STORM: Scrape-off layer turbulence in tokamak fusion reactors

The scrape-off layer of a tokamak fusion reactor carries the plasma exhaust from the hot core plasma to the material surfaces of the reactor vessel. The heat loads imposed by the exhaust are a critical limit on the performance of fusion power plants. Turbulent transport of the plasma regulates the width of the scrape-off layer plasma and must be modelled to understand the intensity of these heat loads. STORM is a plasma turbulence code capable of simulating three dimensional turbulence across the full scrape-off layer of a tokamak fusion reactor, using a drift reduced, collisional fluid model. STORM uses mostly finite difference schemes, with a staggered grid in the direction parallel to the magnetic field. We describe the model, geometry and initialisation options used by STORM, as well as the numerical methods, which are implemented using the BOUT++ plasma simulation framework. BOUT++ has been enhanced alongside the development of STORM, providing better support for staggered grid methods. We summarise these enhancements, including a detailed explanation of the parallel derivative methods, which underwent a major update for version 4 of BOUT++. Program summary: Program Title: STORM CPC Library link to program files: https://doi.org/10.17632/zm3tdfhp9r.1 Developer's repository link: https://github.com/boutproject/STORM Licensing provisions: GPLv3 Programming language: C++ Supplementary material: Configuration and input files and post-processing scripts to run the example code given in Listings 1, 2, and 3. Nature of problem: The scrape-off layer region of tokamak fusion reactors carries the plasma exhaust which escapes from the core, confined plasma and reaches material surfaces along open magnetic field lines. The power and particle loads on the material surfaces are a critical limiting factor for the performance of fusion reactors, but are challenging to simulate due to the large fluctuation amplitudes, complex magnetic geometry, and widely separated time- and length-scales. Three dimensional simulations of plasma turbulence are needed to understand the particle and energy transport in the scrape-off layer and provide predictive capability for the design of future reactors. Solution method: STORM solves a drift reduced, collisional, fluid model for the scrape-off layer plasma. The model is discretised in space using mostly finite difference methods, combined in some places with Fourier methods that take advantage of the toroidal symmetry of the tokamak geometry. The fastest dynamics occur in the direction parallel to the magnetic field, for which a staggered grid is used to avoid the chequerboard instability associated with advective equations [1, sections 6.2, 6.3]. The time solver is a fully implicit, matrix free, variable-step, variable-order method provided by the SUNDIALS library [2]. STORM is implemented using the BOUT++ framework for plasma simulations. References: [1] S. Patankar, Numerical Heat Transfer and Fluid Flow, Hemisphere Publishing Corporation, 1980. [2] A. C. Hindmarsh, P. N. Brown, K. E. Grant, et al., ACM Trans. Math. Softw. 31 (3) (2005) 363–396.