TIME-RESOLVED ELECTRIC-FIELD MEASUREMENTS IN 30 KHZ HYDROGEN DISCHARGES BY OPTICAL-EMISSION STARK SPECTROSCOPY

The temporal behavior of the cathode sheath in 30 kHz 0.4-1 Torr H-2 d ischarges has been investigated by optical emission spectroscopy. Anal ysis of the Stark splitting of plasma-induced H Balmer delta emission was used to measure the electric field with spatial and temporal resol ution in the instantaneous cathode sheath. The location of the plasma/ sheath boundary was determined from the position of the maximum of the H-2 d3PI(u)-->a 3SIGMA(g)+ (0,0) Q1 emission at 622.5 nm. Both method s showed that the sheath width increases as the cathode voltage become s more negative, whereas the width remains constant as the applied vol tage drops off. Analysis of the electric-field profile provided inform ation on the time evolution of the ion density close to the electrode during the cathode half-cycle, in agreement with recent numerical calc ulations. At the beginning of the anodic half-cycle an intense flash o f plasma-induced emission was observed, localized within 3 mm from the electrode.