Ion saturation currents to planar Langmuir probes in a collision-dominated flowing plasma

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Abstract
The transition between diffusive (saturated current) and sheath-convection (currentαvoltage½) behaviour of the current to a negative probe in a high-pressure, flowing plasma has been studied theoretically. By using two physical models, where both effects are considered to be significant, one effect being dominant in one case and the other being dominant in the other case, an approximate relation is derived for the probe current in the transition between these two régimes. This transition is found to cover a variation of two orders of magnitude in ionization density if the required 10% accuracy is to be achieved. It is also found that thermoelectric and nonequilibrium effects due to cooling in the thermal boundary layer do not themselves materially affect the diffusion current.