Influence of the Ion Mass in the Radial to Orbital Transition in Weakly Collisional Low-Pressure Plasmas Using Cylindrical Langmuir Probes

Abstract

This paper presents an experimentally observed transition from the validity of the radial theories to the validity of the orbital theories that model the ion current collected by a cylindrical Langmuir probe immersed in low-pressure, low-temperature helium plasma when it is negatively biased with respect to the plasma potential, as a function of the positive ion-neutral collision mean free path to the Debye length ratio Λ=λ+/λD. The study has been also conducted on argon and neon plasmas, which allows a comparison based on the mass of the ions, although no transition has been observed for these gases. As the radial or orbital behavior of the ions is essential to establish the validity of the different sheath theories, a theoretical analysis of such a transition not only as a function of the parameters Λ and β=T+/Te, T+ and Te being the positive ion and electron temperature, respectively, but also as a function of the ion mass is provided. This study allows us to recognize the importance of the mass of the ion as the parameter that explains the transition in helium plasmas. Motivated by these theoretical arguments, a novel set of measurements has been performed to study the relationship between the Λ and β parameters in the transition that demonstrate that the effect of the ion mean free path cannot be completely ignored and also that its influence on the ion current collected by the probe is less important than the effect of the ion temperature.

Low Electron Temperature Plasma Diagnosis: Revisiting Langmuir Electrostatic Probes

“…Electron temperature values, , are obtained between 0.1 and 10 eV (between 10 3 and 10 5 K). The positive ion temperature is similar to that of the neutral gas [7,[14][15][16][17][18][19][20][21][22]23], since the mean free path of the ion-neutral collision is of the order of 1 mm, this type of collision being the most frequent in this type of discharges and the mean free paths corresponding to the other possible collisions among neutrals, electrons and ions being of the order of magnitude of the characteristic size of the discharge, 0.1 m [3,24].…”

“…In this section, a method for measuring the IVCP, that is used throughout the article and that has been widely used by the authors in other works, is presented [14,15,17,18,22,23,36,37,49,51].…”

“…Finally, it is noted that the probe is located in the diffuse afterglow plasma zone where the value of is only one order higher than , conditions in which the study of the influence of = / ≠ 0 is possible, a situation that is important to be taken into account both in the diagnosis and in the technological processes in which this type of plasmas is normally used [7][8][9][10][11]14,18,22,23,49,52].…”

“…Among the different plasma diagnosis methods, the method based on the Langmuir electrostatic probes is one of the most used diagnostic methods in all types of plasmas [14,15,21,23,[25][26][27][28][29][30][31][32][33][34][35][36][37]. Langmuir himself developed the first diagnostic procedures with Langmuir electrostatic probes in 1929.…”

“…A drawback of this diagnostic method that could be claimed would be that the probe itself, when immersed in the plasma, causes a disturbance in the plasma. However, as shown in Section 3.3, the ion sheath, which, under certain conditions, forms around the probe, shields the plasma from this perturbation, such that it is negligible as we move away a few millimeters from the probe [7,[14][15][16][17]22,23,35,36,38].…”