The operation of a high power density magnetron source during sputtering of a copper target both in standard mode with argon as well as in self-sputtering mode without argon is discussed. Voltage–current characteristics of the source and light emission spectra were taken for various conditions of magnetron operation to understand the transition from standard to self-sputtering mode. A qualitative explanation of the ionization mechanism is offered based on the effect of electron temperature and density on the Maxwellian distribution of electron energy.