A magnetohydrodynamic numerical model with external circuit coupled for pulsed inductive thrusters

Abstract

Pulsed inductive thruster, which employs pulsed inductive magnetic field to ionize propellant and accelerate a bulk of plasma, is accompanied with complicated phenomena such as plasma physics, magnetohydrodynamics and the strong coupling effect between the drive-circuit and plasma load. Simulations employing a snowplow circuit model or present magnetohydrodynamic model might be insufficient to capture these important phenomena simultaneously and self-consistently. Therefore the validity of currently existing numerical models remain to be verified. In this paper, a novel circuit-coupled magnetohydrodynamic model is proposed. The flow process of the plasma in the acceleration channel and the discharge process of the circuit are solved simultaneously in a bi-directionally coupled method by calculating the voltage drop across the drive-coil according to the drive-coil geometry and the temporal electric field distribution. The magnetohydrodynamic field is solved with Navier-Stokes equations coupled with Maxwell equations, while the plasma thermodynamic parameters and transport parameters are calculated by employing the local thermal equilibrium model. And the circuit process is solved with a set of circuit equations based on Kirchhoff's law. All the physics fields are computed by the finite element method in COMSOL MultiphysicsTM. Numerical simulation for American TRW Inc.'s MK-1 thruster successfully reproduces its working process. The numerical magnetic field distribution in plasma, the time-dependent collective Lorentz force and the specific impulse and efficiency of the thruster under varying working voltages agree well with the corresponding experimental data. Numerical results imply that a compact azimuthal plasma current sheet is established in the initial 1-2 s in the near-face region of the drive-coil. This plasma current sheet, which entrains the majority of the propellant, is excluded and accelerated by the Lorentz force derived from the drive-coil magnetic field. Most of the propellant acceleration is accomplished within the first half period of the circuit current, which is about 7-8 s. Furthermore, the bi-directional coupling effect is quantitatively analyzed with the current model. Numerical results indicate that the coupling plasma load generally tends to increase the effective resistance and reduce the effective inductance of the drive-circuit. Moreover, this effect changes as the plasma structure varies. When the plasma current sheet moves away from the drive-coil, the mutual inductance between plasma load and drive-coil decreases monotonically. That implys that the plasma current sheet decouples gradually from the dirve-circuit in the process. In conclusion, bidirectional coupling effect between plasma load and drive-circuit plays an important role in the operation of the thruster. This model could be used to predict the performances of pulsed inductive thrusters and might be helpful in designing a more effective thruster.