预填充模式下同轴枪放电等离子体加速模型分析与优化

Analysis and optimization of acceleration model in coaxial plasma gun in pre-fill mode

雪犁模型是描述预填充模式下同轴枪放电装置加速过程的主要手段,其能够直接给出枪内等离子体轴向速度与加速时间的解析关系式,因此在面向不同应用需求的装置结构设计中成为了重要的参考依据.通过理论推导及光、电、磁信号的测量,本文对比分析了不同充电电压与预填充气压条件下在加速阶段及喷口处等离子体的理论与实验速度,并给出了雪犁模型的优化方法.首先,用实测电流来替代近似的正弦电流,有效消除了磁压力计算误差对模型准确性的影响.其次,随着加速距离的延长,喷口处等离子体速度的饱和现象开始出现,摩擦阻力的存在是其主导的形成原因.相比于现有模型25.8%—53.6%的偏差范围,添加摩擦阻力项后不同电压和气压下的理论与实测速度的差值仅为3.1%—8.4%.

Snowplow model is the main method to describe the acceleration process of coaxial plasma gun in the prefill mode,which can directly give the analytical expression of plasma velocity versus time.So it has become an important reference in designing the device structures satisfying the requirements for different applications.Through measuring the current,magnetic and optical signals,the characteristics of current during the discharge and the motion of plasmoid are investigated.The variation of discharge current with time is close to the damping sine curve,which is an underdamping solution of RLC equivalent discharge circuit.The measured current in lieu of the sine one is used to calculate the theoretical velocity so as to eliminate the error of magnetic pressure.The variation of plasma velocity with discharge voltage and chamber pressure are consistent with those obtained by solving the equation of plasma motion under snow plough model,but the acceleration process is another story.At the initial stage of the discharge,owing to the low sweep efficiency,the plasma is accelerated to a higher velocity than the predicted one,the increase of voltage and the decrease of pressure further enhance the effect.With the extension of the acceleration distance,owing to the friction resistance between plasma and electrodes,the acceleration slows down and the velocity starts to fall below the predicated value,the saturation of plasma velocity at the nozzle is found.The friction resistance term is added to the equation of plasma motion.Compared with the deviation range of 26.8%–53.6%of the existing model,the differences between the theoretical speed and the measured speed under different voltages and pressures are in a range of 3.1%–8.4%after adding the friction resistance term into the equation of plasma motion.The optimization of snow plow model greatly improves the accuracy of velocity prediction,which can provide an effective reference for designing device structure and calculating energy efficiency.