Evolution and Dynamics of pellets and dust in dynamic gas-plasma systems

核聚变提供了几乎无限的电力生产和最小的环境影响的前景。托卡马克（托卡马克是一种磁室，可以将等离子体保持在比太阳核心温度更高的温度下）中的磁聚变已经展示了一条通过JET发电的实用途径，下一代聚变电站ITER的建设正在顺利进行。实现实际核聚变能的关键要素是(1)保持等离子体稳定性和(2)给反应堆换料。低温固体燃料颗粒的注入提供了一种解决这两个方面的方法：这些氘-氚混合物的冰颗粒以高达300米/秒的速度燃烧到燃烧的聚变等离子体中，以补充乏燃料，同时也驱动等离子体的密度分布，从而有助于燃烧过程的稳定性。当颗粒遇到高能等离子体时，它的演变提出了技术挑战，要求严格的建模以优化过程。小球在高能聚变等离子体的猛烈轰击下蒸发，沿着轨道在云中释放燃料气体。作为与预先存在的反应器等离子体平衡过程的一部分，这些云被电离和极化（电荷分离），同时颗粒也被充电；随着弹丸轨迹从纯粹的弹道（即由发射初始条件控制的轨迹）演变为更复杂的动力学，复杂的电磁力和流体力决定了密度沉积剖面。此外，现有等离子体对新材料的响应会影响反应器条件，并且在两个方向上解释瞬态反馈（超过几毫秒）是一个具有挑战性的问题。本研究计划旨在通过利用处理气磁等离子体动量和能量交换以及电离机制的新技术，建立关于等离子体条件和球团演化的蒸发、电离和流体/电磁反馈的新模型。通过这样做，希望这项拟议的研究可以解决当前方法中的差距，并帮助创建新的，优化的托卡马克颗粒换料和稳定性设计。这门科学的影响比无碳能源生产的直接技术目标更广泛：类似的等离子体-固体界面物理发生在许多情况下，从彗星对恒星大气的影响到等离子体催化和气体修复。事实上，在许多情况下，等离子体直接或间接地影响到表面可以促进表面的有益变化，例如使其防水（疏水）或杀菌（抑制细菌附着）或铺设新的表面涂层（等离子体气相沉积）。虽然冷冻颗粒的聚变分析和建模最初似乎与这些领域无关，但在涂料或催化等应用中，探索悬浮中的小液滴或颗粒是否比传统等离子体处理技术具有优势仍有很大的空间；这里提出的研究提供的见解将是评估这些新的潜在用途的关键。Kyle的博士项目将专注于烧蚀区气体等离子体边缘电离锋的计算模拟和理论建模，开发我们内部的PIC代码来研究中性气体屏蔽模型中非均匀和各向异性磁化等离子体条件下电离不稳定性的发展。

Nuclear fusion offers the prospect of almost limitless power production with minimal environmental impact. Magnetic fusion in tokamaks (which are magnetic chambers that can hold plasma at a temperature in excess of that in the solar core) has demonstrated a practical route to power generation via JET, and the construction of the next generation of fusion power station, ITER, is well under way. Key elements in the realisation of practical fusion power are (i) maintaining plasma stability, and (ii) refuelling the reactor. The injection of cryogenically solid fuel pellets offers a way to address both aspects: these icy pellets of Deuterium-Tritium mixture are fired at speeds of up to 300m/s into the burning fusion plasma in order to replenish spent fuel, and also to drive the density profile across the plasma, so aiding the stability of the burning process. The evolution of the pellet as it encounters the energetic plasma presents technical challenges, demanding stringent modelling in order to optimise the process. The pellet evaporates under the intense bombardment of the energetic fusion plasma, shedding fuel gas in clouds along its trajectory. These clouds become ionized and polarized (charge-separated) as part of the process of equilibrating with the pre-existing reactor plasma, and at the same time the pellet also becomes charged; complex electromagnetic and fluid forces then determine the density deposition profile as the pellet trajectory evolves from being purely ballistic (ie with trajectory governed by initial conditions at launch) to a more sophisticated dynamics. Moreover, the response of the existing plasma to the new material affects the reactor conditions, and accounting for the transient feedback (over a few milliseconds) in both directions is a challenging problem. This research proposal aims to create new modelling of the evaporation, ionization and fluid/electromagnetic feedback on both the plasma conditions and the pellet evolution by exploiting new techniques in handling gas-magnetized plasma momentum and energy exchanges, and ionization mechanisms. By so doing, it is hoped that this proposed research can address gaps in the current approaches, and assist in the creation of a new, optimised design of pellet refuelling and stability for tokamaks. The impact of this science is broader than the immediate technological goal of carbon-free energy production: similar plasma-solid interface physics occurs in many situations, from cometary impact on stellar atmospheres to plasma catalysis and gas remediation. Indeed, there are many circumstances in which plasma impacting directly or indirectly onto a surface can promote beneficial changes to that surface, such as making it waterproof (hydrophobic) or biocidal (inhibiting bacterial attachment) or laying down new surface coatings (plasma vapour deposition). Though the analysis and modelling of frozen pellets for fusion may not seem initially to be relevant to these areas, there is major scope to explore whether or not small droplets or pellets in suspension may provide advantages over classical plasma processing techniques in applications such as coatings or catalysis; the insight offered by the research proposed here will be key to evaluating these new potential use. Kyle's PhD project will concentrate specifically on the computational simulation and theoretical modelling of the ionization front at the gas-plasma edge of the ablation region, developing our in-house PIC code to study the development of ionization instabilities in inhomogeneous & anisotropic magnetized plasma conditions in the neutral gas shielding model.