What is the term used for the "cloud of electrons" held by the focusing cup?

The term used for the "cloud of electrons" held by the focusing cup is known as space charge. In the context of x-ray tubes, when a filament is heated, it emits electrons through a process called thermionic emission. These emitted electrons become concentrated in the region around the filament, creating a cloud of electrons, which is specifically referred to as the space charge.

The focusing cup plays a crucial role by serving as a negative electrode that helps to confine and direct this cloud of electrons toward the anode, where they will hit and produce x-rays. Understanding the concept of space charge is essential in grasping how x-ray tubes function, as it affects the efficiency and quality of the x-ray production process.

The other terms provided may relate to concepts in electronics or radiology, but they do not specifically describe the situation of the electron cloud around the filament in an x-ray tube context as accurately as space charge does.