Verification of BOUT++ by the Method of Manufactured Solutions

TL;DR

This paper verifies the correctness of BOUT++ plasma simulation software by applying the Method of Manufactured Solutions to test individual components and complete models, ensuring accuracy across various plasma physics scenarios.

Contribution

It introduces a comprehensive verification of BOUT++ using MMS, including novel tests of components and models in complex geometries and magnetic field configurations.

Findings

Successful verification of time-integration and advection schemes

Validation of the Flux Coordinate Independent approach

Accurate simulation of complex plasma models

Abstract

BOUT++ is a software package designed for solving plasma fluid models. It has been used to simulate a wide range of plasma phenomena ranging from linear stability analysis to 3D plasma turbulence, and is capable of simulating a wide range of drift-reduced plasma fluid and gyro-fluid models. A verification exercise has been performed as part of a EUROfusion Enabling Research project, to rigorously test the correctness of the algorithms implemented in BOUT++, by testing order-of-accuracy convergence rates using the Method of Manufactured Solutions (MMS). We present tests of individual components including time-integration and advection schemes, non-orthogonal coordinate systems and the shifted metric procedure which is used to handle highly sheared grids. The Flux Coordinate Independent (FCI) approach to differencing along magnetic field-lines has been implemented in BOUT++, and is here verified using the MMS in a sheared slab configuration. Finally we show tests of three complete models: 2-field Hasegawa-Wakatani, 3-field reduced MHD in 3D toroidal coordinates, and 5-field reduced MHD in slab geometry.