Numerical Simulations Of Negative Hydrogen Plasma Sources With Global Model And Experimental Validation

Negative Hydrogen Ion Sources(NHIS)play a critical role in future neutral beam injection for fusion which is an efficient method for heating plasma and driving plasma currents in fusion reactors.The neutralization efficiency of negative hydrogen ions remains acceptable for high beam energy.One of the key issues on the NHIS is how to optimize the production of negative hydrogen ions.A verified and validated model is required to reveal the mechanism of volume production of negative hydrogen ions and to further improve the NHIS performance.Compared with fluid simulation and particle in cell simulation,global model is more suitable for simulating hydrogen discharges with complicated chemical kinetics,because it ignores spatial variations of all plasma properties.In addition,global model can fast evaluate the importance of different reactions and further simplify the chemical kinetics.In low-pressure regime of NHIS,the electron kinetics is non local.The production of negative hydrogen ion depends on electron kinetics,however,the global model does not cover the electron kinetics.Therefore,the coupling of different models and the combination with experiments will be helpful for studying the characterization of NHIS.In Chapter 1,the background as well as simulated and experimental research progresses of the NHIS is introduced,and the related issues are analyzed.Finally,the main research contents in this thesis are presented.In Chapter 2,a global model coupled with the electromagnetic field equations and the linearized electron Boltzmann equation,is developed to study the effects of RF frequency,coil current and gas pressure on the distributions of induced electric field and power density,and the anomalous skin effect is revealed.The simulation results show that the power absorption by the plasma can be significantly improved at low RF frequency,gas pressure,and coil current.This is because a lower plasma density under these conditions leads to a deeper penetration of electric field into the plasma.In addition,the simulated electron density and electron temperature achieve reasonable agreement with experimental results.In Chapter 3,a global model with an arbitrary Electron Energy Probability Function(EEPF)has been developed to study the effect of EEPF on the plasma parameters.The hydrogen atom heating equation is coupled into the global model.The bi-Maxwellian EEPF with more low energy electrons increases the production of negative hydrogen ions in the low-pressure regime.The contributions of different reactions to the production and destruction of different vibrational states are evaluated under two different EEPFs.The simulated electron density and electron temperature agree well with other simulations and experimental results.In Chapter 4,both benchmarking and validation of Global Model for Negative Hydrogen Ion Source(GMNHIS)are performed.The simulated densities of all species and electron temperature using the GMNHIS are in good agreement with the results calculated by the other global model.The validation of the GMNHIS is performed by comparing the simulated negative hydrogen ion density with the experimental measuments operated at an electron cyclotron resonance plasma source.The simulation and experimental results are in qualitative agreement.The saturation of negative hydrogen ion density as a function of pressure is mainly due to the saturation of cold electron density that surpresses the dissociative attachment.In Chapter 5,complicated vibrational kinetics is classified and is simplified through evaluating the importance of them using the global model.The simplified model for fast calculation of Vibrational Distribution Function(VDF)is deduced under the assumption that the probabilities of vibrational states of hydrogen molecules deexcited to any lower states are equal.The VDF is almost independent of the assumption of the averaged repopulation probability.The VDF calculated using the simplified model agrees well with the results obtained by the GMNHIS.Finally,the main conclusions,innovations and prospects of this dissertation are presented.