Kenotron tube

The kenotron tube is a vacuum diode made for rectifying alternating current at high voltage.

As by definition the tube has two electrodes (a cathode and an anode) that let the current flow from the positive electrode to the negative electrode. The negative electrode (cathode) emits electrons, and the positive electrode (anode) is the “plate” which is usually of a larger size. The anode is attracting the current of electrons by voltage difference. Current cannot go in the reverse direction.

The kenetron tube can be made to have capacitor features. With an appropriate design the plate anode can be made as a capacitor attracting and store electrons to be discharged in phase with the alternating cycle.