Space charge is three dimensional concept. It is a cloud of electrons which are distributed evenly over the 3D space rather than being at one place. As we know cathode emits electrons and this electron take some finite amount of time to travel and due to this a region near cathode is created which create repulsion for electrons which are coming cathode. This phenomenon is called space charge effect. This decrease the plate’s current.

This Space charge has no effect on the rate of emission of thermions, as rate depend on how much energy we are providing to it. Also it is not related to saturation current or plate’s voltage.