RAYS, a simplified code for exploration of design options for a modern ray tracing code

Ray tracing is a method for obtaining approximate solutions to the wave equation in non-uniform media which, if applicable, can be orders of magnitude less computationally demanding than direct solution of the wave equation. While there have been great strides in full wave solution of RF fields in fusion relevant plasmas, geometrical optics (or ray tracing) is still an important tool for analysis of experiments and will continue to be needed in the workflows of whole-device models. The present code is a complete rewrite and considerable simplification of the venerable RAYS code. The primary objectives are 1) to provide flexibility for exploration of different code organization strategies, modern programming paradigms, and modern computer architectures, and 2) to build in modularity at every level so as to enable A/B comparisons between different implementations of kernel functions, such as differential equation solvers, equilibrium models, dispersion relation models and approaches to generating the Hamiltonian derivatives. Criteria we will be targeting include: ease of use, flexibility - easy support for multiple geometries, dispersion models, equilibrium models, extensibility - ease of adding new features without disturbing existing ones, verifiability - ease of comparison with simple situations for which rigorous answers are available, speed and accuracy. We will present comparisons between different implementations of ODE solvers, derivative computation methods, and equilibrium models.