Description
Gyrokinetic codes are currently the most advanced numerical tools for simulating turbulence in toka-mak plasmas. The code GYSELA [1], written in Fortran 90 and developed for more than 20 years, is one of the flux-driven gyrokinetic codes available worldwide. However, expanding this code to use more complex mathematical methods such as non-uniform points (vital for handling the different magni-tudes of physical quantities in the core and edge regions), and increasingly complex geometries (ge-ometries including both closed and open field lines, and potentially stellarator geometries) has proved to be challenging and sometimes error-prone. The challenges of such extensions are further amplified when trying to structure such a code for use on new GPU architectures, necessary for exascale simu-lations. This is a challenge shared by other gyrokinetic codes.
For these reasons, the development of a new code was started two years ago, named Gysela-X++, from scratch in modern C++, using MPI and Kokkos [2] to handle parallelism. It is built on top of the DDC [3] library providing associative arrays with labelled dimensions to make the source code safer and more expressive. This choice allows the code to run natively on CPUs and GPUs and thus benefit from the capabilities of exascale supercomputers. Some of the algorithms of this new code are directly taken from the legacy code in Fortran and translated to C++/Kokkos, while other parts of the algorithm are completely refactored to alleviate the restrictions of the Fortran code.
The code is split in two parts: Gyselalib++ [4] a library of operators and a set of independent simulation codes built on top of it. A first version of the code is now available. It simulates axisymmetric tokamak plasmas and is therefore 4D. This axisymmetric version, named Gysela-axi, is a major milestone as it will serve as the basis for the parallel development of multiple new physics features, most notably tokamaks with X-points, stellarators and neutral physics. In addition, it will permit optimizations efforts and the development of new numerical capacities, including non-uniform grids, in situ diagnostics and anomaly detection to handle the simulation.
In this contribution, after presenting the main features of the Gysela-axi code, we will report on our extensive study of the impact of plasma shaping on the properties of geodesic acoustic modes (GAMs). The differences between positive and negative triangularity configurations will be particularly scrutinized, in relation to recent theoretical predictions and experimental observations.
Biblio:
1. V. Grandgirard et al., « A 5D gyrokinetic full-f global semi-Lagrangian code for flux-driven ion turbulence simulations », Computer Physics Communications 207 (2016) 35–68
2. C. Trott et al., «Kokkos 3: Programming Model Extensions for the Exascale Era», IEEE TRANSACTIONS ON PARALLEL AND DISTRIBUTED SYSTEMS, VOL. 33, NO. 4, APRIL 2022 805
3. Padioleau et al. (2025). DDC: The Discrete Domain Computation library. Journal of Open Source Software, 10(115), 9122. https://doi.org/10. 21105/joss.09122.
4. E. Bourne et al., «Gyselalib++: A portable C++ library for Semi-Lagrangian Kinetic and Gyrokinetic Simulations», Journal of Open Source Software, 10(113), 8582. https://doi.org/10.21105/joss.08582.