同轴枪放电等离子体电流片的运动特性研究

同轴枪放电可以产生高速度、高密度的等离子体射流,在天体物理、核物理等研究领域具有广泛的应用.基于同轴枪放电等离子体运动的"雪犁模型"分析,本实验通过对等离子体光电信号和磁信号的测量及放电照片的拍摄,研究了不同放电电流和气压对同轴枪放电等离子体电流片的运动特性、电流通道分布的影响.实验结果发现:一次放电过程中,气压为10 Pa、放电电流为35.7—69.8 kA时,随着放电电流的增加,等离子体喷射速度增加,输运距离与离子携带的轴向动能成正比,大电流条件下,等离子体喷出枪口时易于在枪底端形成新的电流通道;气压为5—40 Pa、放电电流为49.8 kA时,随着气压的增加,等离子体喷射速度减小,输运距离缩短,高气压下,等离子体喷出枪口时在枪底端未产生新的放电通道,这与放电过程中遗留在枪底端的带电粒子和电流片渗漏残留在枪内的中性粒子共同形成的阻抗通道有关;电流反向时,二次放电击穿位置发生在电极头部,放电过程中存在多次放电现象.

Motion characteristics of coaxial gun discharge plasma current sheet

The coaxial gun discharge, used as plasma jet with high density and velocity, has a wide variety of applications such as plasma space propulsion, simulation experiment of thermal transient events in the International Thermonuclear Experimental Reactor, plasma refueling for fusion reactors and a laboratory scale platform for studying astrophysical phenomena. The plasma produced in the coaxial gun can be accelerated by self-induced Lorentz force, and the ionization in the transport process can be based on “snow-plow model” in which a coaxial current sheet moves forward and sweeps a large amount of the gas between two electrodes to cause the plasma dump. Based on the measurements of discharge current, voltage, photocurrent and magnetic signal, the experimental investigation on the characteristics of plasma motion and current sheet channel distribution in the gun operated under different discharge conditions and various pressures is carried out. In this paper, it is emphasized to explore the electrical and dynamic properties about plasma in the first half-cycle of current. The results show that the plasma velocity increases with the increase of the current amplitude, and that the transport distance is proportional to the axial kinetic energy of ions when the pressure is fixed at 10 Pa and discharge current is adjusted from 35.7 kA to 69.8 kA. Moreover, in the case of high current, the plasma jet from the nozzle tends to form a new current path at the bottom of the gun. However, when the discharge current is fixed at 49.8 kA and the gas pressures range from 5 Pa to 40 Pa, the plasma motion velocity and transport distance are continuously reduced. Meanwhile, it is not found that new current paths are generated at the bottom of the coaxial gun under high pressure. The generation of the new current path is relevant to the channel impedance formed by more charged particles left at the bottom of the gun and neutral particles leaking from current sheet during discharge. Besides, a multiple discharge phenomenon is presented in experiment and the secondary discharge breakdown position occurs at the head of the electrode when the current is reversed to a positive value. Therefore, this study provides a reasonable choice of electrical parameters to obtain optimal plasma characteristics during the discharge of the coaxial gun.