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The experimental study of the pulsed-mode operation of the PHOENIX-V2 electron cyclotron resonance ion source at 28 GHz has clearly demonstrated, when increasing the repetition rate of the high-frequency power injection at frequencies higher than 1 Hz, the reality of a transient current peak occurring at the very beginning of the plasma discharge. This regime was named Preglow as an explicit reference to the classical Afterglow occurring at the microwave pulse end. After the transient Preglow peak, the plasma regime relaxes to the classical steady-state one. Experimental argon pulses for charge states from 2+ to 8+ are presented. The current observed during the Preglow peak can reach intensities on the order of 1 mA for low charge states (Ar4+). A zero-dimension theoretical model of electron cyclotron resonance (ECR) gas breakdown in a magnetic trap is presented in detail. Results of the simulation are compared with the experimental Preglow peaks and discussed.