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The pressure dependence of the mean electron energy ( E ) was investigated in discharges in Ar experimentally by sampling the plasma potential with an ion mass spectrometer, and theoretically by solving the electron Boltzmann equation for the bulk plasma. Both results show a decrease in ( E ) with increasing pressure in the regime between 0.2 and 40 Pa although this behaviour is much more pronounced in the experimental results. The results are compared to those of single-probe measurements of other authors.In one case they are in good agreement [lo], while there is a discrepancy in another case [11]. Possible reasons for this behaviour are discussed. The advantage of the mass spectrometric sampling method is, that it is absolutely non-invasive, so that also in sensitive processes no contamination will occur.

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