Influence of electrons reflected from target on operation of diode and triode electron sources

When an electron source and a target are immersed in an external magnetic field electrons reflected from a target do not disappear but move along magnetic force lines to the source region where they are rereflected back to the target by the source electric field. Penetration of reflected electrons into the source can lead to distortion of the source electric field and through this to change of the limiting current density emitted by a cathode. Results of calculations of the limiting current density in the diode and triode schemes of an electron source in the presence of reflected electrons are presented. Density of the space charge of reflected electrons was calculated taking into account their real energy distribution obtained by means of Monte Carlo simulation. It was found that penetration of reflected electrons in the diode can decrease essentially the limiting current density. When electrons are reflected with the same energy the maximum lowering of the current density is as much as 3 for reflection coefficient k=1 and 2 for k=0.5. Real lowering of the current density for tungsten target is 1.5. Results of calculations are in a good agreement with experimental data. The analysis performed shows that consideration of reflected electrons is necessary for correct calculation of the beam power density at the target and the distribution of the energy density deposited into the target.