Experimental Study On High Current Pulsed Discharge Plasmas In Coaxial Guns And Its Damage Effects On Tungsten Materials

Plasma wall interaction（PWI）will occur inevitably during the operation of Tokamak.PWI will damage plasma face materials（PFMs）,which affect the performance of PFMs and shorten its lifetime.Tungsten is one of the most promising PFMs in Tokamak because of its high melting point,high thermal conductivity,low sputtering yield and low tritium retention.The coxial gun device has low operation cost and parameters of plasma produced by the gun are close to those of type I ELM,therefore,the coaxial gun is suitable in simulation experiment as heat-flux sources of transient events such as type I ELM under the condition of H-mode in ITER.Such simulation research has not been carried out with coaxial gun in China.Discharge parameters and electrode structure of coaxial gun need to be further optimized,the characteristics of plasma and damage effect on target need to be deeply studied.In view of this,a coaxial gun high current pulsed discharge plasma generator and diagnosis system are built in this dissertation.The characteristics of plasma in deflagration mode discharge are studied,and deflagration mode discharge and pre-filled mode discharge are compared in experiment.The plasma characteristics of tapered gun discharge are studied with helium and hydrogen.The process of plasma thermal shock on target surface is analyzed,and the damage effect of target after thermal shock is deeply studied.The details are as follows:In chapter 1,the background and significance of using coaxial gun as plasma source in simulation experiment for type I ELM under the condition of H-mode in ITER are introduced.The discharge principle,development and discharge mode of coaxial gun are summarized.The research status of coaxial gun at home and abroad is analyzed,and the research purpose of this dissertation is clarified.In chapter 2,a coaxial gun high current pulsed discharge plasma generator is introduced.The cylindrical gun and tapered gun are designed independently.Cylindrical gun is used to analyze the characteristics of discharge plasma,and tapered gun is used in simulation research for type I ELM.The optical signal of plasma is measured by photodetector,and spectral line of plasma is measured by spectrometer.The measurement methods of velocity and electron density of coaxial gun discharge plasma,the manufacturing principle and application method of magnetic probe and calorimeter are described.The equipment used to study the damage effect,such as metallographic microscope,scanning electron microscope（SEM）,energy dispersive spectrometer（EDS）and X-ray diffractometer（XRD）,are introduced.In chapter 3,the characteristics of deflagration mode discharge plasma are studied.It is found that the total discharge loop impedance is dominated by the linear impedance of external circuit,the plasma nonlinear impedance effect is very small in discharge loop,the current waveform is a damped quasi sine wave.When there are multiple breakdown discharges,the change of plasma impedance has an effect on the current waveform and magnetic probe signal.Under condition of same charging voltage in deflagration,the discharge circuit current is similar,and Lorentz force on plasma is basically same,more neutral particles will ionize into plasma with increase of gas mass,so the electron density of the plasma increases and the axial velocity decreases.The increase of gas mass leads to increase of breakdown voltage at bottom of coaxial gun during discharge,so phenomenon of multiple discharges gradually disappears.With the increase of charging voltage,Lorentz force on plasma increases,and more neutral particles ionize into plasma,so axial velocity and electron density of plasma increase both.With the increase of charging voltage,the voltage at the bottom of coaxial gun increases in discharge process,therefore,multiple discharges are more likely to occur.With the increase of discharge cycle,phenomenon of multiple discharges gradually occur at bottom of coaxial gun.At same charging voltage,the loop current curves of pre-filled mode and deflagration mode discharge are similar.In pre-filled mode,plasma will collide and ionize the neutral particles in front,and there is viscous resistance between plasma and electrodes.Therefore,at same charging voltage,axial velocity in pre-filled mode is lower than that in deflagration mode,and electron density in pre-filled mode is higher than that in deflagration mode.In chapter 4,plasma parameters of high current pulse discharge in tapered coaxial gun at different charging voltage and pressure and its damage effect on tungsten target are studied and analyzed.It is found that the axial velocity and electron density of plasma increase with charging voltage.When charging voltage is 16 k V and the helium pressure in vacuum chamber is 50 Pa,the energy density at center of plasma is 1.4 MJ/m²,the power density is 22 GW/m²,and the heat flux factor is 170 MWs^1/2m^-2.When pressure rises from 10 Pa to 50 Pa in same charging voltage,single ion kinetic energy of plasma gradually decreases,while the electron density gradually increases,resulting in increase of energy density.When pressure reaches 70Pa,the collision frequency in plasma increases,energy loss increases,single ion kinetic energy and electron density of the plasma decrease,resulting in gradual decrease of energy density Plasma will ablate whole target surface due to thermal diffusion.The cumulative energy in the ablation process is the highest within 2 cm of center diameter of target surface,and there are obvious ablation marks,which is equal to the muzzle diameter of tapered gun.It can be found that the target surface after thermal shock is mainly tungsten by observing energy dispersive spectrum,which is due to the low melting point of copper.Copper melts and vaporizes during thermal shock,so it does not remain on the target surface.there areμm-order cracks and craters at center of target surface.The formation of craters is due to local heating when plasma thermal shock on target surface.Plasma thermal shock on tungsten target will cause mass loss to tungsten target.In chapter 5,the characteristics of pulsed gas pre-filled discharge plasma in tapered coaxial gun are studied.When charging voltage is 16 k V and gas mass is 1.56 mg,the plasma velocity at target can reach 41.7 km/s.The single ion kinetic energy is 9.2 e V,which is far less than the sputtering threshold of tungsten.The energy density at plasma center is about 1.5 MJ/m²,the power density is 30 GW/m²,and the heat flux factor is 212.1 MWs^1/2m^-2.According to the plasma spectrum,there are a small amount of copper ions in the plasma except hydrogen ions.The damage effect of plasma on tungsten target after 20 pulse thermal shock at different angles is analyzed.Droplets splashing and vaporization occur on the surface of target in process of plasma thermal shock on target surface,which results in mass loss of the target.With decrease of angle,area of plasma thermal shock on target increases and energy accumulation per unit area decreases.The heating and cooling process of target surface is intensem,and there is a large temperature gradient on target surface and within a certain depth,resulting in large residual stress on the surface.The damage of plasma to target includes cracks,craters,cross-section faults,mass loss and variation of surface residual stress.These damage are mainly due to increase of target temperature caused by plasma heat load.