Comparison of the new BOUT++ fluid edge code Hermes-3 to SOLEDGE2D-EIRENE in an example ST40 predictive scenario*

Axisymmetric tokamak edge transport simulations are essential tools for interpretation of experimental data and the design of future devices and operating scenarios. Hermes-3 is a new open-source fluid edge plasma code written under the BOUT++ framework. Combining the features of SD1D and Hermes-2, it allows for self-consistent simulation of edge plasma in 1D, 2D or 3D with an arbitrary number of species for both steady state and unsteady/turbulent regimes.

In this study, Hermes-3 is configured in steady-state 2D axisymmetric transport mode and compared to the edge code SOLEDGE2D-EIRENE in an example predictive double null diverted configuration of ST40, the privately-funded spherical tokamak owned and operated by Tokamak Energy. Although available in both codes, drifts, currents and impurities are not evolved for simplicity. The equations in each code are currently under review to confirm applicability to spherical tokamaks.

While both codes feature similar plasma models, Hermes-3 features fluid neutrals and a grid terminating at an arbitrary magnetic surface while SOLEDGE2D has kinetic neutrals with a grid extending to the wall. The impact of this in the large ST40 vessel region has been found to be significant and the performance of Hermes-3 has been enhanced through improved neutral boundary conditions. In addition, the impact of the Hermes-3 field-orthogonal grid is compared to the SOLEDGE2D grid which supports both field and target orthogonality.