Plasma turbulence in transport barriers of magnetic confinement fusion devices

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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.