脉冲感应式电推进中平面型线圈激励等离子体的流动特征分析

Analysis of Plasma Flow Characteristics under Excitation of Planar Coil in Pulsed Inductive Electric Propulsion

关键词:  感应等离子体  高温热力学  电流片  流动特征  数值计算

Key words:  Inductive plasma  High temperature thermodynamics  Current sheet  Flow characteristics  Numerical calculation

摘要:

感应等离子体推力器是一种具有较好应用前景的空间推进方式，受到复杂瞬态电磁场的影响，流场中热力学和动力学参数的时变特性难以实验测量。采用单流体磁流体力学理论，结合高温气体的热力学和输运模型，数值计算初始静止且无预电离的情况下，等离子体参数的二维分布，着重分析等离子体在前半周期内的流动特征。对内径r_i为0.05m，外径r_o为0.15m的线圈计算表明，其等离子体-线圈耦合距离约为0.032m，以Ar为工质情况下，有效冲量产生时，高价离子（Ar³⁺，Ar⁴⁺）与低价离子（Ar⁺，Ar²⁺）由于受电磁力影响不同，产生了局部分离电场，推动了电流片运动，且在电流片内部，前缘主要为低价离子，后缘主要为高价离子。单脉冲能量210.7J、峰值径向磁感应强度为0.5T、有效作用时间约12μs条件下，5μs时刻的Ar²⁺，Ar³⁺的最大数密度大于6×10²¹m^-3，且大于Ar⁺，Ar⁴⁺的数密度。电流片运动大于耦合距离后，受径向运动以及激励电流反转的影响，线圈表面等离子体磁场非线性特征显著，而前缘磁场维持规则分布。对外径为0.15m，0.3m和0.5m的线圈计算发现，r_o为0.5m时，电流片径向均匀长度达到0.3m，表明较大线圈尺寸除增加耦合距离外，可提高径向电流密度的均匀性。

Abstract:

Pulsed inductive thruster is a promising plasma accelerator for space propulsion. The thermodynamic and kinetic parameters of the flow are difficult to measure in experiment due to complex transient electromagnetic field. In this work, the two-dimensional flow characteristics of the plasma, especially in the first half period, are numerically studied, by employing the single fluid MHD theory, in which the high temperature thermodynamic and transport models are included. In the simulation, the initial gas is static and without ionization. The calculations find that, for the excitation coil of r_i=0.05m and r_o=0.15m, the decoupling distance is about 0.032m and the local electric field driving the current sheet is generated by high level ionization ions (Ar³⁺,Ar⁴⁺) and low level ions (Ar⁺,Ar²⁺ ) caused by the difference in electromagnetic force when Ar is used as propellant. The front of the current sheet is comprised of low order ions, while the back is of high order ions. Maximum densities of Ar²⁺，Ar³⁺ exceed 6×10²¹m^-3,which are greater than Ar⁺,Ar⁴⁺ at 5μs for single pulsed energy of 210.7J, peak radial intensity of 0.5T, period of about 12μs. As the current sheet moves far away from the coil, the magnetic field varies non-linearly on the coil surface and distributes regularly within the sheet. Analysis of the effect of coil dimension on the current uniformity suggests that, when r_o=0.5m, the radial uniform dimension can reach 0.3m, which is better than outer radius of 0.15m and 0.3m, which shows that as coil dimension increases, the uniformity of the current sheet in radial direction is enhanced, besides increasing the decoupling distance.