| The high confinement mode(H-mode)features a steep pressure gradient of plasma in the pedestal region.The H-mode is the baseline operation regime for the high-performance requirements of future steady state tokamaks such as ITER.The diffusion and convective transports caused by the steep gradient of pressure and current engender a large amount of power outflowing across the separatrix and going toward the divertor target,which leads to corrosion and damage to the divertor or other material surfaces.The peak of heat flux is reversely proportional to a critically important parameter—the scrape-off-layer(SOL)heat flux width(λq).The SOL heat flux width is determined by the competition between cross-field and parallel transports in the SOL.The heuristic drift model proposed by Goldston,supported by an experimental scaling λq ∝ 1/Ip(or 1/Bpol)(with no strong dependence on other SOL parameters).nredicts λq～1 mm for ITER.The divertor target power loads with such narrow width would in reality be impossible to be handled.However,more complicated gyro-kinetic simulations and turbulence simulations at the ITER scale indicate λq～5-6 mm.Up to date,no consensus has been reached for the magnitude of the heat flux width for ITER and future reactors,about which there still exists great uncertainty.No latter which prediction is right,there is no doubt that the heat flux width definitely needs to be broadened by any applicable techniques.Under such backgrounds,I have carried out my PhD project,part of which has led to the present dissertation.The thesis is composed of six chapters.Chapter 1 first briefly reviews the background of this PhD project,and introduces some basic concepts used in the later chapters,and outlines the whole thesis.The BOUT++ code package and edge plasma transport model are introduced tersely in Chapter 2.In Chapter 3,a model developed by reducing Braginskii’s transport equations under the frame of BOUT++ was employed to investigate how lithium(Li)species transport in the edge plasma in the real-time Li aerosol injection experiments in EAST.The simulation results show that Li atoms propagate inwards since the Li injection,and their penetration depth depends on both the local plasma conditions along their path and initial injection velocity.It is also found that Li ions accumulate rapidly in the edge,and only a small fraction of Li species can transport cross the separatrix into the core.In the poloidal direction,Li ions drift swiftly downwards along the field line,and transport much faster at the high field side than at the low field side.The strong interaction between background plasma and Li ions plays a key role in lowing the temperature of background plasma and raising the density of background plasma in the edge.In Chapter 4,the BOUT++ fluid transport code(trans-Er)is developed with all drifts and the sheath potential in the SOL.The steady state radial electric field(E:)is calculated by coupling a plasma transport model with the quasi-neutrality constraint and the vorticity equation within the BOUT++framework.Based on the experimentally measured plasma density and temperature profiles in Alcator C-Mod discharges,the effective radial particle and heat diffusivities are inferred from the set of plasma transport equations.Given these diffusivities,the electric field can be calculated self-consistently across the separatrix from the vorticity equation with sheath boundary conditions coupled to the plasma transport equations.The sheath boundary conditions act to generate a large and positive Er in the SOL,which is consistent with experimental measurements.The effect of EXB drift is not obvious in our simulation,while the effect of magnetic particle drifts is shown to play a significant role on local particle transport and the radial electric field.The net particle flow due to the magnetic drift causes the charge separation in both the edge and SOL regions,yielding large modification of the Er across the separatrix.In Chapter 5,the BOUT++transport code(trans-Er)with all the cross-field drifts is used to evaluate the heat flux width.In order to understand the relative role of cross-field drifts vs.turbulent transport playing in setting the heat flux width,four C-Mod EDA(enhanced Da)H-mode discharges with lower single null divertor configuration are simulated.A turbulence diffusivity scan {χ⊥)identifies two distinct regimes:a drift dominant regime when X⊥is small;a turbulence dominant regime when X⊥ is large.The heuristic drift model yields a lower limit of the width λq.These studies indicate that drifts and turbulent transport compete to determine the heat flux width in C-Mod EDA H-mode discharges.The transition from a drift dominant regime to a turbulence dominant regime depends on whether the real thermal diffusivity is over the critical value of thermal diffusivity Xνc.The critical value of thermal diffusivity is associated with both machine size and plasma operating regime.From C-Mod to ITER and CFETR,the critical value of thermal diffusivity χ⊥c reduces significantly because of the larger machine sizes,stronger magnetic field and higher current.Correspondingly,the drift-based radial transport weakens remarkably,and turbulence-based radial transport turns relatively much stronger.BOUT++ simulations indicate that both ITER and CFETR will run in a turbulence dominant regime and divertor heat flux width no longer follows the experimental scaling.Finally,the conclusions and innovation points are summarized,and prospects for the future work are discussed. |
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