Explanation: The charge accumulation described is known as **space charge**. In a vacuum tube, when the cathode (emitter) heats up, it emits electrons (thermionic emission). If the plate (anode) potential is not high enough to attract all these electrons immediately, they accumulate in the region around the cathode, forming a cloud of negative charge. This "space charge" repels newly emitted electrons and restricts the flow of current to the plate, effectively controlling the tube's operation. * **Getter** is a material used to absorb residual gases in a vacuum tube to maintain the vacuum, not related to electron accumulation. * **Plate saturation** occurs when the plate voltage is high enough to attract *all* available electrons from the cathode, and further increases in voltage no longer increase current. This is the opposite of the scenario described, where not all electrons are attracted.