Comparative study of positive and negative pulsed discharge plasma characteristics of coaxial gun

Abstract

The coaxial gun plasma generated by pulsed discharge possesses the characteristics of high speed and high density, and has potential application value in the field of fusion, space propulsion and astrophysics. In this paper, the effect of positive and negative pulsed discharges on plasma characteristics are investigated and a theoretical model for analyzing the morphology of positive and negative pulsed current sheets is proposed. Positive and negative pulsed discharges are realized by changing the direction of the rectifier diode in the pulse power supply to change the direction of the recharging current. Through theoretical analysis, and measurements by using photodiode, Pearson probe, magnetic probe, HD camera, fast-framing camera and RGB image processing, the plasmas generated by positive and negative pulsed discharges are compared and investigated. Most of experimental diagnoses concentrate on investigating the plasma behavior in the coaxial gun muzzle on a microsecond-order time scale. Because radial and axial transport characteristics of plasma change little, we think, the plasma characteristics in the muzzle still depend on the characteristics of plasma in the coaxial gun. Therefore, the conclusion of the theoretical analysis of the current sheet in the coaxial gun is still valid for the plasma in the muzzle. The theoretical analysis shows that the positive pulsed current sheet presents a parabolic shape and the negative pulsed current sheet displays a convex shape, which makes the negative pulsed current sheet sweep more efficiently and a large amount of plasma is concentrated near the inner electrode, namely the cathode, so the negative pulsed plasma is denser. For the positive pulsed plasma, near the inner electrode the plasma is thin and the magnetic pressure is powerful, and near the outer electrode, the plasma is dense and the magnetic pressure is weak. Therefore, the positive pulsed plasma is faster in movement speed but easier to split, and because of its dispersion, its transport stability is not so good as that of the negative pulsed plasma. The experimental results accord with the theoretical analyses. The final conclusion shows that under the same discharge parameters, the positive pulsed discharge produced plasma is faster in movement speed but more likely to split, and the negative pulsed discharge created plasma is denser in density and more stable. Therefore, for obtaining a higher density plasma, the negative pulsed discharge is recommended, and for achieving a high-speed plasma source, the positive pulsed discharge is advised to be adopted.