Prospects of core–edge integrated no-ELM and small-ELM scenarios for future fusion devices

One of our grand challenges towards fusion energy is the achievement of a high-performance plasma core coupled to a boundary solution. The high confinement mode (H-mode) provides such a high-performance fusion core due to the build-up of an edge transport barrier leading to a pedestal. However, it usually features type-I edge localized modes (ELMs) which pose a threat for long-duration plasma operation in future fusion devices as they induce large energy fluences onto the plasma facing components and typically are projected to damage the first wall. For future fusion devices, the integration of a stationary no-ELM regime with a power exhaust solution is indis- pensable. Several no-ELM and small-ELM regimes have extended their operational space in the past years, with the ultimate goal of providing an alternative core-edge solution to ITER and EU-DEMO. Prominent no-ELM or small-ELM alternatives include the I-mode, QH-mode, EDA H-mode, quasi-continuous exhaust (QCE) and ‘grassy’ ELM regimes, X-point radiator scenarios and negative triangularity L-mode. The state-of-the-art, including access conditions and main signatures, of these alternative regimes is reviewed. Many of these regimes partly match the operational space of ITER and EU-DEMO, however, knowledge gaps remain. Besides compatibility with divertor detachment and a radiative mantle, these include extrapolations to high Q operations, low core collisionality, high Greenwald fractions, impurity transport, amongst others. The knowledge gaps and possible strategies to close these gaps to show their applicability to ITER and EU-DEMO are discussed.

我们在聚变能领域面临的重大挑战之一，是实现高性能等离子体芯部（plasma core）与边界解的耦合。高约束模（H-mode）可提供此类高性能聚变芯部，其通过形成边缘输运垒（edge transport barrier）催生台基（pedestal）。但该运行模式通常伴随I型边缘局域模（type-I edge localized modes，ELMs），会对未来聚变装置的长时等离子体运行构成威胁：此类模体会向面向等离子体部件（plasma facing components）投射大能量通量，且预计会损坏第一壁（first wall）。对于未来聚变装置，将稳态无ELM运行模式与排热方案（power exhaust solution）相整合是不可或缺的。过去数年间，多种无ELM及小ELM运行模式已拓展了其运行区间（operational space），最终目标是为国际热核聚变实验堆（ITER）与欧洲演示聚变堆（EU-DEMO）提供替代的芯-边解决方案。典型的无ELM或小ELM替代方案包括：I模（I-mode）、QH模（QH-mode）、EDA高约束模（EDA H-mode）、准连续排热（quasi-continuous exhaust，QCE）与‘草状’ELM运行模式、X点辐射器方案（X-point radiator scenarios），以及负三角度低约束模（negative triangularity L-mode）。本文综述了这些替代运行模式的当前研究前沿（state-of-the-art），包括其可达条件与主要特征。其中多数模式可部分匹配ITER与EU-DEMO的运行区间，但仍存在认知空白（knowledge gaps）。除了与偏滤器脱靶（divertor detachment）及辐射幔（radiative mantle）的兼容性之外，这些认知空白还包括向高Q运行（high Q operations）、低芯部碰撞率（low core collisionality）、高格林沃尔德份额（high Greenwald fractions）、杂质输运（impurity transport）等场景的外推。本文还讨论了这些认知空白，以及填补这些空白以验证其在ITER与EU-DEMO中适用性的可行策略。