gkylzero: the Lightweight Library Underpinning the Gkeyll Simulation Framework

The Gkeyll simulation framework and the variety of plasma physics-relevant solvers contained in the framework, from multi-fluid multi-moment to full-f electromagnetic gyrokinetic, have been used to tackle problems in a wide variety of plasma systems, from global modeling of planetary magnetospheres to turbulence in the scrape-off layer of tokamaks. We present here an overview of the lowest level of the Gkeyll simulation framework, gkylzero, which constitutes a lightweight library underpinning the whole framework and which can be compiled and linked to independent of the rest of the simulation framework. This software design not only permits significant runtime flexibility in everything from choice of solver to choice of architecture, but also provides a means of utilizing Gkeyll's core solvers outside of the rest of Gkeyll's infrastructure should one desire. We focus on a number of improvements to the overall code structure this design permits, including the practical consequences for the diverse array of applications Gkeyll is used for, as well as how one can utilize gkylzero as a library, thus allowing other codes access to the high quality solvers for multi-fluid multi-moments, Vlasov-Maxwell, and gyrokinetics for applications such as code coupling.