| With the rapid industrialization and urbanization,energy is considered as the essential base of modern activities,and its demand is continuously increasing.According to the Global Energy Overview,renewable alternatives and energy replacement will be the key developments in improvements in energy supply systems and enduse energy structures,respectively.Fusion is a promising option for providing sustainable clean and base-load energy to meet the rapidly increasing demand while having a relatively low environmental impact.The tokamak is still regarded to be the most promising approach to realize controlled nuclear fusion and overcome the global energy crisis.However,instabilities such as magnetohydrodynamic(MHD)instabilities and some possible operational errors may trigger tokamak disruption.During the tokamak disruption event,the plasma's thermal and magnetic energy will be released.The released thermal energy will impose a substantial heat load on the plasma facing components(PFCs).Besides that,the intense electromagnetic stress induced in the vacuum wall can cause significant damage to the internal structure.Plasma disruption also leads to the generation of a large amount of highly energetic runaway electrons.Due to the small impact area,fast time scale,and deep penetration into the bulk of the machine components,the runaway electrons(REs)are considered high risk for tokamaks and are a critical safety problem for future tokamak operations.Therefore,understanding the mechanisms behind RE generation,runaway suppression,and runaway dissipation is of vital importance.As a consequence,while building large-scale devices such as the China Fusion Engineering Test Reactor(CFETR)and the International Thermonuclear Experimental Reactor(ITER),we should have specific strategies in order to minimize the possibility of major plasma disruptions.If the plasma disruption can not be completely avoided,we must take measures to mitigate it,thereby minimizing the damage posed by the disruption.Massive gas injection(MGI)is the artificial injection of a significant amount of no-ble gas into the plasma before an anticipated disruption to immensely minimize the heat load and electromagnetic forces through physical processes such as radiation,ionization,and recombination.During the prethermal quench phase,the mixing of injected impurities and plasma at the edge produces cooling.The impurities are transported through the magnetic flux in the toroidal direction during the thermal quench stage,hence leading to the toroidal spreading of the radiation.Under certain conditions.the instabilities triggered by MGI can produce enormous heat convection between the core plasma and the impurity at the outer edge.Finally,it will result in increased thermal radiation in three-dimensional space,and the injected massive gas impurity will produce convection in the radial direction to mitigate plasma disruption.In tokamak disruption,plasma current converted into runaway current.One way to deter such conversion is magnetic field stochasticity due to MHD activity and therefore,the majority of primary electrons are lost from this randomized regime.Magnetic perturbation can modify the magnetic topology structure during the plasma disruption triggered by MGI.More stochastic magnetic typologies are highly beneficial for primary RE loss during the TQ phase and may suppress a runaway current plateau by significantly reducing the generation of secondary runaways by the avalanche mechanism.In this study,NIMROD simulations of rapid shutdown scenarios by MGI in EAST and CFETR are performed.In EAST,simulations are carried out to investigate the effects of massive Helium gas injection level on the induced disruption.It is demonstrated in simulations that two different scenarios of plasma cooling(complete cooling and partial cooling)take place for different amounts of injected impurities.For the impurity injection above a critical level,a single MHD activity is able to induce a complete core temperature collapse.For impurity injection below the critical level,a series of multiple minor disruptions occur before the complete thermal quench(TQ).MGI simulations are carried out in CFETR using high Z gas(Neon),a typical MGI process is demonstrated,and its results are compared with EAST in certain aspects.In the second part of this thesis,NIMROD simulations are performed to investigate the runaway electron(RE)loss mechanism during the disruption process induced by massive gas injection on EAST and CFETR.The simulation results indicate that the injected gas decreases the plasma edge temperature initially and enhances plasma resistivity,therefore,contract the current profile which destabilizes MHD modes and leads to magnetic field stochasticity.The inner flux surfaces remain intact until complete collapse of core temperature.The onset of thermal quench(TQ)is characterized by a significant decrease in thermal energy that occurs due to dominant(n=1)mode saturation.During the TQ phase,a significant fraction of primary REs is lost due to magnetic field stochasticity,which takes place in three steps in EAST.First.a small fraction of REs is lost due to the magnetic field stochasticity in the edge region,followed by a rehealing phase as a result of continued ohmic heating,when no RE loss occurs.Finally,almost all of the remaining REs are lost due to the complete stochasticity of the magnetic field.It is demonstrated that in a lower amount of He injection,the complete collapse of thermal energy takes a longer time than the case without ohmic heating.These results signify that the magnetic field stochasticity may be possibly responsible for the RE deconfinement.The amount of He injection and ohmic heating may significantly modify the RE loss time.It is found that in each EAST simulation,more than 90%of the RE is lost,whereas in CFETR,even after achieving the full stochastic phase,only a small fraction(20-25%)of the RE is lost.This comparison of the two devices(with different sizes)reveals a trend of increased RE confinement with increasing device size. |
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