Low-density plasmas generated by electron beams passing through silicon nitride window

Abstract

In general, more attention is paid to how to improve the characteristic parameters of plasma in plasma applications. However, in some cases, it is necessary to produce plasma with low-electron density, such as in the laboratory simulation of ionospheric plasma in space science. In this study, a low-density plasma is generated by electron beams passing through a silicon nitride transmission window under low pressure condition. The transmission properties of electron beam passing through silicon nitride films are investigated by Monte Carlo simulation, and the plasma feature is studied by a planar Langmuir probe and a digital camera. It is found that the plasma exhibits a conical structure with its apex located at the transmission window. At a constant pressure, the cone angle of conical plasma decreases with the electron energy increasing. This is qualitatively consistent with the Monte Carlo simulation result. The frequency of electron-neutral collisions increases as the working pressure rising, which leads the plasma cone angle to increase. When the beam current is reduced from 10 μA to 0.5 μA at 40 keV, the electron density decreases, in a range between 10⁵ and 10⁶ cm^–3, while the electron temperature does not change significantly but approaches 1 eV. It can be inferred that the electron density decreases with the distance <i>z</i> from the transmission window in the incident direction of the electron beam. A low-density plasma of less than 10⁵ cm^–3 can be obtained further away from the transmission window.