Various points of view on the mechanism of evolution of an open discharge are analyzed. This problem is important for understanding the physics of generation of an electron beam, whose efficiency achieves 90% and even higher. For this purpose, a technique is proposed for calculating the factors of electron emission from a cathode whose surface is bombarded by fast gas particles and ions. The calculation has shown that this process fails to provide for the efficiency observed in experiments. The calculated results gave an additional proof of the dominating effect of photoelectron emission from the cathode on dynamics of the open discharge and e-beam generation in it.