Student Excellence Award Finalist: Collisional electron momentum loss in low temperature plasmas: On the validity of the classical approximation*

Understanding electron power absorption in low temperature plasmas is of great importance due to their numerous industrial applications. One of the main power absorption mechanisms, ubiquitous to all types of plasma, is Ohmic heating. 

The electron momentum loss, fundamental to its calculation, is usually approximated using the collision frequency. In this work, the electron momentum loss obtained from PIC/MCC simulations, and the classical approximation based on the electron-neutral collision frequency, are calculated and compared in low pressure CCPs in various gases. 

We find that the classical approximation, commonly used in fluid models, exaggerates the role of low-energy electrons and can lead to a significantly lower momentum loss compared to the exact value, even if the exact electron distribution function is known. This leads to an underestimation of the Ohmic power absorption and a change in the harmonic content of the momentum loss. For gases with a Ramsauer-Townsend minimum (e.g. argon), the classical approximation is found to be particularly poor due to the decreased number of collisions for low energy electrons. This is confirmed by using a ``fake'' argon gas where the Ramsauer-Townsend minimum is artificially removed. The results are of broad general relevance to low-temperature plasmas, and can be useful for assessing errors in plasma fluid models