Development of Compact Tokamak Fusion Reactor Use Cases to Inform Future Transport Studies

The OMFIT STEP [O. Meneghini et al., Nucl. Fusion 10 1088 (2020)] workflow has been used to develop inductive and steady-state H-mode core plasma scenario use cases for a B0 = 8T, R0 = 4m machine in order to help guide and inform future higher- fidelity studies of core transport and confinement in compact tokamak reactors. Both use cases are designed to produce 200 MW or more of net electric power in an up- down symmetric plasma with minor radius a = 1.4 m, elongation κ = 2.0, triangularity δ = 0.5, and effective charge Zeff ≃ 2. Additional considerations based on the need for compatibility of the core with reactor-relevant power exhaust solutions and external actuators were used to guide and constrain the use case development. An extensive characterization of core transport in both scenarios is presented, the most important feature of which is the extreme sensitivity of the results to the quantitative stiffness level of the transport model used as well as the predicted critical gradients. This sensitivity is shown to arise from different levels of transport stiffness exhibited by the models, combined with the gyroBohm-normalized fluxes of the predictions being an order of magnitude larger than other H-mode plasmas. Additionally, it is shown that although heating in both plasmas is predominantly to the electrons and collisionality is low, the plasmas remain sufficiently well-coupled for the ions to carry a significant fraction of the thermal transport. As neoclassical transport is negligible in these conditions, this situation inherently requires long-wavelength ion gyroradius-scale turbulence to be the dominant transport mechanism in both plasmas. These results are combined with other basic considerations to propose a simple heuristic model of transport in reactor-relevant plasmas, along with simple metrics to quantify coupling and core transport properties across burning and non-burning plasmas.OMFIT STEP工作流[O. Meneghini等人，《核聚变（Nuclear Fusion）》，10卷，1088页（2020年）]已被用于为B0=8特斯拉、R0=4米的聚变装置开发感应型与稳态高约束模（H-mode）芯部等离子体场景用例，以助力未来紧凑型托卡马克（tokamak）反应堆芯部输运与约束特性的高保真研究，并为其提供指导参考。两组用例均旨在在小半径a=1.4米、拉长比κ=2.0、三角度δ=0.5、有效电荷（Zeff）≈2的上下对称等离子体中产生200兆瓦及以上的净电功率。此外，结合芯部需与反应堆级排热方案及外部执行机构兼容的要求，本次研究对用例开发进行了约束与引导。本文对两组场景中的芯部输运进行了全面表征，其最核心的特征在于：计算结果对所用输运模型的定量刚度水平以及预测的临界梯度具有极强的敏感性。研究表明，这种敏感性源于不同模型展现出的输运刚度差异，同时本次预测的gyroBohm归一化通量比其他高约束模等离子体高出一个数量级。此外，研究还发现：尽管两组等离子体的加热主要作用于电子且碰撞率较低，但等离子体仍保持足够良好的耦合，使得离子承担了相当比例的热输运。由于在该工况下新经典输运（neoclassical transport）可忽略不计，因此该场景本质上要求长波长离子回旋半径尺度湍流成为两组等离子体中的主导输运机制。本研究将上述结果与其他基础考量相结合，提出了一种适用于反应堆级等离子体的简化输运启发式模型，并给出了可量化燃烧等离子体与非燃烧等离子体耦合特性及芯部输运性质的简易指标。