Langmuir paradox revisited

Electron energy distribution functions (EEDF) have been measured in the positive column of dc discharges under the condition of the Langmuir paradox (1 and 3 mTorr of argon gas) using a high resolution probe measurement system developed by the authors. An essential deviation of the measured EEDFs from Maxwellian distributions was found for both, slow and for fast electrons. The low energy peaks found in the measured non-Maxwellian EEDFs are attributed to non-local electron kinetics in the axially non-uniform electric field caused by standing striations. Similarly, in rf capacitive and inductive discharges, the formation of the low energy peak is a result of spatially localized electron heating combined with trapping of low energy electrons outside of the heating zone.