Experimental investigation of double layers in expanding plasmas

TL;DR

This study experimentally investigates the formation, stability, and characteristics of double layers in expanding plasmas across electropositive and electronegative gases, revealing their dependence on external parameters and magnetic fields.

Contribution

It provides new experimental insights into the behavior of double layers in expanding plasmas, including their formation conditions, stability, and voltage characteristics under various gas compositions and magnetic influences.

Findings

DLs form at low pressures with magnetic field influence in electropositive gases.

Voltage drop across DLs varies with pressure and electron temperature.

Electronegative DLs are sensitive to external parameters and propagate at high negative ion fractions.

Abstract

Double layers (DLs) have been observed in a plasma reactor composed of a source chamber attached to a larger expanding chamber. Positive ion beams generated across the DL were characterized in the low plasma potential region using retarding field energy analyzers. In electropositive gases, DLs were formed at very low pressures between 0.1 and 1 mTorr with the plasma expansion forced by a strongly diverging magnetic field. The DL remains static, robust to changes in boundary conditions, and its position is related to the magnetic field lines. The voltage drop across the DL increases with decreasing pressure, i.e., with increasing electron temperature around 20 V at 0.17 mTorr. DLs were also observed in electronegative gases without a magnetic field over a greater range of pressure 0.5 to 10 mTorr. The actual profile of the electronegative DL is very sensitive to external parameters and intrusive elements, and they propagate at high negative ion fraction. Electrostatic probes measurements and laser-induced photodetachment show discontinuities in all plasma parameters electron density, electron temperature, negative ion fraction at the DL position. The voltage drop across the electronegative DL is about 8 V, is independent of the gas pressure and therefore of the electron temperature.