where α is the ionization probability for a sputtered atom, β the probability for this ion to return to the sputter target and γSS the self-sputtering yield. For Π = 1, a sustained discharge without any additional gas is possible, for Π < 1 gas molecules, respective ions are necessary to maintain the sputter process,whereas at Π > 1 a runaway discharge only limited by the capacity of the power supply or the pulse length is encountered. Parameters α and β are always less than 1, thus γSS has to be larger than 1 to achieve unity for Π. For the self-sputtering yield, a strong dependency on the elemental composition is observed, with especially copper and silver exhibiting a very high self-sputter yield .