| Magnetized Target Fusion(MTF) is the most developed technology in MagnetoInertial Fusion concept. There are three main steps in MTF: a magnetized equilibriumtarget plasma is firstly preheat and compressed in the formation step and then betranslated to the compression region in the translation step, the magnetized target plasmais compressed by implosion in a solider liner in the compression step. Field ReversedConfiguration(FRC) is an ideal approach for magnetized target plasma, in FRC, Pre-ionization(PI) plasma is formed by discharging the static low pressure hydrogen. So far,the insufficient of PI plasma density and lifetime are one of the main difficulties for MTFprogram. In our research,we adopted numerical simulation method to study the gasbreakdown process in PI plasma generation and to provide optimized dischargeparameters for MTF experiment.The electron in PI plasma is strongly nonlocal and non-thermalequlibrium, so generalFluid model is not suitable for simulating PI Plasma. It is appropriate to adopt Particle incell/Monte Carol(PIC/MC) model in this research, and the excitation and ionization ofneutral gas must be calculated self-consistent. In addition, the compression step must bedeal with Two-fluid model and the implosion step must be deal with Radiate Fluid model.In our simulation,1D/2D implicit PIC/MC code are implemented with the parameters ofthe FRC device to study how different bias magnetic field，inductive electric field andgas pressure influence the density, temperature and lifetime of the PI plasma. We assumedthat the bias magnetic field was uniform in space and the electromagnetic field generatedby plasma could be ignored in PI phase, neutral gas was discharge by vortex electric fieldonly. We adopted electrostatic and magnetostatic model. Periodic and Dirichlet boundarycondition are employed in our1D planar case and2D cylindrical case(Zdirection),respectively. For simplicity, we adopted Ar gas discharge instead of Hydrogendischarge.Simulation parameters of1D case are as follows:0~0.5T bias magneticfield,0~100kV/m electric field,10~100mTorr gas pressure and the length is30cm.Benchmark parameters are0.2T,100kV/m and20mTorr. It was found that higher bias magnetic field led to longer plasma lifetime but lower plasma density; higher electricfield and gas pressure led to lower lifetime and higher density. That means plasma lifetimeand density are hardly to reach the scheduled goal at the same time, and our simulationgives the optimal interval of discharge parameters.In2D simulation, Type B/Type A are two different models in which neutral gasdepletion are considered/not considered, respectively. Parameters are as follows:0.1~0.5T bias magnetic field,0~200kV/m electric field,20~100mTorr gas pressure. Wesimulated the time evolution of electron density, electron temperature, electric potential,ampere density and so on. It was found in Type A model that: PI plasma density increasedfrom1017m-3to1022m-3in60nanoseconds and electron temperature increased from3eV to200~300eV. For a given bias magnetic field, there is a minimum breakdownelectric field. Higher bias magnetic field required higher breakdown electric field. Inaddition, higher electric field led to faster discharge and shorter discharge period. It wasfound in Type B model that: PI plasma firstly emerged near the wall and then oscillatedbetween the wall and axis. Higher bias magnetic field led to longer plasma period ofoscillation and shorter plasma lifetime near axis; higher electric field and gas pressure ledto shorter plasma period of oscillation and longer plasma lifetime near axis.Our simulation provide a theoretical foundation for selections of dischargeparameters in FRC experiments. In future work, we will implement our simulation inrealistic geometries of FRC devices, and to calculate the whole discharge process in FRCwe will generalize our code from electrostatic and magnetostatic model to fullyelectromagnetic implicit PIC model. Monte Carol and neutral gas model for Hydrogenwill also be developed. |
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