Quantifying, modelling and interpreting edge plasma turbulence in tokamak and stellarator fusion experiments

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Turbulence in the edge region of magnetically confined plasmas is a central topic in fusion science. Its character helps to determine, and also responds to, the radial profiles of temperature and density which govern the overall confinement properties of the plasma. Together with any magnetohydrodynamic instabilities at the edge, the edge turbulence also determines the outward flux of plasma and energy across the last closed magnetic flux surface, and hence to the divertor or to the first wall. Edge plasma turbulence thus plays an essential role in both the physics of the confined plasma and the associated quasi-engineering constraints such as plasma exhaust and first wall loading. This proposal outlines a systematic study of statistical features of fusion edge plasma turbulence for different plasma regimes and different confinement concepts, namely tokamaks and stellarators. This multi-device study is aligned with the experimental plan to exploit resonant magnetic perturbation mechanisms on the Mega Amp Spherical Tokamak (MAST) at UKAEA, Culham due for commision in 2008. This mechanism generates stochastic magnetic field at the edge region of MAST which is similar to that found in a stellarator. Statistical techniques will allow us to make new direct comparisons between statistical properties of edge plasma in stellarator and a tokamak with, and without, the ergodised magnetic field, building on our existing collaboration with the large Helical Device (LHD) team at National Institute for Fusion Science (NIFS), Japan. This research addresses fundamental issues, such as the extent of universality exhibited by the edge plasma turbulence, quantifying complex turbulent fluctuations with a robust single figures-of-merit and addressing the practical problem of modelling the distribution of radial particle flux in different confinement systems.There are three closely coupled research projects which will address these broad goals. Project 1 will investigate statistical features of transport in the presence of ergodic magnetic field in two different confinement systems, namely MAST spherical tokamak and LHD stellarator. We will address the fundamental role of electromagnetic fluctuations in the evolution of edge plasma turbulence. The project will build on our ongoing collaboration with UKAEA Culham and NIFS, Japan.The second project concerns systematic quantitative characterisation of edge plasma turbulence in MAST. Edge plasma turbulence will be studied under different operating regimes, using the techniques of complex systems science. This high-volume project will require analysis of many datasets in order to build a properly interlinked global picture, which is hitherto lacking. The project will validate the idea that the distribution of large particle flux events can be described with the universal functional form that does not depend on confinement method or plasma parameters. The modelling of the probability density function for the radial fluxes is one of the topical questions in fusion studies. Radial fluxes can be dramatically modified by poloidal zonal flows that interact with edge turbulence. We will use reduced model to quantify the reduction in particle fluxes in reduced turbulence model with zonal flows.The third project is dedicated to the quantitative comparison between the outputs of plasma turbulence simulations and the measurements of plasma turbulence from fusion experiments.This is an underexplored aspect of fusion science; The applicant will use the 2D Hasegawa-Wakatani model and its extension (magnetic field gradient, temperature, zonal flows and magnetic field randomisation will be included), multi-fluid codes and the gyrokinetic code of A. Peeters (CFSA University of Warwick). The reduced model will be extended to 3D drift-Alfven turbulence simulation which will incorporate magnetic effects.