Experimental investigation of double layers in expanding plasmas

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.

The influence of magnetic-field gradients and boundaries on double-layer formation in capacitively coupled plasmas

“…case 1A refers to setup 1, configuration A. Case 1A will be used to reproduce prior works 11,14,16,12,13,15,17 , since the MFG is located close to the diameter change. Case 2B is quiet similar to case 1A, therefore, it is possible to study the influence of the distance to the source.…”

“…However, DLs observed in space plasmas are usually current free. Although, a number of theoretical 7,8,9 and experimental 10,11,12,13,14,15,16,17 studies of these current-free DLs have been performed in the last decade, the exact formation mechanism is still under debate 18 . Nevertheless, applications such as ion thrusters 19,20,21 are already under development.…”

Ion acceleration in a solenoid-free plasma expanded by permanent magnets

“…Recent experiments and theories have shown the DL formation and/or the ion acceleration in expanding plasmas. [3][4][5][6][7][8][9][10] Charles and Boswell have suggested that the new type of electric thruster, named the helicon double layer thruster, can yield a long-lifetime because it does not require ion extraction electrodes. Recent measurements of the electron energy distribution function upstream of the helicon double layer showed that only a single upstream plasma source is required to maintain the formation of the double layer, 10,11 hence the above-mentioned ion accelerator is considered to be applicable to the thruster.…”

Flow dynamics and magnetic induction in the von-Kármán plasma experiment

“…While the first scheme could be obtained with only one plasma source, the second one requires two independent sources. Note that in this configuration, axial flows could be driven from the magnetic expansion of the plasma (Charles and Boswell 2004;Plihon et al 2007). Alternative schemes such as single grid acceleration of the plasma (Dudin and Rafalskyi 2009) may also be implemented to independently control the ratio of the poloidal (s) and toroidal (t) flows.…”