Axial variation of line emission from surface wave sustained discharges

Abstract

The intensities of ArI emission lines 750.4 and 811.5 nm and of H (hydrogen appearing as a small admixture to the discharge in Ar) were measured along the column of a stationary, surface wave sustained plasma. Simultaneously, electron densities along the discharge were determined, probing the spatial phase characteristics of test surface waves. Using test waves of different frequencies, the evaluation of electron densities was based on the dispersion relation, which connects densities with axial wavenumbers of the m = 0 mode. The emitted line intensities plotted against the electron density, particularly those of H and the 750.4 nm line, reveal that the discharge economizes on dissipated RF power in fulfilling the local energy and particle balance. A theoretical model of the diffusion dominated plasma confirms the conclusion drawn from the spectroscopically observed trends. According to this model, the lowering of the RF field intensities is correlated with the transition from ground-state excitation/ionization (prevailing at low electron densities) to additional, less power consumptive, stepwise excitation/ionization, which is enabled due to enlarged densities of metastables.