Pitfalls in the evaluation of cathode properties from I-V characteristics

It is shown that three-halves power dependence between current and applied voltage does not guarantee sufficient electron emission all over the entire cathode area, but deviations from this dependence do not necessarily imply poor emission. The derivation of the work function from a measured I-V characteristic is obscured not only by space-charge and patch size distribution, but also by the beamlet effect. However, this effect is used to determine the barium diffusion length over a surface from a measurement of electrical current. It is also found that to obtain planar diode conditions, the distance between cathode and anode has to be smaller than about one tenth of the cathode diameter. A method that can be used to demonstrate changes in cathode series resistance curing cathode life is considered. For patchy cathodes the zero field current density is not equal to the current density obtained from Schottky extrapolation. To determine the latter properly from feasible experiments, the space charge in the saturation region has to be taken into account, i.e. space-charge correction is essential to the determination of Richardson constant and work function from a Richardson plot.<
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