A new method to estimate the negative ion density in reactive gas plasmas with a Langmuir probe is proposed. This method has the advantage that the negative ion density is evaluated only by taking the ratio of the ion saturation–electron saturation current ratio obtained from the I–V curve of the Langmuir probe measured in an electronegative-gas mixture plasma to that measured in a reference noble gas plasma. The negative ion density in a SF6/Ar double plasma is estimated utilizing this method. Furthermore, the negative ion density measured with this method is confirmed to agree with that calculated from the measured phase velocity of the ion acoustic wave (fast mode) in the SF6/Ar double plasma, where positive and negative ion masses are obtained from the spectrum analysis with a quadrupole mass spectrometer.

Ion acoustic waves in multi-ion plasmas including two negative ion species are investigated both numerically and experimentally. Numerically, the kinetic dispersion relation in two-negative ion plasmas is investigated. There are three modes of the ion acoustic waves in two-negative ion plasmas. In an Ar+–F−–SF6− plasma, only one of the three modes is dominant, regardless of the values of the electron and the ion temperatures. In a Xe+–F−–SF6− plasma, on the other hand, two modes can be important for a certain range of the electron–ion temperature ratio. The results also imply the possibility of the coexistence of the fast mode and the slow mode in one-negative ion plasmas. Experimentally, ion acoustic waves are observed in an Ar+–F−–SF6− plasma and are found to show a mode transition that agrees with the theoretical prediction for one of the three ion acoustic modes.

The newly developed method of the negative ion density measurement in a plasma by laser Thomson scattering (LTS) was checked by comparing the obtained results against an independent technique, namely the Langmuir probe method. Both measurements were performed at the same position of the same inductively coupled plasma. The results agree quite well with each other and this has given confidence in the LTS method of negative ion density measurement. At the same time, both methods are complementary to each other, because the Langmuir probe measurement requires knowledge of the positive ion mass number.

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Dynamics of microscopic charged dust particles in cryogenic condition is studied experimentally. Dust particles were free falling through the liquid helium (LHe) vapor as well as through LHe. In the liquid, particles start to go up toward the surface. It was found that dust particles are charged negatively in vapor plasma, but lose the negative charge and became charged positively in the liquid, which is confirmed by the examination of the particle dynamics. Based on dust trajectories observed, the charge values for the dust particles were calculated.

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