Dependence of the boundary heat flux width on core and edge profiles in Alcator C-Mod

This work presents new evidence that the heat flux width, λ q , in the Alcator C-Mod tokamak scales with the edge electron pressure, as observed in the ASDEX Upgrade (AUG) tokamak (Silvagni et al 2020 Plasma Phys. Control. Fusion 62 045015), but the scaling with volume-averaged pressure, p¯, from the plasma stored energy, found by Brunner et al (2018 Nucl. Fusion 58 094002), is a better predictor of λ q in Alcator C-Mod than the edge electron pressure. These previous studies, which find that λ q decreases with increasing plasma pressure, imply that a high performance core at high pressure will lead to challenging heat and particle exhaust due to very small λ q . This concern has led to our significant enlargement of the C-Mod database with the electron density, temperature, and pressure profile data from the Thomson scattering and electron cyclotron emission diagnostics. Using the C-Mod database augmented with new profile data, we find that λ q decreases with increasing edge electron pressure as λq∝ pe,95-0.26, similar to results from AUG, and showing the strength of cross-machine comparisons. We also find that λq∝ pe,core-0.56, consistent with the original finding from C-Mod that the heat flux width scales as p¯-0.48 (Brunner et al 2018 Nucl. Fusion 58 094002). The scalings of λ q with separatrix pressure and gradient scale length are found to match the AUG results qualitatively. The C-Mod scalings with edge plasma quantities have more scatter than the p¯ scaling, and, importantly, show different trends for H-modes relative to L- and I-mode. Investigating the source of this discrepancy presents an opportunity for further study that may improve our ability to predict the heat flux width in different confinement scenarios in the pursuit of optimizing core-edge performance in future reactors.

本研究提供了新证据，表明阿尔卡特C-Mod托卡马克（tokamak）中的热流宽度λ_q与边缘电子压强存在标度关系，该现象与ASDEX升级（ASDEX Upgrade, AUG）托卡马克中的观测结果一致（Silvagni等, 2020, 《等离子体物理与控制核聚变》62卷045015期）；但相较于边缘电子压强，基于等离子体储能得到的体平均压强p̄的标度关系，能更准确地预测阿尔卡特C-Mod中的λ_q，该标度关系由Brunner等（2018, 《核聚变》58卷094002期）提出。过往研究发现λ_q随等离子体压强升高而减小，这意味着高压强下的高性能芯部等离子体，会因λ_q过小面临极具挑战性的热流与粒子排灰问题。这一顾虑促使我们大幅扩充了C-Mod数据库，新增了来自汤姆逊散射与电子回旋辐射诊断得到的电子密度、温度及压强剖面数据。利用新增剖面数据后的C-Mod数据库，我们发现λ_q随边缘电子压强升高而减小，满足标度律λ_q∝p_e,95^(-0.26)，与ASDEX升级装置的研究结果类似，也印证了跨装置对比研究的重要价值。同时我们还得到λ_q∝p_e,core^(-0.56)，与C-Mod最初的研究结果一致，即热流宽度与体平均压强的标度关系为p̄^(-0.48)（Brunner等, 2018, 《核聚变》58卷094002期）。研究还发现，λ_q与分界面压强及梯度标度长度的标度关系，在定性上与ASDEX升级装置的结果相符。相较于体平均压强的标度关系，基于边缘等离子体参数得到的C-Mod标度关系离散性更强，且尤为重要的是，高约束模（H-mode）与低约束模（L-mode）、I模的变化趋势存在差异。探究这一差异的来源，可为后续研究提供方向，有助于我们在优化未来聚变堆芯部-边缘性能的目标下，提升对不同约束模式下热流宽度的预测能力。