Recent studies have demonstrated that the two-ion-stream-instability occurs near the plasma boundary and makes the ions reach the 'modified Bohm velocity' at the sheath edge. In most low-temperature plasmas, however, the ion-neutral collisions can disturb the growth of instability to occur frequently, and the ions exit the plasma boundary with their own Bohm velocities. We report some experimental observations regarding this issue. The spatial variations of the ion drift velocities and the space potential near the sheath edge were measured in Ar/Xe mixture plasmas by increasing the total pressure in the range of 0.5-2.1 mTorr. The results show that the instability cannot occur above a certain pressure condition and that is consistent with the theoretically driven pressure criteria for the onset of the instability.

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The generality of the non-Maxwellian electron energy distribution function (EEDF) is demonstrated by using optical emission spectroscopy (OES) and Langmuir probe measurements in inductively-and capacitively-coupled low-pressure argon plasmas to analyze the shape factor of the EEDF. To measure the shape factor of the EEDF, we propose a corona -equilibrium (CE)based analysis model operating at low density, which uses the line intensity ratio of the Ar I to the Ar II emission lines. The Ar I line is chosen to represent the relatively low-energy state, and the Ar II line is chosen to represent the high-energy state. Thus, an analysis of the shape factor is equivalent to monitoring the variation in the high-energy electron fraction represented in the tail of the EEDF. Results show a depleted tail for the Maxwellian distribution in most of the low-density argon plasmas. The analysis reveals that the generation and the stepwise ionization of metastable argon atoms by inelastic collisions with high-energy (∼ 10 eV) electrons are dominant processes of argon plasma generation and cause serious high-energy electron loss in a low-density system compared to the loss in an ideal Maxwellian plasma. The existence of argon metastable states is inevitable; thus, the general shape of the electron energy distribution in low-pressure argon plasmas is non-Maxwellian.PACS numbers: 52.25. Kn, 52.25.Tx,

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