Electron heat transport in tokamak edge pedestals

在托卡马克等离子体的高限制区域-实现燃烧等离子体所必需的-在等离子体边缘形成传输势垒，据信是由于压力梯度驱动的剪切流抑制了离子尺度的湍流。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限制属性，因此对整个设备至关重要。底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)和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颗粒和热通量反过来决定了梯度的大小。一个与密度梯度成正比的临界电子温度梯度是驱动电子尺度ETG湍流的线性不稳定性[5]所必需的，因此热流是有限的。因此，电子温度基座的形成与密度基座的形成密切相关。非线性陀螺动力学模拟[6，7]表明，在比线性阈值陡峭得多的梯度上，在实验工作点以上，热流密度随驱动梯度的增加比线性增加的速度更快，即传输变得“僵硬”，将轮廓限制在这个非线性阈值附近。这一阈值的存在被认为与具有高平行波数的模式的出现有关，这些模式与电子的平行运动共振。这个项目的目的是理清这些现象背后的基本物理，从而找出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。这些信息经过适当的参数处理后，可以用来开发基座的简化模型，这是设计未来燃烧等离子体设备所需的，例如STEP。简要说明该项目如何与EPSRC的战略/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的长期投资的主要组成部分，因此也是EPSRC的“弹性国家”成果的关键要素。CCFE是世界领先的聚变研究中心之一，主持着桅杆和喷气式飞机项目，也是英国聚变计划的中心。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(“维护”地位)，通过该主题强调了对能源安全的贡献。EPSRC 2016年对裂变与聚变的审查指出了大学与CCFE互动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，有助于更广泛地提高英国在数据科学方面的能力，从而促进EPSRC的“互联国家”雄心1。A.R.菲尔德等人，“JET-C和JET-ILW H模等离子体中排气功率分量对边缘梯度的依赖”，PPCF 62,055010(2020年)2.C.J.Ham等人，“朝向了解反应堆相关托卡马克基座”，Nucl。Fusion 61,096013(2021年)3.G.J.Colyer等人，“ETG湍流中电子热通量的碰撞性标度”，PPCF 59,055002(2017年)4.F.Jenko等人，“环形电子温度梯度模式的临界梯度公式”，Phys.等离子体8,4096(2001)5.J.Parisi等人，“JET-ILW基座线性谱中的环面和板条ETG不稳定性优势”，Nucl.Fusion 60,126045(2020年)6.D.R.Hatch等人，“具有碳和ITER样壁的喷流中底座传输的直接回转动力学比较”，PPCF 59,086056(2019年)7.W.GuttenFelder等人，“在两个DIII-D Elmy H模式中测试电子尺度湍流底座传输的预测”，Nucl。融合61,056005(2021年)

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)