SPACE AND TIME RESOLVED ELECTRIC-FIELD VECTOR DISTRIBUTION IN RADIOFREQUENCY DISCHARGES BETWEEN UNEQUAL AREA ELECTRODES

Two dimensional space and time-resolved electric field vector distributions have been determined in asymmetric, capacitively coupled, low frequency (35 kHz) and radio-frequency (4 MHz) discharges in Ar/2.9 %K mixtures. The electric field was measured using a non perturbative spectroscopic technique based on the Stark mixing observed in the laser induced fluorescence of NaK molecules. Sheath oscillations are clearly visible, and show the different discharge conditions which prevail close to the different electrodes. The field direction is almost stationary in time. The electric field distribution is nearly radially uniform in front of the large electrode, while geometrical effects seem to affect the discharge close to the small electrode. Edge effects are visible at a distance of 2 mm from the electrode edges. In low frequency discharges the ionic current is about the same at both electrodes, whereas the current density is very different, due to the different charge densities. In RF discharges conditions were found for which different discharge sustaining regimes occur at each electrode : << alpha >> regime near the large electrode, << gamma >> regime near the small electrode.