Development Of Dust Module Under BOUT++ Framework And Its Application In Simulation

Dust,resulting from plasma-wall interactions,poses a serious threat to tokamak operation and safety.The impurities from dust ablation processes in the core region causes severe plasma radiation,which in turn degrades tokamak confinement and reduces operation efficiency.In some occasions,high-Z dust penetrates into the core plasma and leads to the termination of plasma discharge.In addition,some dusts are toxic or radioactive,which is a potential threat to the safety of maintenance staffs.Therefore,it is important to study in the tokamak discharge environment the behavior of dust grains,which depends heavily on their size and equilibrium charge.The existing simulation studies up tp now mainly address the issues on dust grains with radii larger than 1μm,in which case,the drift effect due to electromagnetic field can be reasonably ignored.For nano-meter scale dust grains,however,the drift effect becomes a dominant factor and a new model based on guiding-center system needs to be established.During the H-mode discharge,the ELM(Edge Localized Mode)triggers radial transport of particles and energy,which has a great impact on dust transport behaviors.No studies have been found in this field.Under such circumstances,I have carried out my Ph.D.project,which presents a systematic research on the nano-dust behavior and impact of ELMs on it.The charging process that includes contributions from plasma ions and electrons,SEE(Secondary Electron Emission)and TEE(Thermionic Electron Emission)of the dust grains is evaluated in tokamak plasma environment.Dust ablation is also assessed considering particle bombardment,electron emission,recombination processes,neutral particle emission and radiative cooling mechanisms.For the dust transport,The Hamiltonian guiding center equation of motion by Littlejohn and others is utilized to describe the velocity and acceleration of dust grains at arbitrary time step.The drift effect of gravity and Lorentz force are included in the Hamiltonian,while the normalized ion drag force is added in the acceleration equation since it is parallel to the magnetic field.The developed DS(Dust simulation)code deals with dust charging and ablation.Furthermore,the NDS(Nano Dust Simulation)module is developed under BOUT++framework to model nano-dust transport and its property evolution along the trajectory.In addition,the newly added NDS+module is utilized to simulate the impacts from ELMs.The thesis contains six chapters.In Chapter 1,the Tokamak configuration and its reaction regime is briefly reviewed.The dust impacts on tokamak operation efficiency and safety issue is addressed in detail.Besides,the plasma-wall interactions that bring large amounts of dusts into fusion plasma are analyzed.The structure of this thesis is outlined at the end.Chapter 2 introduces the OML(Orbital Motion Limited)theory in the dust charging and ablation model.BOUT++framework structure,along with its coordinate system and common operators are presented as well.In Chapter 3,the dust charging and ablation model is established.The simulation has been done on carbon and tungsten dusts with different sizes in common fusion plasma environment.Results show that the charging time,which is inversely proportional to plasma density,is between the order of nanosecond and ten microseconds.The carbon dust ablation becomes severe and has a positive floating potential when the plasma temperature and density is above 15 eV and 1019 m-3,respectively.As the plasma temperature and density increases,the carbon dust can be fully ablated.Tungsten dust possesses a higher equilibrium charge when the plasma density hits 1020 m-3.The equilibrium charge on the dust grain is on the order of 10 to 104 elementary charges.Besides,the magnetic field in tokamaks can lower the charge on the dust.In Chapter 4,the NDS model,which simulates the behaviors of spherical dusts with a radius below micrometer scale,is demonstrated.And the code module is developed accordingly under BOUT++framework.Results show that the dust with a radius of 10 nm launched from midplane region can be trapped in the SOL and oscillates until fully ablated in C-Mod tokamak.Larger ones with a radius of 100 nm and a initial speed of 100 m/s,however,cannot be fully constrained by the electromagnetic field and transported out of the simulation domain directly to the wall after launching.For the case of injecting from divertor region,a 50 nm-radius tungsten dust experienced severe ablation processes and was fully ablated within several milliseconds when it reaches the Separatrix.In Chapter 5,a random walk model is developed based on NDS module to simulate the influence of ELMs on dust transport behaviors.The updated NDS+module is utilized to run simulation on dusts in HL-2A and EAST tokamak under BOUT++framework and the following results are obtained.When there’s no ELM present,HL-2A SOL(Scrape Off Layer)region cannot ablate injected dust grains fully.The nano-dust launched in the SOL region is trapped and oscillates along the magnetic field during the entire tokamak discharge.The 200 nm-radius dust injected from inner divertor of EAST Tokamak with upper null configuration,in contrast,can be ablated completely.The impact on dust transport from the magnetic field is more obvious when the dust radius is below 100 nm.And the 10 nm-radius tungsten dust injected from the inner midplane will travel along the magnetic field and hit the inner divertor target at the velocity of～2 km/s,which might cause damages on the plate and bring more dusts into the tokamak.When ELMs are present in HL-2A,the number of dusts collected from the upper region increases with larger transport coefficient.This indicates that the ELMs are beneficial for the removal of existing dusts during tokamak discharge.In Chapter 6,conclusions and innovation points are summarized.Plan for future work is outlined as well.