Study On Positive And Negative Pulse Discharges Of High-Current Plasma In A Coaxial Gun

The coaxial plasma gun can generate plasma with high directional energy and dense electron density,which has great applications in simulated experments of space debris protection,plasma-material interaction,Tokamak fuel injection and magneto inertial fusion.Therefore,no matter for fundamental research and applicaitons,it is pivotal to clarify the involved mechanism and understand how to enhance the performance of the coaxial gun plasma.In previous work,the plasma parameters were mainly controlled by the structure and size of the coaxial gun,but for a fixed pair of coaxial electrodes,the pulse polarity also posses important effect on the plasma characteristics,besides charged voltage and methods of gas injection.However,only a few experimental results were observed so yet,in order to further understand the effect of pulse polarity,the plasma characteristics in coaxial gun between positive and negative pulse discharges was investigated from both theoretical and experimental perspectives.Positive and negative pulse polarity has different effect on the spatial distribution of ions and electrons,one of the most direct impact is the shape of the current sheet.At first,equilibrium equation was established by magnetic pressure and plasma pressure.Then the ideal shapes of positive and negative pulse current sheet were obtained through analyzing the equilibrium states.Finally,it was found that the negative pulse current sheet has a higher sweep efficiency and more compact axial distribution,ions are mainly distributed near the inner electrode.Compared to the positive pulsed plasma,negative pulsed plasmas have a denser electron density but a slightly slower velocity.Two photodetectors with different axial positions were used to observe the plasma shape.At the same time,calculating the velocity of the coaxial gun plasma by the climbing points of the two photocurrent signals.Then the result shows that the positive pulsed plasma is faster under the same conditions.As for the electron density,it was obtained through calculating the Stark broadening of the H_?line which detected by a spectrograph.And then it was found that in the same conditions,the electron density of negative pulsed plasma is denser.In addition,the distribution of positive and negative pulsed plasma in the transport channel,the transport state,the shape of the current sheet and the central bright column in the process of negative pulsed discharge were observed by HD cameras and fast-framing camera.And it is proved that the transport state of negative pulsed plasma is more stable and the axial distribution is more compact,which is consistent with the theoretical analysis.Moreover,pulse repetition periods and peak currents of positive and negative pulse discharges with different Electrode erosion conditions had been compared,and the result shows that electrode erosion will decrease the energy efficiency of plasma and result in a decrease in plasma velocity and electron density.The influence of different polarity discharge on plasma is due to the shape of current sheet and ions distribution.On the one hand,ions are mainly distributed near the outer electrode in the positive pulse discharge,and the plasma near the inner electrode is thinner,so it is easier to be accelerated,which leads to faster velocity.On the other hand,the axial distribution of the negative pulse current sheet is more compact,and the ions are mainly distributed near the inner electrode,which leads to higher ionization efficiency,denser density and better transport stability.