Sheath Expansion Measurement of Planar and Cylindrical Electrodes

Abstract

Stable argon plasma is generated in multi-dipole chamber by hot filament discharge. Parameters of the plasma are electron temperature T_e ~ 1.35-1.69 eV, plasma density n_e~ 1-2×10^10/cm^3 and Debye length λ_D~59-89 μm. Planar electrode and cylindrical electrodes with radius 3.17 mm, 0.65 mm and 0.06 mm are used. Negative high voltage pulses are applied to the electrode and electron saturation current is measured by changing probe position to find the sheath arrival time. Sheath expansions are measured with various plasma conditions and the applied voltage. For each electrode except wire of radius 60 μm, the expansion of sheath fits very well to the model of transient pre-sheath developed by Brutscher. Just after the pulse is launched, sheath expansion speed is fast enough to ignore pre-sheath. When the sheath expansion speed slows down, pre-sheath is formed slowly. After the end of pulse, sheath shrinks and ion acoustic wave is observed beyond sheath edge. For the case of 60 μm radius wire, dimension comparable to the electron Debye length, the current flow through the electrode is less than space charge limited current. The final sheath thickness of this electrode is smaller than that of the model of transient pre-sheath.