The Study On The Characteristics Of The W7-X Scraper-Off Layer With Three-dimensional Magnetic Topology

Stellarator and tokamak are the two main types of fusion devices,of which Wendelstein 7-X stella rator(W7-X)in Germany is one of the most advanced and la rgest stellarators.Compared with traditional tokamak,W7-X is characterized by an optimized magnetic field,which could reduce the fast alpha particles loss produced in deuterium-tritium fusion reactions,and reduce the neoclassical transport caused by the helical ripples.A divertor design has been used in tokamak to exhaust heat and impurity particles,while a island divertor concept has been used in W7-X instead of the traditional poloidal divertor by forming an island chain whose toroidal mode number n=5 at the edge of W7-X.The scrape-off layer(SOL)width in W7-X stellarator is expected longer than that in tokamak because of the long magnetic connection length,thus the island divertor has its natural advantage on exhausting heat and pa rticles.As one of the key parameters on the SOL region,SOL width determines the wetted area where the heat flux deposited on divertor plate.It is therefore important to study the SOL physical properties on a better understanding of the divertor performance.A mutli-purpose manipulator has been installed on W7-X to carry various probe systems which could serve for various measurement,such as electron density,electron temperature,ion flow velocity and turbulence characteristic.In this thesis,the setup and theory of the multi-channel Mach probe system has been introduced.The Mach probe head could measure the plasma flow velocity by the asymmetry between the upstream ion saturation current and downstream ion saturation current.In this thesis,the first measurement of the radial profile of parallel and perpendicular ion flow velocity using this multi-channel Mach probe has been given in the SOL of the W7-X.A low frequency mode whose main frequency is 1kHz has been observed by the radial arrays,and this mode is radially outward propagated with a speed of around 200 m/s.In this thesis,we also introduce the setup of a combined probe system as well as the theory of four point probe.By the four point probe,the influence of the three dimensional magnetic topology on the profile of electron density,the profile of electron temperature and the characterized width of SOL in various magnetic configurations and discharge parameters has been studied.The result shows the decay length is in positive correlation with the connection length of SOL.The density and heat flux decay length in standard and low iota configuration with a connection length being above 250 m is longer than that in limiter and high iota configuration with a connection length being below 100 m.This thesis finally makes a comparison of the model used to describe the edge localized modes(ELMs)in tokamak with the model used to explain the ELMs in stellarator.In this thesis,a current relaxation model has been used to predict the ELM frequency distributions on EAST tokamak,and the result shows the ELM frequency is very sensitive to the edge current density and the edge safety factor.In the case of a high edge current density,the ELM frequency tends to obey a single-peak distribution;while in the case of a moderate and low edge current density,the ELM frequency tends to obey a multi-peak distribution.