ExCALIBUR Fusion Use Case: Project NEPTUNE (NEutrals & Plasma TUrbulence Numerics for the Exascale)

Project NEPTUNE seeks to develop knowledge, capability and prototype infrastructure targeting one of the Grand Challenge problems in modelling the behaviour of a thermonuclear tokamak plasma – a region of the plasma known as “the edge”, where hot plasma comes into contract with cold neutral gas and the plasma wall itself. The systems of equations form a classic multi-physics, multiscale grand challenge problem that has long been referred to as an “exascale” challenge.

Year 1 activities are designed to form an exploratory phase, similar in many ways to the early stages of the Met Office Gung-Ho project (where work for example concentrated upon optimal gridding of the atmosphere) and how to deploy a separation of concerns philosophy which has resulted in the LFRic platform – NEPTUNE will for example explore the efficacy of high order meshing in order to meet requirements around field line alignment and conformality with plasma facing components. A separation of concerns philosophy (as built into the ExCALIBUR pillars) is also paramount as there is a great deal of uncertainty around the models themselves at this juncture – it will be essential to be able to easily adapt the systems of coupled PDEs quickly as new knowledge is forthcoming from machines like the EPSRC MAST-U tokamak and ITER, a €25B reactor class experiment being built in the South of France.

UKAEA’s Activities this year focus upon a) Continuing requirements capture, Project Management and Collaboration Management, b) Down selection or creation of suitable algorithms for delivering the eventual coupled system of models to specified (and evolving) requirements and c) development of suitable Data Structures & Standards in order to deliver an exascale scalable, performant and easy to adapt framework for studying turbulent plasma physics at the first wall and in the divertor region of a burning tokamak. 60-70% of the project is being defrayed to 3rd parties, in order to build a UK wide team needed to deliver the UK’s Fusion Roadmap (with a focus upon the STEP programme to deliver fusion power to the National Grid in the 2040’s). Year 1 exploratory/design work is largely being carried out through the development of low order models embedded within Proxyapps. The Fusion Use Case Science plan contains more detail, however a short summary of Y1 exploratory tasks and objectives is as follows:

  1. Performance of Spectral Elements: Confirmation of the efficacy of the high order spectral/hp method for achieving exascale performance and scalability together with grid alignment requirements for next generation coupled fluid + gyro/drift kinetic solvers.
  2. Optimal Use of Particles: The coupled systems will most likely involve close coupling of a fluid method (inside plasma) to a particle-based method (PIC, possibly gyro or drift kinetic). Work is needed to identify issues around noise propagation, scalability/performance etc.
  3. Study of UQ techniques: Uncertainty Quantification will be built into the infrastructure from the ground up – advances in multi-level techniques across coupled models, semi-intrusive methods etc. will be down selected and deployed to ensure that the eventual code is “actionable”.
  4. Study of Model Order Reduction techniques: MOR will likely be essential for making UQ tractable – many possible solutions exist which will require down selection and tuning to the Fusion Use Case.
  5. Development of Fluid Referent Model: This task will develop the baseline fluid model for the core plasma, building up the systems of equations and solvers/library solutions by starting at low order, with exploratory work taking place through the development of Proxyapps.
  6. Investigation of Matrix preconditioning: A modest amount of work is required to explore the optimal solutions for Matrix preconditioning.
  7. Development of gyro-averaged model: A more “ambitious” approach to solving for the confined plasma is to adopt a kinetic model, ideally gyro-kinetic but possibly drift-kinetic. An exciting new model that will hopefully work in the edge plasma region (where radial gradients in plasma parameters tend to break the gyro-kinetic model assumptions) is under development – this year, we will deploy this model in slab geometry to explore the scalability and performance of the model and chosen solvers at low order (again, through the development of Proxyapps).
  8. Investigation of DSL and Code Generators: To embed a separation of concerns philosophy in the eventual software stack, Domain Specific Languages (DSLs) and Code Generator technologies will be explored and down selected in order to instantiate the higher order systems of equations that will be the eventual coupled code/infrastructure.

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