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Quasi-stochastic and sudden formation of plasma in a vacuum gap and subsequent high-voltage discharges between electrodes often limits the reliability of high-voltage vacuum electronic devices. Electronic field emission (FE) from the negatively charged electrode is assumed a precursor for a subsequent explosive electron emission (EEE) discharge of the macroscopic gap. The reason for the sudden development of discharge events after long periods of reliable operation is still matter of debate. This paper postulates a relatively slow growth process of carbon and metal based field enhancing structures, which eventually cause avalanche effects of vacuum breakdown and EEE to occur. The slow growth process explains why the undesired events often occur quasi stochastically, despite of prior conditioning the high voltage gap. Means to improve the reliability of vacuum electronic high voltage devices can be derived from the model.
