Core Transport Modeling and Characterization for Compact Tokamak Reactor Scenarios

Motivated by the current interest in compact, high-field approaches to fusion power plants, the OMFIT STEP integrated modeling workflow has been used to generate self-consistent core plasma transport solutions representative of potential compact tokamak reactor operating scenarios. In this study, solutions for an idealized Rmaj = 4 m, B0 = 8 T tokamak “use case reactor” (UCR) were developed, with the intention of providing starting parameters for more comprehensive future transport studies in the spirit of the CYCLONE base case. Both inductive pulsed (UCR-P) and steady-state (UCR-SS) solutions potentially capable of producing 1 GW of fusion power and 200 MW or more net electric power have been identified. A common feature of both scenarios is that the core confinement time is long enough for the plasmas to be well- coupled, even though core collisionality is low. This situation leads to significant core ion thermal transport, despite the heating being predominantly to the electrons, and a corresponding dominance of long-wavelength ion temperature gradient modes. A similar situation is found to hold for ITER and SPARC plasma scenarios, and is argued to be an inherent property of power plant-relevant burning plasmas. For both UCR scenarios, the EPED code predicts peeling-limited pedestals with extremely weak sensitivity to core pressure values, enabling use of a fixed boundary condition in core transport modeling. With this constraint, another key finding of this study is the extreme sensitivity of the results to the quantitative stiffness level of the transport model used as well as the predicted critical gradients, with outcomes ranging from runaway ignition to radiative collapse possible depending upon the choice of TGLF saturation rule. Given this uncertainty, new analysis presented in the paper details initial benchmarking of TGLF against linear and nonlinear gyrokinetic simulations. The gyrokinetic results are broadly consistent with the relevant TGLF predictions, but highlight the need to improve the accuracy of transport stiffness and particle flux predictions, especially at larger radii.

受当前紧凑型强磁场聚变电站技术路线研究热潮的启发，本研究采用OMFIT STEP集成建模工作流（OMFIT STEP integrated modeling workflow），生成了代表潜在紧凑型托卡马克反应堆运行工况的自洽芯部等离子体输运解。本研究针对理想化主半径$R_{ ext{maj}}=4 ext{m}$、环向磁场$B_0=8 ext{T}$的托卡马克“基准工况反应堆（UCR）”开发了相关输运解，旨在参照CYCLONE基准案例（CYCLONE base case）的研究范式，为后续更全面的输运研究提供初始参数。本研究已识别出两类输运解：感应脉冲型（UCR-P）与稳态型（UCR-SS），二者均可实现1吉瓦的聚变功率输出与200兆瓦及以上的净电功率产出。两类工况的共同特征为：尽管芯部碰撞率较低，但芯部约束时间足够长，可使等离子体实现良好耦合。此种工况下，即便加热主要作用于电子，仍会产生显著的芯部离子热输运，且长波长离子温度梯度模占据主导地位。类似的工况在国际热核聚变实验堆（ITER）与SPARC托卡马克的等离子体工况中同样存在，且被认为是与电站相关的燃烧等离子体的固有属性。针对两类UCR工况，EPED程序预测其边界台基为剥离模限制型台基，且对芯部压强值的敏感性极弱，因此可在芯部输运建模中采用固定边界条件。基于这一约束条件，本研究的另一关键发现为：计算结果对所采用输运模型的定量刚度水平以及预测的临界梯度极为敏感；根据TGLF饱和规则（TGLF saturation rule）的不同选择，计算结果可能会从逃逸点火到辐射崩溃不等。鉴于这一不确定性，本文呈现的新分析详细介绍了TGLF与线性及非线性回旋动理学模拟的初始基准测试工作。回旋动理学模拟结果与相应的TGLF预测结果大体一致，但同时也凸显出提升输运刚度与粒子通量预测精度的必要性，尤其是在更大径向半径区域。