Experimental Study On The Characteristics Of Electromagnetically Driven Plasma Jets And Spheromaks In A Coaxial Gun

The coaxial gun high-current pulsed discharge can generate plasma with ultra-high velocity,high density and high energy density,which has potential applications in the fields of plasma space propulsion,dust particle acceleration and nuclear fusion.With the increasingly prominent energy crisis and environmental problems,developing fusion energy and exploring its engineering application are of great importance for solving energy problems.Magnetic Inertial Confinement Fusion(MIF)combines the long-time confinement of Magnetic Confinement Fusion(MCF)with the high-density feature of Inertial Confinement Fusion(ICF).It reasonably avoids the problem that some parameters of MCF and ICF are too high,and it is a flexible fusion scheme with reaction conditions between MCF and ICF.In the research of MIF scheme based on plasma collision and merging,the magnetized target is formed by spheromaks collision,and then the fusion is realized by inertial compression of multiple plasma jets.As the plasma generating device in this scheme,coaxial gun discharge can generate spheromaks and plasma jets.The process of plasma generation by coaxial gun is divided into acceleration stage and ejection stage.The physical process of MIF is directly affected by plasma parameter selection and optimization.However,the effects of different electrode structures,discharge modes and discharge parameters on plasma formation,ejection,and their correlation are still currently unclear.In this view,a coaxial gun discharge platform and diagnosis system are independently developed in this dissertation.The plasma characteristics of coaxial gun discharge under static gas-filled are studied,and the effects of different discharge conditions on plasma parameters are analyzed.The discharge process and spheromak formation in magnetized coaxial plasma gun are studied.The characteristics of plasma jet and spheromak generated by coaxial gun are compared whether a bias magnetic field is applied or not,and the interaction mechanism between plasma and magnetic field is analyzed.The details are as follows:In chapter 1,the application background of coaxial gun discharge plasma in the field of magnetized inertial confinement fusion is introduced.The structure design,discharge principle and discharge modes of the coaxial gun are reviewed.The overseas and domestic research status of coaxial gun plasma jet and spheromak generation technology are analyzed,and the motivation of this research is clarified.In chapter 2,the self-built coaxial gun high-current pulsed discharge experimental platform is introduced in detail.Combined with the discharge sequence control,the technical scheme of generating plasma with different characteristics is given.Photodiodes and highspeed camera are used to measure the optical properties of the plasma.High-voltage differential probe and Pearson current probe are used to measure the electrical properties of the discharge circuit,and the magnetic probe is used to measure the current distribution inside the gun and the plasma magnetic field characteristics outside the gun.The diagnostic methods of velocity,momentum,plasma electron density and temperature are described,the manufacturing principle and application method of magnetic probe,momentum measurement system and triple probes are described.In chapter 3,the effects of different discharge parameters on the internal current distribution,plasma current sheet motion and its external transport characteristics of coaxial gun in snow-plow mode are studied.The results show that the high-current pulse discharge of coaxial gun is a rapid energy release process,and the discharge circuit is a typical RLC oscillation circuit.The plasma is completely ionized during the discharge process.The resistance and inductance of the plasma account for a small proportion compared to that of the outer circuit and have little effect on the discharge loop.Discharge parameters have little effect on the circuit current oscillation characteristics.During the process of primary discharge,the plasma moves forward in the form of a current sheet,and is prone to secondary breakdown under high current and low pressure.Under high current,the plasma pressure rises at the moment of breakdown,resulting in a greater number of charged particles remaining near the insulation layer,forming a low impedance path.When the plasma is ejected from the muzzle,it is easy to build a new current channel at the bottom of the gun.Under high pressure,the number of neutral particles remaining in the gun grows due to leakage of the current sheet,and the impedance increases,inhibiting the formation of a new current channel at the bottom of the gun during discharge.The plasma velocity is a function of the neutral gas density and the magnitude of the current at different times,it is proportional to the magnitude of the discharge current and inversely proportional to the square root of the neutral gas density.The axial transport distance of the plasma after ejection primarily determined by the axial kinetic energy of ions.In chapter 4,the discharge process of the magnetized coaxial plasma gun and the characteristics of the ejected spheromak are studied.It is found that primary discharge and secondary discharge belong to two different discharge modes.During the first half-cycle of the current,the plasma sweeps the neutral particles ahead in the form of a current sheet,acting as a snowplow mode.The plasma carries all the discharge current,resulting in a single spheromak.In the second half-cycle of the current,the diffuse distribution of current paths indicates a deflagration mode.The Lorentz force on the leading plasma decreases,preventing it from overcoming the tension of the bias magnetic field.Thus,the plasma produced by the secondary discharge is restrained inside the gun and only a single spheromak is ejected during the entire discharge.Under the discharge conditions satisfying the formation threshold of the spheromak,the ejected spheromak is a magnetized plasma ring with an axisymmetric magnetic field structure,the toroidal magnetic field and the poloidal magnetic field are approximately equal and symmetrically distributed.The variation of spheromak velocity with the discharge current and the gas-puffed mass is consistent with the "snow-plow model".The spheromak velocities are in the range of 44 to 90 km/s when the discharge currents and gas puffed masses varied from 183 to 253 k A and 0.46 to 0.61 mg,respectively.The spheromak’s toroidal and poloidal magnetic fields both increase with discharge current,reaching a maximum of 0.2 T.The analysis suggests that a toroidal electric field is induced when the plasma passes through the radial magnetic field,which changes the plasma current direction from poloidal to toroidal,thereby introducing an additional poloidal magnetic field.Increasing the discharge current leads to an increase in plasma velocity and the additional poloidal field,resulting in a better field constraint.In chapter 5,the influence of the bias magnetic field on the discharge characteristics and plasma parameters of the coaxial gun under different discharge conditions is studied.The results show that depending on whether a bias magnetic field is applied during the discharge of a coaxial gun,two types of plasma with different ejection characteristics,spheromak or jet,are generated.During the ejection of plasma jet,the current path at the inner electrode is stretched by the axial Lorentz force,and the axial component of the current increases,resulting in a pinch effect of the plasma at the center.During the formation of spheromak,when the accelerated plasma compresses and stretches the radial magnetic lines,the axial magnetic field is enhanced,and the charged particles cannot pass through the magnetic lines to connect with the inner electrode head,thus eliminating the pinch effect.Comparing the plasma parameters with and without the bias magnetic field,it is found that the velocity and momentum trends of the plasma jet are consistent with the snow-plow model.The bias magnetic field slows down the axial motion of plasma,resulting in a spheromak with lower velocity and momentum than the jet.For the same discharge conditions,the plasma jet has a higher electron density but a lower electron temperature than the spheromak.The analysis suggests that there is a pinch effect during the ejection of the jet,which leads to a further increase in the plasma density.During the formation of spheromak,the stretched magnetic lines prevent the plasma current path from concentrating at the head of the inner electrode to avoid the pinch effect.The electron density of the spheromak is lower than that of the jet.In the case of a bias magnetic field,the plasma loses part of the axial kinetic energy during its interaction with the magnetic field and converts it into magnetic energy,which is dissipated by heating the plasma through magnetic reconnection.Therefore,the electron temperature of the spheromak is higher than that of the jet.