load optimization workflows, and deep memory hierarchy–aware algorithms.
The resulting exascale application will be unique in its computational capabilities and will have potentially transformational impact in fusion science, for example, by studying a much larger and more realistic range of dimensionless plasma parameters than ever before and by assessing the rich spectrum of kinetic micro-instabilities that control the quality of energy confinement in a toroidal plasma (e.g., tokamaks, stellarators), with the core and the edge plasma strongly coupled at a fundamental kinetic level based on the gyrokinetic equations.
The exascale science challenge problem is the high- fidelity simulation of whole device burning plasmas applicable to a high-confinement (i.e., H-mode) advanced tokamak regime, specifically, an ITER steady-state plasma that aims to attain a tenfold energy gain. The physics objective is to predict one of the most important indicators for energy confinement in the H-mode—the plasma pressure “pedestal” height and shape. Realization of the H-mode with high-edge plasma pressure and mild pedestal gradient is critical to ITER’s performance and success. Efficiency of the fusion burn is virtually determined by the height of the pressure pedestal at the edge. The strategy will involve using WDMApp, which is focused on coupling the core and the edge regions seamlessly.