用可微编程优化核聚变装置线圈设计,并测试机器学习解偏微分方程的可靠性。
Differentiable Programming for Computational Plasma Physics
- 用反向自动微分实现线圈优化,提升设计效率与灵活性。
- 提出误差修正算法,确保机器学习求解器保持守恒性与稳定性。
- 发现多数文献存在基线过弱和报告偏差,质疑ML解PDE的实际潜力。
可微编程通过自动微分(AD)实现代码函数导数的自动计算。本文探讨其在计算等离子体物理中的两项应用。首先,构建基于梯度优化的托卡马克线圈设计工具FOCUSADD,利用反向模式AD实现有限制造能力的线圈设计,具有高效、灵活优势。其次,研究机器学习(ML)替代或改进求解偏微分方程(PDE)的数值方法,重点关注流体力学中与等离子体相关的时变PDE。可微编程使神经网络嵌入数值框架成为可能。本文回答两个核心问题:第一,能否设计出具备守恒性、稳定性与正性保证的ML-PDE求解器?答案是肯定的,提出误差校正算法以维持时变PDE的不变量。第二,哪些类型的ML求解器表现最优?通过对科学文献的系统回顾发现,多数研究存在基线过弱与报告偏差,影响结果可复现性。结论指出,尽管前景诱人,但目前使用机器学习求解PDE的实际效果尚未达预期。
原文摘要 · Abstract (English)
Differentiable programming allows for derivatives of functions implemented via computer code to be calculated automatically. These derivatives are calculated using automatic differentiation (AD). This thesis explores two applications of differentiable programming to computational plasma physics. First, we consider how differentiable programming can be used to simplify and improve stellarator optimization. We introduce a stellarator coil design code (FOCUSADD) that uses gradient-based optimization to produce stellarator coils with finite build. Because we use reverse mode AD, which can compute gradients of scalar functions with the same computational complexity as the function, FOCUSADD is simple, flexible, and efficient. We then discuss two additional applications of AD in stellarator optimization. Second, we explore how machine learning (ML) can be used to improve or replace the numerical methods used to solve partial differential equations (PDEs), focusing on time-dependent PDEs in fluid mechanics relevant to plasma physics. Differentiable programming allows neural networks and other techniques from ML to be embedded within numerical methods. This is a promising, but relatively new, research area. We focus on two basic questions. First, can we design ML-based PDE solvers that have the same guarantees of conservation, stability, and positivity that standard numerical methods do? The answer is yes; we introduce error-correcting algorithms that preserve invariants of time-dependent PDEs. Second, which types of ML-based solvers work best at solving PDEs? We perform a systematic review of the scientific literature on solving PDEs with ML. Unfortunately we discover two issues, weak baselines and reporting biases, that affect the interpretation reproducibility of a significant majority of published research. We conclude that using ML to solve PDEs is not as promising as we initially believed.
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