Electron-rich sheath dynamics. II. Sheath ionization and relaxation instabilities

Abstract

Instabilities in an electron-rich sheath on a plane electrode in a discharge plasma have been investigated experimentally. The high-frequency sheath-plasma instability near the electron plasma frequency is observed. With increasing dc voltage, the instability exhibits bursty amplitude and frequency jumps. The electrode current shows spikes and jumps, and the plasma potential near the electrode shows large fluctuations below the ion plasma frequency. Sheath-ionization has been identified as the cause for these low frequency instabilities. Electrons energized in the sheath produce ions which reduce the space charge in the sheath and the electric field and the ionization rate. Ions are ejected from the sheath which increases the charge density, electric field, and ionization rate. The positive feedback between these processes leads to a relaxation instability whose time scale is determined by ion inertia and ionization rates. The associated density and potential fluctuations affect the amplitude and frequency of the sheath-plasma instability. When the sheath ionization rate exceeds the ion losses, the sheath expands into an anode plasma or “fireball.” The potential drop across the sheath decreases and the sheath-plasma instability vanishes. The electrode current-voltage characteristics develop a region of negative conductance. For short grid voltage pulses, the ionization effects can be avoided.