In a kV X-ray unit, what helps to accelerate the electrons?

In a kV X-ray unit, electrons are accelerated by a constant or pulsating positive direct current (d.c. potential). This voltage creates a strong electric field between the cathode and anode within the X-ray tube. When the filament at the cathode is heated, it releases electrons through thermionic emission. The positive potential at the anode attracts these electrons, accelerating them toward the anode at high speeds. Upon striking the anode, the sudden deceleration of these high-energy electrons produces X-rays through interactions with the target material.

In contrast, the other options do not effectively serve this function in the context of a kV X-ray unit. A rotating magnetic field is typically related to applications in induction motors and does not apply to electron acceleration in X-ray production. High-frequency alternating current is more commonly associated with RF (radio frequency) in other contexts, such as in MRI or certain types of radiation therapy, rather than directly accelerating electrons in a kV X-ray tube. Static electric charge would not create an ongoing force needed to maintain the acceleration of electrons, as it lacks the dynamic nature of a continuous potential difference that is required in the operation of an X-ray unit.