Electron heat transport in tokamak edge pedestals

在托卡马克等离子体的高约束状态下——这是实现等离子体燃烧的必要条件——等离子体边缘形成了一个传输屏障，据信这是由压力梯度驱动的剪切流抑制离子尺度湍流引起的。由此产生的边缘“基座”区域的物理特性决定了整个等离子体的约束特性，因此对整个装置至关重要。基座是一个复杂的非线性系统，其特征是陡峭的平衡梯度（密度和温度）与它们引发的湍流之间的相互作用[1,2,3]——由这种湍流引起的湍流粒子和热通量反过来决定了梯度的大小。驱动电子尺度ETG湍流的线性不稳定性[5]需要一个与密度梯度[4]成正比的临界电子温度梯度[4]，从而导致有限的热通量。因此，电子温度基的形成与密度基的形成密切相关。非线性陀螺动力学模拟[6,7]表明，在明显大于线性阈值的梯度处，在实验工作点以上，热流密度随驱动梯度增加的速度大于线性，即输运变得“僵硬”，将剖面固定在该非线性阈值上。这个阈值的存在被认为与具有高平行波数的模式的出现有关，这些模式与平行电子运动共振。这个项目的目的是整理出这些现象背后的基本物理学，因此它们对平衡参数的定量依赖。可以在一系列条件下将结果与JET-ILW和MAST-U的实验基座剖面数据进行详细比较。这些信息，适当地参数化，然后可以用于开发基座的简化模型，这是设计未来燃烧等离子体设备（例如STEP）所需要的。简要说明该项目如何与EPSRC战略/优先研究领域保持一致：磁约束聚变是英国在能源安全和效率方面长期投资的主要组成部分，因此也是EPSRC“弹性国家”成果的关键要素。CCFE是世界领先的核聚变研究中心之一，主持MAST和JET项目，是英国核聚变计划的中心。上述与CCFE的合作项目直接解决了聚变科学中的一些关键挑战。它属于EPSRC的“等离子体和激光”研究主题（“保持”状态），通过该主题强调对能源安全的贡献。EPSRC 2016年对裂变和聚变的回顾表明了大学- ccfe相互作用的有效性以及英国在建模方面的优势。该项目还提供高水平和相互补充的理论和实验/数据分析/高级计算技能培训，为英国更广泛的数据科学能力做出贡献，从而实现EPSRC的“互联国家”雄心。A. R. Field等，“射流- c和射流- ilw h模等离子体边缘梯度对排气功率成分的影响”，物理学报，42(2):2023 - 2023。C. J. Ham等人，“走向理解与反应堆相关的托卡马克基座”，核。核聚变61,096013 （2021）郭金杰，“ETG湍流中电子热流密度的碰撞标度”，物理学报，59(2),2017。F. Jenko et al.，“环形电子温度梯度模式的临界梯度公式”，物理学。[4]等离子体学报，2001,19(4):444 - 444。J. Parisi et al.，“JET-ILW基座线性谱中的环形和平板ETG不稳定性优势”，核。核聚变60,1260 （2020）D. R. Hatch等，“碳和iter类壁面射流基座输运的直接陀螺动力学比较”，环境工程学报，59,086056（2019）。W. Guttenfelder等人，“两种DIII-D ELMy h模式中电子尺度湍流基座输运的测试预测”，核子。聚变61,056005 （2021）

In the high-confinement regime of tokamak plasmas---essential to achieve a burning plasma---a transport barrier forms at the edge of the plasma, believed to be caused by suppression of ion-scale turbulence by pressure-gradient-driven sheared flows. The physics of the resulting edge "pedestal" region sets the confinement properties of the whole plasma and is thus pivotal for the entire device. The pedestal is a complex nonlinear system characterised by an interplay between steep equilibrium gradients (of density and temperature) and the turbulence that they trigger [1,2,3]---the turbulent particle and heat fluxes caused by this turbulence in turn decide the size of the gradients. A critical electron temperature gradient, proportional to the density gradient [4], is required for the linear instability [5] that drives electron-scale ETG turbulence and hence a finite heat flux. The formation of the electron-temperature pedestal is therefore intimately related to that of the density pedestal. Nonlinear gyrokinetic simulations [6,7] show that at significantly steeper gradients than the linear threshold, above the experimental operating point, the heat flux increases faster than linearly with the driving gradient, i.e., transport becomes "stiff", clamping the profiles to this nonlinear threshold. The existence of this threshold is thought to be related to the appearance of modes with a high parallel wavenumber, which are resonant with the parallel electron motion. This project's aim is to sort out the fundamental physics behind these phenomena and hence their quantitative dependence on the equilibrium parameters. Detailed comparisons of the outcome with experimental pedestal profile data from JET-ILW and MAST-U can be made over a range of conditions. Such information, suitably parameterised, can then be used to develop a reduced model of the pedestal, which is required to design future burning-plasma devices, e.g., STEP.Brief statement on how the project aligns with EPSRC strategy/priority research areas: Magnetic confinement fusion is a major component of UK's long-term investment in energy security and efficiency, and thus a key element for EPSRC's "Resilient Nation" outcome. CCFE is one of the world's leading centres for fusion research, hosting the MAST and JET projects, and is the centre of the UK Fusion Programme. The collaborative project with CCFE described above addresses directly a number of key challenges in fusion science. It falls within EPSRC's "Plasma and lasers" research theme ('maintain' status) through which the contribution to energy security is emphasised. EPSRC's 2016 review of Fission and Fusion flagged the effectiveness of university-CCFE interactions and the UK's strengths in modelling.The project also provides training in high-level and mutually complementary theoretical and experimental/data analysis/advanced computing skills, contributing to UK capabilities in data science more widely and thus EPSRC's "Connected Nation" ambition.1. A. R. Field et al., "The dependence of exhaust power components on edge gradients in JET-C and JET-ILW H-mode plasmas," PPCF 62, 055010 (2020)2. C. J. Ham et al., "Towards understanding reactor relevant tokamak pedestals", Nucl. Fusion 61, 096013 (2021)3. G. J. Colyer et al., "Collisionality scaling of the electron heat flux in ETG turbulence", PPCF 59, 055002 (2017)4. F. Jenko et al., "Critical gradient formula for toroidal electron temperature gradient modes", Phys. Plasmas 8, 4096 (2001)5. J. Parisi et al., "Toroidal and slab ETG instability dominance in the linear spectrum of JET-ILW pedestals", Nucl. Fusion 60, 126045 (2020)6. D. R. Hatch et al., "Direct gyrokinetic comparison of pedestal transport in JET with carbon and ITER-like walls", PPCF 59, 086056 (2019)7. W. Guttenfelder et al., "Testing predictions of electron scale turbulent pedestal transport in two DIII-D ELMy H-modes", Nucl. Fusion 61, 056005 (2021)