Principle of Operation

Date: 2015-10-07; view: 382.

Consider a plate of conductive material containing electrons that are free to move and stationary positive charges. When this cathode is heated, electrons begin to move in a random jostling fashion until a number escape from the surface of the material. Facing the cathode and separated from it in an evacuated space, is the anode; an external circuit is connected between them, Fig.5a.

As the cathode is heated, electron activity increases and electrons escape across the vacuum to the anode. The electrons then flow through the load and through the return circuit to the cathode, thus producing electric power. The concept in this simplified diagram is not new, since emission of electrons from the surface of a heated cathode is a process long used in electron tubes.

A more quantitative picture is offered by a potential diagram that corresponds to the schematic arrangement of the thermionic con­verter, Fig.5b. Here the potential energy of the electron is plotted at each point in the diagram. The potential inside the cathode mate­rial is taken as zero. The electrons inside the metal are normally pre­vented from escaping by a potential barrier, , which exists at the surface of the metal.

As the electrons become heated, a few have sufficient energy to surpass the potential barrier and escape into the space between the cathode and anode. When the electron reaches the anode, it falls down the potential barrier corresponding to the anode work function, . The energy thus released is converted into heat at the anode and is lost in the process. If the anode work function is less than that of the cathode, the remaining amount of energy, , is avail­able to do useful work in the external circuit and to supply the electri­cal losses in the return circuit.