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.