THE POSITIVELY BIASED PLANAR LANGMUIR PROBE IN A HIGH-PRESSURE PLASMA

The properties of a positively biased planar (electron) Langmuir probe , immersed in a high-pressure flowing plasma are investigated both the oretically and experimentally. A theoretical model is proposed for a o ne-dimensional convection supported sheath, which is very similar to t he existing model for negatively biased (ion) probes. The latter model is the basis of existing ion probe relations for flush, cylindrical a nd spherical configurations, which relate probe current to plasma velo city, plasma ionization and probe blas. It is shown that these relatio ns can be converted to electron probe relations if two steps are taken : first the calculated ion current to the probe must be multiplied by a factor equal to the ratio of the electronic to ionic mobilities and second, an additional velocity-induced electric field in the plasma ou tside the sheath must be taken into account. Measurements in a flame p lasma utilizing a large planar probe specifically designed to generate planar sheaths of measurable thickness (up to 6 mm) show that the she ath thickness, the plasma electric field and the electron-to-ion curre nt ratio are close to the values predicted by the theoretical model. T his direct experimental evidence in support of this model reinforces s imilar recently published conclusions on cylindrical probes. In the ca se of these new results, however, the theoretical model is more closel y replicated by the experiment, the theory is less approximate, and mo st importantly, systematic measurements of sheath thickness and plasma electric field establish the integrity of the model in a way that was not possible in the earlier work.