Reduced transport modelling of fast ions in MAST Upgrade

在聚变等离子体中，快离子的能量远远高于热等离子体背景。快离子是由外部辅助加热产生的，如中性束注入(NBI)和离子回旋共振加热(ICRH)，或者由聚变反应本身产生。在前一种情况下，快离子是氢同位素，能量范围从几十个凯夫到几个梅夫。除了氢同位素外，聚变反应还会产生能量在MeV范围内的阿尔法粒子。快离子在加热等离子体、维持维持聚变反应所需的高温方面发挥着重要作用，对实现燃烧的等离子体至关重要。NBI加热对电流驱动也很重要，也就是托卡马克在感应区域以外的长脉冲运行，从而实现聚变反应堆。因此，在等离子体中进行足够长的时间限制快离子，使它们能够将能量传递到背景等离子体，对于实现基于热核聚变反应的发电厂的目标至关重要。然而，快离子约束被等离子体不稳定性降低，其中一些不稳定性是由快离子本身触发的。在这种情况下，快离子与不稳定性之间的能量交换导致快离子的重新分布和损失，最终降低聚变堆的性能。此外，等离子体中快离子的损失可能导致反应堆第一壁的破坏，这对将在ITER和DEMO中产生的非常高能量的阿尔法粒子来说尤其是一个问题。FIS和MHD不稳定性之间的相互作用是一个活跃和激烈的研究领域。最近的模拟发展包括MHD和粒子动力学程序(具有不稳定性幅度和空间结构的真实描述和全轨道计算)，如HALO(在CCFE开发)和TRANSP/NUBEAM的简化输运“Kick模型”(在PPPL开发)。特别是对于桅杆升级来说，特别重要的是对FIS全轨道的建模，以验证FIS和MHD不稳定性之间相互作用的理论预测，例如锯齿、鱼骨、环形Alfvén本征模、长寿命模和边缘局域模。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的限制是至关重要的。该项目旨在系统地比较全轨道和引导中心约化输运计算与一组在桅杆升级时存在锯齿、茶叶和鱼骨引起的等离子体平衡扰动的快离子实验测量结果。将特别侧重于使用安装在桅杆升级上的升级的中子照相机对准直中子通量测量进行建模和解释，并与其他FI诊断方法(FILD、FIDA、COMPACT NPA和带电聚变产物探测器)进行比较。这是一个需要良好的数值计算技能的建模项目。这项研究将主要在达勒姆大学进行，与CCFE(针对HALO)、普林斯顿等离子体物理实验室(针对TRANSP/NUBEAM)和阿尔托大学(针对ASCOT)合作进行。该项目的成果是一个简化的快速离子传输模型的框架，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FIS动力学的综合理解。反过来，该项目的成果将有助于开发运行场景，其中将抑制性能限制分布的影响，从而改善FI限制和无感电流驱动。由于轴上/轴外Nb注入的灵活性，该项目的主要重点是主杆升级，但开发的框架将适用于传统托卡马克，并将与STEP、热核实验堆和演示相关。

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.