Efficient production of O+ and O− ions in a helicon wave oxygen discharge

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Abstract
High density oxygen plasmas (ne=1010–1013 cm−3) are produced by a helicon wave discharge source with rf powers of 0.1–3 kW. Positive and negative ion species in the plasmas are measured by a time‐of‐flight mass spectrometer. The intensity ratio of O+ to O+2 increases with the electron density ne and is almost proportional to it in the region of ne=1010–1012 cm−3. When the electron density increases up to 8×1012 cm−3, the ratio becomes about 4. In a high density plasma of 1.3×1013 cm−3 obtained by mixing Ar gas, about 83% of the positive oxygen ions becomes O+. By using pulse modulation of the rf power, O− ions are mainly observed with remarkable increase in the afterglow. The maximum density of O− is about 3×1011 cm−3 at 30 μs after turning off the rf power of 0.85 kW and the decay time of O− is about 60 μs.

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