Impact of ionization peak location on measured opaqueness in DIII-D H-mode plasmas

This study investigates the relationship between electron pedestal density and the location of the ionization peak on neutral penetration in DIII-D H-mode plasmas, utilizing a database of Lyman-α emission measurements. The electron density leads to neutrals being ”screened” and the ionization front being pushed out into the Scrape-Off Layer (SOL). This is also referred to as the neutral opaqueness, which is heuristically expected to scale with edge plasma density and machine size. However, at lower electron pedestal density, the penetration depth of the neutrals varies, and measured opaqueness deviates from the heuristic scaling. The database reveals that at low density, when the ionization peak is located in the pedestal region, the linear relationship between the electron density and neutral penetration holds. However, when the peak is located inside the separatrix, the penetration of the neutrals (λn0) is much wider ∼ 3.0 − 3.5 cm, breaking the heuristic opaqueness approximation. These findings provide valuable insights into fueling efficiency and plasma behavior, with implications for Fusion Pilot Plants (FPPs) where high pedestal densities are anticipated and where the neutral opaqueness behaves like its heuristic approximation. This analysis offers a framework to refine neutral opaqueness approximations, enhancing the predictive capability for advanced tokamak operations.

本研究依托莱曼-α（Lyman-α）发射测量数据库，探究了DIII-D装置高约束模（H-mode）等离子体中，电子基座密度与中性粒子穿透过程中电离峰位置之间的关联。电子密度升高会使中性粒子被“屏蔽”，并将电离前沿推至刮削层（Scrape-Off Layer, SOL）内，该现象也被称为中性不透明度，其经验上被认为与边缘等离子体密度及装置尺寸成定标关系。然而在较低电子基座密度下，中性粒子的穿透深度存在波动，实测不透明度与经验定标律出现偏差。该数据库分析表明，在低密度工况下，当电离峰位于基座区域时，电子密度与中性粒子穿透深度间的线性关系成立；但当电离峰处于分界面内侧时，中性粒子穿透长度（λn0）可达约3.0~3.5 cm，远高于常规水平，打破了经验不透明度近似模型的预测。上述研究结果为理解燃料注入效率与等离子体行为提供了重要参考，对聚变试点工厂（Fusion Pilot Plants, FPPs）具有指导意义——这类装置预期将采用高基座密度运行，其中性不透明度符合经验近似规律。本分析为优化中性不透明度近似模型提供了框架，可提升先进托卡马克运行的预测能力。