Inference of Experimental Radial Impurity Transport on Alcator C-Mod: Bayesian Parameter Estimation and Model Selection.

We present a fully Bayesian approach for the inference of radial profiles of impurity transport coefficients and compare its results to neoclassical, gyrofluid and gyrokinetic modeling. Using nested sampling, the Bayesian Impurity Transport InferencE (BITE) framework can handle complex parameter spaces with multiple possible solutions, offering great advantages in interpretative power and reliability with respect to previously demonstrated methods. BITE employs a forward model based on the pySTRAHL package, built on the success of the well-known STRAHL code [Dux, IPP Report, 2004], to simulate impurity transport in magnetically-confined plasmas. In this paper, we focus on calcium (Ca, Z=20) Laser Blow-Off injections into Alcator C-Mod plasmas. Multiple Ca atomic lines are diagnosed via high-resolution X-ray Imaging Crystal Spectroscopy and Vacuum Ultra-Violet measurements. We analyze a sawtoothing I-mode discharge for which neoclassical and turbulent (quasilinear and nonlinear) predictions are also obtained. We find good agreement in diffusion across the entire radial extent, while turbulent convection and density profile peaking are estimated to be larger in experiment than suggested by theory. Efforts and challenges associated with the inference of experimental pedestal impurity transport are discussed.

本文提出一种用于杂质输运系数径向分布推断的全贝叶斯方法，并将其结果与新经典、回旋流体（gyrofluid）及回旋动理学（gyrokinetic）建模结果进行对比。借助嵌套采样（nested sampling），贝叶斯杂质输运推断（Bayesian Impurity Transport InferencE, BITE）框架可处理存在多解的复杂参数空间，相较于已报道的方法，在解释能力与可靠性上具备显著优势。BITE采用基于经典STRAHL程序[Dux，IPP报告，2004]开发的pySTRAHL软件包构建正向模型，以模拟磁约束等离子体中的杂质输运过程。本文聚焦于注入至阿尔卡特C-Mod（Alcator C-Mod）等离子体中的钙（Ca，原子序数Z=20）激光吹除注入实验。通过高分辨X射线成像晶体光谱学与真空紫外测量，对多条钙原子谱线完成诊断。本文针对一次存在锯齿振荡的I模放电开展分析，该放电同时具备新经典与湍流（准线性及非线性）理论预测结果。研究发现，全径向范围内的扩散系数与实验结果吻合良好，但实验中测得的湍流对流与密度分布峰化程度均高于理论预测值。本文还讨论了实验等离子体台基（pedestal）杂质输运推断相关的工作与挑战。