EPOC++ a future-proofed kinetic simulation code for plasma physics at exascale

在等离子体物理学的许多研究计划的核心是需要捕捉等离子体的全部动力学特性。这是最常用的粒子在细胞（PIC）的代码，移动样品的等离子体粒子在自洽的电磁场。十多年来，沃里克，最初与牛津大学和约克合作，一直在为英国等离子体研究人员开发这样的代码-EPOCH代码。核心PIC方案处理的是忽略粒子间碰撞的等离子体--无碰撞等离子体。EPOCH已经扩展到包括二元碰撞，量子电动力学（QED）过程和辐射。这使得EPOCH成为全球使用最广泛的PIC代码之一，应用于粒子加速器，下一代光源，高功率激光QED研究以及磁和惯性约束聚变的聚变科学。有关键的科学扩展需要英国EPOCH，以满足英国用户群的需求。以上目标部分对此进行了详细说明。然而，维持EPOCH变得越来越困难。这是因为EPOCH是从一个较老的Fortran PIC代码演变而来的，当添加更复杂的边界、诊断或物理包时，对于无碰撞等离子体来说，这是相对容易修改的，但Fortran的有限功能集使这些添加变得越来越麻烦。也许可以再坚持使用旧的遗留Fortran一段时间，但还有其他可能更紧迫的问题。现代高性能计算机（HPC）系统正在摆脱完全由同一种处理器类型构建的模式。相反，混合使用GPU加速的传统CPU的HPC系统正变得越来越普遍。正在安装使用ARM处理器甚至FPGA的其他系统。这种向异构HPC系统和复杂HPC环境的转变是由计算能力持续增长的需求驱动的。用于处理这些较新的异构系统的工具集主要使用C++，而不是Fortran。将EPOCH迁移到C++实现（因此重命名为EPOC++）为这些通往Exascale HPC的新途径开辟了道路。同时，它使代码更易于维护，并且对未来的更改更加灵活。该项目将完成所有这些变化，并提供一个EPOC++代码，能够满足英国和国际EPOCH用户的需求，具有数十年的性能和并行代码。

Central to many research programmes in plasma physics is the requirement to capture the full kinetic properties of the plasma. This is most commonly done with a particle-in-cell (PIC) code which move samples of plasma particles in the self-consistent electromagnetic field. For over a decade Warwick, initially in collaboration with Oxford and York, has been developing such a code for UK plasma researchers - the EPOCH code. The core PIC scheme deals with a plasma in which collisions between particles are ignores - a collisionless plasma. EPOCH has been extended to include binary collisions, Quantum Electro-Dynamics (QED) processes and radiation. This has made EPOCH one of the most widely used PIC codes worldwide with application to particle accelerators, next generation light sources, high-power laser QED studies and fusion science for both magnetic and inertial confinement fusion. There are key science extensions required to UK EPOCH to meet the needs of the UK user base. These are detailed in the Objectives section above. However, maintaining EPOCH has become increasingly fraught. This is because EPOCH evolved out of an older Fortran PIC code and while this was relatively easy to modify for the collisionless plasma when adding more sophisticated boundaries, diagnostic or physics packages the restricted feature set of Fortran made these additions increasingly cumbersome. It may be possible to persevere with the old legacy Fortran for a while longer but there are other perhaps more pressing problems. Modern high-performance computer (HPC) systems are moving away from being built all from the same processor type. Instead HPC systems with a mixture of traditional CPUs accelerated with GPUs are becoming prevalent. Other systems are being installed which use ARM processors or even FPGAs. This move towards heterogeneous HPC systems and a complex HPC landscape is been driven by the need for continued increases in computing power. The toolsets for working with these newer heterogeneous systems are primarily in C++, not Fortran. Moving EPOCH over to a C++ implementation (renamed EPOC++ as a result) opens up access to these newer routes towards Exascale HPC. At the same time it makes the code easier to maintain and more flexible for future changes. This project will complete all these changes and deliver an EPOC++ code able to meet the needs of UK and International EPOCH users with a performant and parallel code future proofed for decades.