The negative oxygen ion density is studied experimentally in the high-density helicon device VINETA by laserinduced photodetachment. The measurements indicate a negative ion density being higher than the electron density in contrast to previous experiments in helicon oxygen discharges. The dependency of the relative negative ion density on the oxygen gas pressure and rf-power is in good qualitative agreement with observations in low temperature RF inductive and capacitive discharges, but differ quantitatively by one order of magnitude. To understand the basic physics in the helicon discharges, the measurements are compared to a zero dimensional reaction rate model of the electronegative oxygen discharge. The particle densities are calculated via rate equations estimated from collisional cross sections assuming Maxwellian electron energy distribution functions. The model shows that the electron temperature is the most sensitive parameter governing the formation of negative oxygen ions. Additionally, the differences to previous experimental measurements in rf discharges can be explained by different electron temperature and plasma losses to the walls.

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