Influence of electrons reflected from a target on the operation of triode-type electron sources

An external magnetic field between the electron source and a target causes the reflected electrons to move along the magnetic force lines towards the source region. There they get reflected back to the target by the electric field of the source. These electrons distort the electric field in the sour ce region, and change the source's operational parameters. Penetration of r eflected electrons back into the source region reduces essentially the spac e charge limited emission current density of a cathode and the minimum curr ent density to form a potential minimum and a virtual cathode. How much the emission current density decreases depends on the target material's atomic number, Z, which determines the reflection coefficient and the energy spec trum of the reflected electrons. The calculations are in good agreement wit h experimental data. The analysis shows that reflected electrons must be co nsidered for a correct calculation of the beam parameters at the target for the distribution of the energy density deposited into the target, and for a better understanding of the electron source operation. (C) 2000 American Institute of Physics. [S0021-8979(00)02320-3].