Abstract:Spatially resolved plasma induced emission spectroscopy (PIE) is applied to a symmetric parallel plate discharge in helium under rf excitation at 13.56 MHz. The optical emission features are studied in the pressure range of 10-360 Pa and power densities from 6 mW/cm3 to 90 mW/cm3. At pressures above 60 Pa and power densities exceeding 50 mW/cm3 the transition from the a-to the y-mode is observed, which, in helium, is accompanied by the formation of a disk-shaped brightly luminous glow. A marked difference betw… Show more
The electron energy distribution function (EEDF) is measured with a Langmuir probe in a symmetrically capacitively-coupled RF (13.56 MHz) helium discharge over a range of gas pressures from 0.1 to 1.0 Torr while keeping the discharge current constant at 100 mA. The bulk electrons are Maxwellian over this range, while the average electron energy decreases with increasing pressure, together with an increase in electron density. The power dissipation calculated from the EEDF and the discharge current-voltage relation agrees with that measured.
The transition from the a to the y mode in a low-pressure helium RF discharge beween parallel plates is investigated by spatic-temporally resolved plasma induced emission spectroscopy. The time development of several levels of atomic helium and of the n = 4 level of ionic helium is presented over a wide range of discharge pressure (40-300 Pa) and power input (75-300 W). Single photon counting is applied to resolve the time dependence. The transition to the y or secondary electron dominated state is obselved directly in space and time dependent excitation of the helium ion level. The effect of the extreme lowering of the electron temperature resulting from the non-locai sustaining mechanism is ObseNed in the light of atomic helium lines, which show substantial differences between the singlet and the triplet systems due to spin changing collisions (singlet to triplet) coupling the metastable levels. The cross section of these collisions increases with a decrease of electron temperature below the threshold value of 0.79 eV. In addition, probe measurements in a similar, but symmetrically driven parallel plate discharge optimized for use with Langmuir probes are presented to give independent evidence to the rapid decrease of electron temperature when the transition occurs.
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