| In tokamak fusion devices,the transition from the low-confinement mode(L mode)to the high-confinement mode(H mode)is usually accompanied with a periodic burst of boundary disturbance,which is called the edge localized mode(ELM).Along with the ELM events,plasma particles and energy will be ejected out,which will bring a very high local transient heat load on the first wall material of the fusion device,causing cracks,melting,evaporation,and ablation of the first wall material,reducing the service life of the first wall material.However,ELM also has its beneficial effects.For example,ELMs have a strong transport capacity in the pedestal area of the H-mode,which can effectively expel the core impurities and avoid plasma collapse caused by the accumulation of core impurities.Therefore,research on how to achieve effective control of ELM to avoid the disadvantages caused by ELM and make full use of the beneficial aspects of ELM is of great significance to the stable and highly confinement long pulse operation in EAST and future fusion devicesAiming at the problem of ELM control under the H-mode operation mode,first of all,the implementation of H-mode,the generation mechanism of various types of ELM and the existing ELM control methods were thoroughly investigated.Secondly,the technologies of generating and cleaning sub-millimeter scale lithium granules was developed.the lithium granules has good sphericity and high purity,which provides support for the subsequent use of lithium granules in ELM control experiments;the existing real-time lithium powder injection system on EAST has been optimized and upgraded in structure and control module,and calibrated under vacuum conditions,which has broadened the lithium powder injection mode and improved the precise control of the lithium powder injection amount;A new multi-impurity injection system was developed,with the ability to real-time inject various powder particles(such as boron powder,lithium powder)and granule impurities(such as lithium,carbon granules,etc.).Finally,the new multi-impurity injection system and the mid-plane lithium projectile injection system were used to real-time inject lithium granules,boron powder and boron powder-lithium granules simultaneous injection in physical experiments on EAST.Specifically,real-time lithium granules injection from upper divertor X point mitigated type-I ELM and the effect of ELM on core impurities triggered by spontaneous ELM-free H-mode lithium granules injection was investigated;Obtain the effect of long pulse ELM The completely suppressed ELM was achieved by read-time boron powder injection;Under the condition that boron powder completely suppresses ELM,lithium granules injection achieved complete control of ELM frequency.The experimental results of real-time injection of lithium granules show that lithium granules injection can effectively mitigate type-I ELM.The size of ELM gradually decreases with the injection of lithium balls,but the ELM frequency does not change significantly during mitigation phase.Deeper analysis by ELITE codes found that during lithium granules injection mitigating type-I ELM,the pressure gradient and bootstrap current in the pedestal region were reduced and the operating point was away from the thresholds of peeling mode and ballooning mode,but it was closer to the threshold of the peeling mode instability threshold.Under spontaneous ELM-free H mode discharge conditions,the injection of lithium granules successfully triggered ELM,and the triggering success rate reached 100%.The impurities in the plasma core were effectively expelled out,and the core radiant power was significantly reduced,Finally the plasma energy increases about 10%.The experimental results of real-time boron powder injection show that real-time boron powder injection can achieve complete suppression of ELM,and the effective suppression time reaches 100 times the energy confinement time.There was no impurity accumulation phenomenon in core plasma during the ELM suppression period,and there was no significant reduction in edge recycling.Off-target of the inner target was achieved during ELM suppression.The injection of boron powder stimulates a kind of harmonic oscillation,which was located in the pedestal region of the plasma boundary,within the last closed magnetic surface.This harmonic oscillation has the ability to transport net particles,which is why there is no impurity aggregation during ELM suppression.In addition,there is a boron flow rate threshold for the suppression of ELM by boron powder.Only when the flow rate of boron powder is higher than this threshold,the complete suppression of ELM can be achieved.The study found that the flow threshold is related to the amount of the auxiliary heating power.The greater the heating power,the higher boron flow rate threshold was needed.The experimental results of simultaneous injection of boron powder and mid-plane lithium granules show that under the condition that boron powder completely suppressing ELM,the injection of lithium granules successfully triggers the ELM,and the triggering success rate reached 100%,achieving complete control of the ELM frequency(ie,the ELM frequency and the frequency of lithium granules injection is exactly the same),and the triggered ELM size is reduced by about 50%compared to the natural ELM size under the same plasma parameters.The presence of triggered ELM keeps the core tungsten impurities at a low level.In addition,there is a certain"competitive relationship" between the triggered ELM and the harmonic oscillation excited by the injection of boron powder,and the triggered ELM will weaken the harmonic oscillation.Through the use of boron powder and lithium granules injection to control ELM,the method of ELM suppression and mitigation has been broadened.Among them,real-time boron powder injection can achieve repeatable,"wide window" complete ELM suppression,which provides a new way for realizing high-parameter and steady-state operation of fusion devices in the future.In addition,when the boron powder completely suppresses ELM,the ELM is successfully triggered by lithium granules injection,which achieves complete control of the ELM frequency,and the size of the triggered ELM is reduced by 50%.It is a constructive preliminary exploration to avoid ELM disadvantages,and use its advantages.The physical mechanism of the above impurity injection for ELM control requires more in-depth simulation research.However,due to the complexity of the impurity injection process,it is difficult to directly simulate the ELM control by impurity injection at this stage.The fuelling particle injection is relatively simple,so this thesis used BOUT++to simulate the effect of fuel particle injection on plasma,and the simulation results were in good agreement with the experimental results,which laid a solid foundation for the future simulation study of impurity injection. |
