**Computational Challenges and Optimization in Kinetic Plasma Physics**

## Stellarator profile prediction using the T3D+GX+KNOSOS multi-scale gyrokinetic framework

**
Noah Mandell, Princeton Plasma Physics Lab
**

**Wednesday, February 21, 2024**

**Abstract**:

As we progress down the stellarator path to a fusion pilot plant, a key need will be the ability to accurately and robustly predict the equilibrium profiles (and hence fusion power) attainable by a given FPP design configuration. In modern optimized stellarators, neoclassical transport can be reduced to the point that turbulent transport becomes the dominant confinement loss mechanism. Thus a transport solution that includes first-principles turbulence modeling is required. T3D is a framework that leverages multi-scale gyrokinetic theory to efficiently model macro-scale profile evolution in fusion plasmas (tokamaks and stellarators) due to micro-scale turbulent and neoclassical processes. To model micro-turbulence we use the GX gyrokinetic code, which has been developed as a GPU-native code that uses an efficient pseudo-spectral discretization scheme to target fast turbulence calculations for reactor design and optimization. Neoclassical transport is modeled by the KNOSOS neoclassical solver, which uses orbit-averaging to solve the drift kinetic equation very efficiently at low collisionality. Additionally, both GX and KNOSOS use a radially-local approach, taking advantage of the reactor-relevant limit of small orbit widths and small turbulent eddies. This enables a series of GX and KNOSOS calculations to be embedded in parallel in the Trinity3D transport solver for tractable fusion profile prediction (and evolution) calculations. An implicit Newton method is used to time-evolve the transport equations, allowing large timesteps on the timescale of the energy confinement time. Ion temperature profile predictions subject to ion temperature gradient turbulence can be completed in less than an hour, while multi-channel predictions including electron temperature and density profiles can be completed in roughly a day. We will demonstrate a validation of the framework via modeling of W7-X plasmas and discuss future plans for using the framework in experimental studies as well as stellarator FPP design and optimization.