This must be a rater basic question about terminology.
I frequently come across the following statement: when vacuum diode is working in space charge mode (as opposed to saturation mode), the cathode emission current is larger than anode current
Iemission > Ianode
This is is supposedly caused by the potential barrier created by the space charge around cathode, which prevents some emitted electrons from reaching the anode.
I would immediately understand that inequality if we were talking about the initial stages of vacuum diode operation, when the space charge is still being accumulated, i.e. when emitted electrons leave the cathode and just stay in space charge area around it.
However, it seems that the above inequality is also widely applied to vacuum diodes working in well-established mode, when the space charge is already fully formed, i.e. the number of electrons that enter the space charge equals the number of electrons that leave it.
In that case, how it is possible to have such current inequality? Where does the extra emission current "disappear", if it does not reach the anode?
I suspect that the answer is simple: Iemission, by definition, is intended to include only the current that leaves the cathode, but it does not include the current that returns to the cathode from the space charge. If we designate that return current as Ireturn and assume that it has negative value, then the following equality will hold
Iemission + Ireturn = Ianode, where Ireturn < 0
That would explain the inequality when Ireturn is left out of the picture.
Is this the proper understanding of what is meant by the inequality in question? Or am I missing something else?