What type of transformer must be used in the filament circuit in order for thermionic emission to be possible?

In the filament circuit of an X-ray tube, a step-down transformer is essential for facilitating thermionic emission. This process relies on heating the filament, which is typically made of tungsten, to a high temperature. The filament transformer decreases the voltage to a level suitable for heating the filament, providing the necessary current to achieve that elevated temperature.

The step-down transformer lowers the voltage while increasing the current, which is crucial because the filament must reach a specific temperature to release electrons through thermionic emission effectively. The thermionic emission occurs when the filament is heated enough, generating a cloud of electrons around it, which are then utilized in the production of X-rays.

In contrast, a step-up transformer is designed to increase voltage, which would not be suitable for heating the filament. An autotransformer is primarily used for varying voltage levels in a circuit but does not specifically facilitate the conditions needed for thermionic emission. A rectifier is involved in converting AC to DC and does not pertain directly to the heating function of the filament. Therefore, the step-down transformer is the appropriate choice in this context, as it directly enables the conditions necessary for thermionic emission to take place.