Spatially- and vector-resolved momentum flux lost to a wall in a magnetic nozzle rf plasma thruster

Kazunori Takahashi,Akira Ando,Takeharu Sugawara

doi:10.1038/s41598-020-58022-6

Abstract

Most of the artificial low-pressure plasmas contact with physical walls in laboratories; the plasma loss at the wall significantly affects the plasma device performance, e.g., an electric propulsion device. Near the surface of the wall, ions are spontaneously accelerated by a sheath and deliver their momentum and energy to the wall, while most of the electrons are reflected there. The momentum flux of the ions is a vector field, i.e., having both the radial and axial components even if the azimuthal components are neglected in a cylindrical system. Here the spatially- and vector-resolved measurement of the momentum flux near the cylindrical source wall of a magnetic nozzle radiofrequency (rf) plasma thruster configuration is successfully demonstrated by using a momentum vector measurement instrument. The results experimentally identify the spatial profile of a non-negligible axial momentum flux to the wall, while the radially accelerated ions seem to be responsible for the energy loss to the wall. The spatial profiles of the radial and axial momentum fluxes and the energy lost to the wall are significantly affected by the magnetic field strength. The results contribute to understand how and where the momentum and energy in the artificial plasma devices are lost, in addition to the presently tested thruster.

Highlights

Most of the artificial low-pressure plasmas contact with physical walls in laboratories; the plasma loss at the wall significantly affects the plasma device performance, e.g., an electric propulsion device
One of the experiments relating to the magnetic nozzle rf plasma thruster have implied that non-negligible axial momentum flux is lost to the radial source wall of a high density helicon source[31], which has been discovered by the direct measurement of the total axial force imparted to the cylindrical source tube, while it has been assumed to be negligible in previous theories for simplification[12,13,14]
The momentum vector measurement instrument (MVMI) similar to the previous bench test[34], which is described in ‘Method’ section, is mounted on an axially movable motor stage immersed in vacuum and a 20-mm by 30-mm detector plate facing the radial center is inserted into the source tube

Summary

Introduction

Most of the artificial low-pressure plasmas contact with physical walls in laboratories; the plasma loss at the wall significantly affects the plasma device performance, e.g., an electric propulsion device. It is crucial to understand and characterize the momentum transport, conversion, gain, and loss mechanisms for clarifying the structural formation of plasmas such as the astrophysical jets[1], the particle acceleration in auroras at the Earth and Jupiter[2,3], the coronal mass ejection from the Sun[4], the interaction between the plasmas and the geomagnetic field[5], and so on In these naturally appearing plasmas, the processes involving the momentum and energy transfer occur due to the interaction with electromagnetic fields, collisions, turbulences, and so on, since they cannot see any physical boundaries in space except for the surface of planets. The energy loss to the wall is assessed from the measured momentum fluxes and the ion current; implying the reduction of the energy flux to the wall by the magnetic field These data are significantly important to understand the performance degradation mechanisms and to lead to a high performance plasma thruster

Methods

Results

Conclusion