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Collisional electron momentum loss in low temperature plasmas: on the validity of the classical approximation

The electron momentum loss obtained from kinetic simulations, as well as the classical approximation based on the electron–neutral collision frequency, are calculated and compared in low pressure capacitively coupled plasmas in argon, helium and oxygen gases. The classical approximation (which is commonly used in theoretical or numerical fluid models) exaggerates the role of low-energy electrons and can lead to a significantly lower momentum loss compared to the exact momentum loss depending on the gas used, 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 as revealed by Fourier analysis. For argon, the classical approximation is found to be particularly poor and is partially related to the presence of a Ramsauer–Townsend minimum in the momentum transfer cross-section at low electron energies: a fact 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.