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 discharges has been investigated by optical emission spectroscopy. Analysis of the Stark splitting of plasma-induced H Balmer delta emission was used to measure the electric field with spatial and temporal resolution in the instantaneous cathode sheath. The location of the plasma/sheath boundary was determined from the position of the maximum of the H₂ d ³Π_u→a ³Σ_g ⁺ (0,0) Q₁ emission at 622.5 nm. Both methods showed that the sheath width increases as the cathode voltage becomes more negative, whereas the width remains constant as the applied voltage drops off. Analysis of the electric-field profile provided information on the time evolution of the ion density close to the electrode during the cathode half-cycle, in agreement with recent numerical calculations. At the beginning of the anodic half-cycle an intense flash of plasma-induced emission was observed, localized within 3 mm from the electrode.