Reduced transport modelling of fast ions in MAST Upgrade

在聚变等离子体中，快离子的能量远高于热等离子体背景。快速离子是通过外部辅助加热，如中性束注入（NBI）和离子回旋共振加热（ICRH）或聚变反应本身产生的。在前一种情况下，快离子是氢的同位素，其能量范围从几十千伏到几兆电子伏。除了氢的同位素外，聚变反应还会产生能量在MeV范围内的α粒子。快离子在加热等离子体、维持聚变反应所需的高温以及实现等离子体燃烧方面发挥着重要作用。NBI加热对于电流驱动也很重要，即对于托卡马克在感应区以外的长脉冲运行，从而实现聚变反应堆。因此，将快速离子在等离子体中限制足够长的时间，使它们能够将能量转移到背景等离子体中，对于实现基于热核融合反应的发电厂的目标至关重要。然而，等离子体的不稳定性降低了快离子约束，其中一些是由快离子本身引发的。在这种情况下，快离子之间的能量交换和不稳定性导致了快离子的重新分布和损失，最终降低了聚变反应堆的性能。此外，等离子体中快离子的损失可能会导致反应堆第一壁的损坏，这对ITER和DEMO中将产生的高能α粒子来说尤其严重。FIs和MHD不稳定性之间的相互作用是一个活跃而激烈的研究领域。最近的建模发展包括MHD和粒子动力学代码（对不稳定性的振幅和空间结构以及全轨道计算的现实描述），如HALO （CCFE开发）和TRANSP/NUBEAM的简化输运“踢打模型”（PPPL开发）。特别重要的是，特别是对于MAST升级，是FIs的全轨道建模，用于验证FIs与MHD不稳定性之间相互作用的理论预测，如锯齿，鱼骨，环面alfv<s:1>特征模态，长寿命模态和边缘局部化模态。这些不稳定性的空间结构和时间演变，有时与FIs的动力学非线性耦合，对于正确预测FIs的限制至关重要。该项目旨在系统地比较全轨道和导向中心减少输运计算与MAST升级上的一组快离子实验测量，这些测量存在由锯齿、tea和鱼骨引起的等离子体平衡扰动。特别的重点将是使用安装在MAST Upgrade上的升级中子相机对准直中子通量测量进行建模和解释，并与其他FI诊断（field、FIDA、紧凑型NPA和带电聚变产物探测器）进行比较。这是一个建模项目，需要良好的数值计算技能。该研究将主要在杜伦大学进行，并与CCFE (HALO)，普林斯顿等离子体物理实验室（TRANSP/NUBEAM）和阿尔托大学（ASCOT）合作。该项目的结果是一个简化的、快速的离子输运模型框架，该模型将用于比较探测相空间不同区域的模拟和测量，从而提供对fi动力学的综合理解。反过来，该项目的结果将使开发操作场景成为可能，其中性能限制分布的影响将被抑制，从而允许改进FI限制和无感电流驱动。该项目的主要重点是MAST升级，因为它具有轴上/离轴NB注入的灵活性，但开发的框架将适用于传统的托卡马克，并将与STEP， ITER和DEMO相关。

In fusion plasmas, fast ions have energies much higher than the thermal plasma background. Fast ions are generated by external auxiliary heating such as Neutral Beam Injection (NBI) and Ion Cyclotron Resonance Heating(ICRH) or by the fusion reactions themselves. In the former cases, fast ions are hydrogen isotopes with energies in the range from tens of keVs up to a few MeVs. Fusion reactions produce, in addition to hydrogen isotopes, alpha particles with energies in the MeV range.Fast ions play an important role in heating the plasma, maintaining the high temperatures necessary to sustain the fusion reactions and crucial in achieving a burning plasma. NBI heating is also important for current drive, that is for long pulse operation of tokamaks beyond the inductive regime and therefore for the realization of a fusion reactor.Confining fast ions in the plasma for time long enough so that they can transfer their energy to the background plasma is therefore crucial for achieving the goal of a power plant based on thermonuclear fusion reactions. However, fast ion confinement is degraded by plasma instabilities some of which are triggered by the fast ion themselves. In this case, energy exchange between the fast ions and the instabilities result in the redistribution and loss of fast ions, ultimately reducing the performances of fusion reactors. Furthermore, the loss of fast ions in the plasma can result in the damage of the reactor first wall, an issue particularly for the very energetic alpha particles that will be produced in ITER and DEMO.The interaction between FIs and MHD instabilities is an active and intense field of research Recent modelling developments include MHD and particle kinetics codes (with realistic description of the instabilities' amplitude and spatial structure and full orbit calculations) such as HALO (developed at CCFE) and the reduced transport "kick-model" for TRANSP/NUBEAM (developed at PPPL). Of particular importance, especially for MAST Upgrade, is the modelling of the FIs full orbits for the validation of theoretical predictions of the interplay between FIs and MHD instabilities such as sawteeth, fishbones, toroidal Alfvén eigenmodes, long-lived modes and edge localized modes. The spatial structure and temporal evolution of these instabilities, at times non-linearly coupled to the dynamics of the FIs, is crucial for the correct prediction of the confinement of FIs. The project is aimed at a systematic comparison of full-orbit and guiding-center reduced transport calculations with a set of fast ions experimental measurements on MAST Upgrade in presence of perturbations of the plasma equilibrium due to sawteeth, TEAs and fishbones. A particular focus will be dedicated to the modelling and interpretation of collimated neutron flux measurements using the upgraded neutron camera installed on MAST Upgrade and the comparison with other FI diagnostics (FILD, FIDA, compact NPA and charged fusion product detector). This is a modelling project for which good numerical computation skills are required. The research will be carried out mainly at Durham University with collaborations with CCFE (for HALO), the Princeton Plasma Physics Laboratory (for TRANSP/NUBEAM) and Aalto University (for ASCOT). The outcome of this project is a framework of reduced, rapid fast ion transport models that will be used to compare simulations and measurements probing different regions of the phase space thus providing an integrated understanding of the FIs dynamics. In turn, the outcome of this project will enable the development of operating scenarios where the effect of performance limiting distributions will be suppressed thus allowing improved FI confinement and non-inductive current drive. The main focus of this project is MAST Upgrade thanks to its on-axis/off-axis NB injection flexibility but the developed framework will be applicable to conventional tokamaks and will be of relevance to STEP, ITER and DEMO.