平行轨道加速器等离子体动力学特性研究

等离子体电磁加速器可产生高速度、高密度等离子体射流而广泛应用于核物理、天体物理等领域.本文通过光电二极管、磁探头研究了不同放电电流和初始气压条件下等离子体在平行轨道加速器内的轴向运动特性.通过电流截断的方法,采用冲击摆测量了首次等离子体射流的动量.平行轨道加速器驱动电源由14级脉冲形成网络组成,每级电容为1.5μF,每级电感约为300 nH,得到振荡衰减型方波的电流波形.实验发现,电流过零时,轨道起始处一般会发生二次击穿,并形成二次轴向运动的等离子体.放电电流为10—55 kA、初始气压为200—1000 Pa时,等离子体的轴向速度为8—25 km/s.实验获得的等离子体的运动速度为雪犁模型理论结果的60%—80%,这主要是理论模型忽略了电极表面对电弧的黏滞阻力以及电极烧蚀引起的质量增加.等离子体动量与电流的平方随时间的积分成正比.放电电流为21-51.6 kA时,首次等离子体射流的动量为1.49—9.88 g·m/s.等离子体运动过程中除了受到洛伦兹力外,还会受到电极表面的黏滞阻力,造成等离子体动量约为理论结果的75%.

Plasma dynamic characteristics of a parallel-rail accelerator

Electromagnetic plasma accelerators which can produce plasma jets with hypervelocity and high density have been widely used in the fields of nuclear physics and astrophysics. Parallel-rail accelerator, a type of electromagnetic plasma accelerator, is usually used to generate high density and compact plasma jets. The axial movements of plasma in a parallel-rail accelerator operated at different discharge currents and initial pressures are reported in this paper. Based on current truncation, the momentum of the first plasma jet is measured by a ballistic pendulum. The axial movement characteristics and velocity of the plasma during the acceleration phase are diagnosed by magnetic probes and photodiodes. The accelerator is powered by 14 stage pulse forming networks. The capacitor and inductor in each stage are 1.5 μF and 300 nH respectively, yielding a damped oscillation square wave of current with a pulse width of 20.6 μs. Plasma sheath is formed upon breakdown at the back wall insulator surface and subsequently accelerated by Lorentz force towards the open end of the accelerator. A secondary breakdown generally occurs at the starting end of the rail when the current reverses its direction, and then a secondary axial movement of plasma is formed. We focus on the first plasma jet accelerated by the first half-cycle of current. According to the snowplow model, the plasma velocity is proportional to the current and is inversely proportional to the square root of gas initial density or pressure. The axial velocity of the plasma is in a range from 8 km/s to 25 km/s when the discharge current is varied from 10 kA to 55 kA and the initial pressure is varied from 200 Pa to 1000 Pa. The experimental results show that the experimental velocities of the plasma are about 60%-80% of the theoretical result. It is likely that the viscous resistance of the electrode surface acting on the plasma and the mass increase of plasma caused by the electrode ablation are neglected in the snowplow model. The momentum of the first plasma jet is nearly proportional to the integration of the square of current over time, which is consistent with the predictions of the theoretical model. The maximum momenta of plasma jet at different currents appear at average velocities ranging from 13 km/s to 14 km/s when the plasma just moves to the outlet of the rail in the end of the first current pulse. The measured momentum of plasma jet is actually the total momentum of the truncated current waveform. The ratio of the momentum of the first plasma jet to the total measured momentum is about 87%. The momenta of the first plasma jet are in a range from 1.49 g·m/s to 9.88 g·m/s at discharge currents ranging from 21 kA to 51.6 kA. The experimental plasma momentum is about 75% of the theoretical result. These results show that the viscous resistance of rail electrode surface is about 25% of the Lorentz force, and thus leading to a lower value of plasma momentum.