Sheath characteristics in a magnetically filtered low density low temperature multicomponent plasma with negative ions

Abstract

Negative ion rich multicomponent plasma is produced in a novel plasma device for sheath studies. The experimental volume is composed of low electron temperature (Te∼0.2−0.4 eV) and low electron density (ne∼105−106 cm−3) argon (Ar) plasma with negative ions. Negative ions are introduced in the argon plasma by injecting sulfur hexafluoride (SF6) gas. Plasma parameters in the device are tunable, very close to the ionospheric plasma conditions specific to the D-region. A magnetic filter is used, which allows selective diffusion of low energy electrons and ions from a DC discharge source. Positive ion and electron/negative ion sheaths formed in front of a biased metal plate are investigated using an emissive probe. Both the positive ion and electron/negative ion sheaths are found to be of similar thickness and symmetric structure. With an enhanced negative ion concentration, the sheath thickness is found to decrease. Poisson's equation is derived considering the electrons as Maxwellian. The experimental results closely follow the prediction of Poisson's equation and the Debye sheath model. Sheath studies as shown here, for laboratory plasmas with near-ionospheric parameters, are important for the understanding of spacecraft-to-plasma interactions and for the design of ionospheric instrumentation.