Plasma turbulence in transport barriers of magnetic confinement fusion devices

磁约束核聚变是基于这样一个事实，即如果磁场的强度足够大，带电粒子就会被束缚在磁场线上。然而，限制并不完美，因为聚变装置中的等离子体梯度驱动电场和磁场的波动，导致粒子和能量泄漏。这些波动被称为等离子体湍流。由于等离子体湍流，维持聚变等离子体的外部功率输入远远大于天真的理论估计。因此，等离子体湍流对聚变发电厂的最小尺寸和价值施加了严格的限制。实验观察到，在最有希望的聚变反应堆--托卡马克--概念中，湍流涨落减少的区域自然出现。在这些被称为传输势垒的区域中，等离子体参数的梯度必须变得非常大，以驱动足够的湍流来排空注入等离子体中的粒子和能量。梯度足够大，即使输运势垒趋于薄，整体等离子体性能也大大提高。人们认为，等离子体流是一个重要的成分，因为差分旋转可以剪切湍流结构。这个哲学博士项目的目的是确定什么时候流动剪切可以形成运输障碍。学生将使用等离子体湍流代码GS 2，维护和牛津大学和卡勒姆中心聚变能开发。首先，学生将研究流动对湍流的影响。该研究将针对大等离子体梯度进行，因为湍流抑制必须是有效的，即使是对于传输势垒中存在的大梯度。先前的湍流模拟表明，由非常大的等离子体梯度驱动的湍流是难以抑制的。为此，考虑磁场线几何形状，特别是磁剪切（磁场线的节距角的导数）将是重要的。磁剪切与流动相结合可以解释实验中观察到的抑制。除了模拟，学生还将获得JET和MAST中多普勒后向散射诊断（DBS）收集的数据。在研究湍流流动的影响后，学生将使用牛津大学最近开发的模型来确定是否可以通过等离子体湍流驱动必要的抑制流。如果这是可能的，学生将开发一个自洽模型的运输障碍。如果没有，学生将学习流动产生的替代机制（等离子体壁相互作用，与中性粒子的碰撞......）。该项目属于EPSRC等离子体和激光研究领域的福尔斯。

Magnetic confinement fusion is based on the fact that charged particles are bound to magnetic field lines if the strength of the magnetic field is sufficiently large. Confinement is, however, not perfect because the plasma gradients in fusion devices drive fluctuations in the electric and magnetic fields that cause particle and energy leakage. These fluctuations are known as plasma turbulence. Due to plasma turbulence, the external power input to maintain a fusion plasma is far greater than naïve theoretical estimations suggest. As a result, plasma turbulence imposes a severe limit on the minimum size and prize of a fusion power plant. It has been experimentally observed that regions of reduced turbulent fluctuations appear naturally in the most promising concept for a fusion reactor, the tokamak. In these regions, known as transport barriers, the gradients of the plasma parameters have to become very large to drive sufficient turbulence to evacuate the particles and energy injected into the plasma. The gradients are sufficiently large that even though transport barriers tend to be thin, the overall plasma performance is greatly improved.The mechanism behind transport barriers is poorly understood. It is believed that plasma flow is an important ingredient because differential rotation can shear turbulent structures. The objective of this DPhil project is to determine when the flow shear can form transport barriers. The student will use the plasma turbulence code GS2, maintained and developed at the University of Oxford and the Culham Centre for Fusion Energy. First, the student will study the effect of flow on turbulence. The study will be done for large plasma gradients, since the turbulence suppression must be effective even for the large gradients present in transport barriers. Previous turbulent simulations show that turbulence driven by very large plasma gradients is difficult to suppress. For this reason, it will be important to consider the magnetic field line geometry, and in particular the magnetic shear (the derivative of the pitch-angle of the magnetic field line). The magnetic shear in conjunction with the flow may explain the suppression observed in experiments. In addition to the simulations, the student will have the data collected by the Doppler Backscattering Diagnostic (DBS) in JET and MAST.After studying the effect of flow of turbulence, the student will use a model recently developed at the University of Oxford to determine whether the necessary flow for suppression can be driven by the plasma turbulence. If this is possible, the student will develop a self-consistent model for the transport barrier. If not, the student will study alternative mechanisms for flow generation (plasma-wall interaction, collisions with neutrals...).This project falls within the EPSRC Plasma and Lasers research area.