The updated ITPA global H-mode confinement database: description and analysis

The multi-machine ITPA Global H-mode Confinement Database has been upgraded with new data from JET with the ITER-like wall and ASDEX Upgrade with the full tungsten wall. This paper describes the new database and presents results of regression analysis to estimate the global energy confinement scaling in H-mode plasmas using a standard power law. Various subsets of the database are considered, focusing on type of wall and divertor materials, confinement regime (all H-modes, ELMy H or ELM-free) and ITER-like constraints. Apart from ordinary least squares, two other, robust regression techniques are applied, which take into account uncertainty on all variables. Regression on data from individual devices shows that, generally, the confinement dependence on density and the power degradation are weakest in the fully metallic devices. Using the multi-machine scalings, predictions are made of the confinement time in a standard ELMy H-mode scenario in ITER. The uncertainty on the scaling parameters is discussed with a view to practically useful error bars on the parameters and predictions. One of the derived scalings for ELMy H-modes on an ITER-like subset is studied in particular and compared to the IPB98(y,2) confinement scaling in engineering and dimensionless form. Transformation of this new scaling from engineering variables to dimensionless quantities is shown to result in large error bars on the dimensionless scaling. Regression analysis in the space of dimensionless variables is therefore proposed as an alternative, yielding acceptable estimates for the dimensionless scaling. The new scaling, which is dimensionally correct within the uncertainties, suggests that some dependencies of confinement in the multi- machine database can be reconciled with parameter scans in individual devices. This includes vanishingly small dependence of confinement on line-averaged density and normalized plasma pressure (β), as well as a noticeable, positive dependence on effective atomic mass and plasma triangularity. Extrapolation of this scaling to ITER yields a somewhat lower confinement time compared to the IPB98(y, 2) prediction, possibly related to the considerably weaker dependence on major radius in the new scaling (slightly above linear). Further studies are needed to compare more flexible regression models with the power law used here. In addition, data from more devices concerning possible ‘hidden variables’ could help to determine their influence on confinement, while adding data in sparsely populated areas of the parameter space may contribute to further disentangling some of the global confinement dependencies in tokamak plasmas.

多装置ITPA全球H模约束数据库（multi-machine ITPA Global H-mode Confinement Database）已完成升级，新增了带有ITER仿真实心壁的欧洲联合环（JET）以及全钨壁的ASDEX升级装置（ASDEX Upgrade）的相关数据。本文对该升级后的数据库进行了介绍，并基于标准幂律开展回归分析，以估算H模等离子体的全局能量约束定标律。研究考量了数据库的多个子集，重点关注壁材料与偏滤器材料类型、约束运行模式（全H模、有边缘局域模的H模（ELMy H-mode）或无边缘局域模（ELM-free））以及类ITER约束条件。除普通最小二乘法外，本文还采用了另外两种稳健回归技术，可兼顾所有变量的不确定性。对单装置数据的回归分析表明，总体而言，在全金属壁装置中，约束效应对密度的依赖关系以及功率退化效应均最弱。基于多装置定标律，本文对标准有边缘局域模H模运行场景下的国际热核聚变实验堆（ITER）的约束时间进行了预测。本文还围绕定标参数的不确定性展开讨论，以期为参数与预测结果提供具有实用价值的误差棒。本文特别针对类ITER数据集下得到的有边缘局域模H模定标律之一进行了研究，并将其与工程形式及无量纲形式的IPB98(y,2)约束定标律进行了对比。研究表明，将该新定标律从工程变量转换为无量纲量后，无量纲定标律会产生较大的误差棒。因此，本文提出采用无量纲变量空间中的回归分析作为替代方案，可得到符合要求的无量纲定标律估计结果。该新定标律在不确定性范围内满足量纲一致性，其表明多装置数据库中的部分约束依赖关系可与单装置的参数扫描结果相契合，其中包括约束效应对线平均密度与归一化等离子体压强（β）的依赖关系近乎消失，以及约束效应对有效原子质量与等离子体三角度存在显著的正相关依赖关系。将该定标律外推至ITER装置时，得到的约束时间较IPB98(y,2)的预测结果略低，这可能与新定标律中对大半径的依赖程度较弱有关（仅略高于线性关系）。未来还需开展更多研究，将更灵活的回归模型与本文所用的幂律模型进行对比。此外，纳入更多装置的相关‘隐变量’数据，将有助于明确这些变量对约束效应的影响；而在参数空间中数据稀疏的区域补充数据，则可进一步厘清托卡马克等离子体全局约束的各类依赖关系。