Langmuir probes in a wide range of plasma density: Emissive and non-emissive probes

Ion saturation current and floating potential were calculated for non-emissive and emissive probes using a kinetic approach. A wide range of plasma pressures was considered: from almost collisionless R/l ≪ 1 up to almost continuous (R/l ≫ 1) cases, where R is the probe radius and l is the ion free path. Calculations were performed for cold ions (ion temperature Ti = 0) and for warm ions (the ratio of the ion temperature to the electron temperature Ti/Te not exceeding approximately 0.3). The Debye radius was assumed to be much narrower than all of the characteristic lengths. Simple approximation formulas are suggested for the ion current and the floating potential for both (non-emissive and emissive) types of probes. Calculations showed that the ion velocity at the sheath-presheath boundary corresponds to the Bohm criterion for all the considered cases.