Propagation Characteristics of Ion Acoustic Waves in an Ar/SF6Plasma

Abstract

There has been a great interest in wave phenomena in a negative ion plasma. Usually sulfur hexafluoride (SF6) gas is used to produce a negative ion plasma, because SF6 gas has a large electron attachment cross-section for electron energies less than 1 eV. Furthermore, the negative ion concentration, r, can be controlled by partial pressure of the gas. However, the introduction of SF6 gas produces several ion species in discharge plasmas. In fact, there are many discussions on ion species in discharge plasmas. In the most of the experiment on the ion acoustic wave in a negative ion plasma, only F− or only SF6 was assumed as the negative ion species. 1, 2) On the other hand, in many experimental studies on the ion acoustic soliton in Ar/SF6 plasmas it was assumed that the light components, that is, F− contributes more effectively to the ion acoustic wave, 4) and the results of these experiments were consistent with this assumption. Cooney et al., however, obtained the soliton velocity that shifted gradually from the theoretical value for Ar and F− to that for Ar and SF6 as r increased. 5) Although they mentioned that this was perhaps due to an increase in the relative concentration of SF6 to F − as r increased, there was no evidence for the phenomenon. On the other hand, it was recently observed by the mass spectrometry that the relative concentration of SF6 to F− increases with increasing r in the double plasma (DP) device. Thus, in order to confirm whether the variations in the relative concentrations of ion species affect the propagation characteristics of the ion acoustic wave or not, we detected dominant ion species in an Ar/SF6 plasma with a quadrupole mass spectrometer (QMS) and measured the phase velocity of the ion acoustic wave for different values of r. While much work on the ion acoustic wave in two-positive ion plasmas have been done, 8) the study on the ion acoustic wave in two-negative ion plasmas hardly has been carried out. The purpose of this paper is to observe the dependence of the ion acoustic wave on ion species, especially negative ion species. The experiments were carried out using a multidipole DP device. The diameter and the length of the device