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Based on the observation of time-resolved avalanche current waveforms in electronegative gases, three different types of avalanches can be distinguished. The first distinction is that between electron avalanches and ion-dominated avalanches. Electron avalanches are further subdivided into electron avalanches with and without delayed electrons. Delayed electrons are the result of consecutive attachment and detachment processes. Experimental identification of the different avalanche types requires an experimental set-up with a time-resolution in the order of 1 ns. The conventional avalanche model, which involves effective ionization and drift, gives an adequate description only for electron avalanches without delayed electrons. An extended model, which also incorporates electron detachment and ion conversion processes, in principle describes all three kinds of avalanches. When the extended model is applied to the evaluation of measured waveforms, the often-quoted abnormal pressure dependencies in the electron drift velocity and the effective ionization coefficient disappear for the gases studied in this paper.

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