The saturation current in a triode valve can be altered by varying

en all the electrons emitted by the cathode are collected by the plate.

According to the Richardson-Dushman equation, the thermionic emission current (saturation current) is given by $I_s = A T^2 e^{-\dfrac{W}{kT}}$, where $T$ is the absolute temperature of the cathode and $W$ is the work function of the cathode material.

Thus, the saturation current depends only on the nature of the material and the temperature of the cathode. It is independent of the plate voltage, grid voltage, or the physical separation between the electrodes.