A comment on the measurement of efficiency of the electron beam formation in an open discharge

Methods used for the measurement of the efficiency of the electron beam formation in an open discharge are critically analyzed. It is shown that a high efficiency determined as the ratio of the beam current to the total current, η=je/j, can serve neither as evidence for one of the open discharge mechanisms nor as a true criterion of the energy efficiency of the beam formation. Under typical open discharge conditions, when electrons in the entire discharge gap occur in the runaway regime, the η value is close to the geometric transparency of the anode grid and is independent of the coefficient of electron emission (γ) from the cathode. It is also demonstrated that recent experiments reported by P.A. Bokhan and D.E. Zakrevsky [Tech. Phys. Lett. 28, 74 (2002) do not prove the photoelectron nature of the discharge studied but, on the contrary, confirm once again that the open discharge is not induced by photoelectrons.