Plasma parameters of Ar-N2 positive column

Electron energy distribution function (EEDF) in a DC Ar-N2 positive column is measured with a Langmuir probe. In a discharge tube with 18 mm in inner diameter and 300 mm in length, the gas mixing ratio dependence of EEDF is investigated over the total gas pressure from 0.3 Torr to 1.3 Torr with keeping the discharge current constant at 100 mA. The EEDF depends strongly on the amount of N2 due to the vibrational collisions between electrons and N2 particles, producing a great deviation from the Maxwellian distribution. The electron density and the average electron energy, which are calculated from the EEDF, slightly increase with the increase of Ar content, while the electric field decreases linearly. The electron density is inversely proportional to the electric, field, which is predicted from the current conservative equation. Since the energy loss rate decreases with the increase of Ar content, the average electron energy increases with the decrease of electric field. There is a large discrepancy between the measured and calculated EEDFs. The EEDF is calculated from the Boltzmann equation. The discrepancy is partially attributed to the ambiguity in estimation of the vibrational temperature for N2 particles and the neglection of the ionization processes related to metastables. On the other hand, the relationship between E/p and pR obtained experimentally is in good agreement with that calculated from the EEDF, where E is the electric field, p the gas pressure and R the tube radius