Which type of tube consists of a series of cylinders with gaps between them?

The linear accelerator is a type of tube characterized by its configuration of a series of cylinders, often referred to as "drift tubes," with gaps between them. This design facilitates the acceleration of charged particles, such as electrons, by allowing them to gain energy from high-frequency electromagnetic fields as they travel through these gaps.

In a linear accelerator, particles are injected into the first cavity and then travel through successive cavities, each providing an additional boost of energy. The use of drift tubes allows for effective acceleration while maintaining space for the particles to travel, thus optimizing the system for efficiency and effectiveness in applications such as medical radiotherapy and industrial radiography.

While other options like the Van de Graff generator and betatron are also used in particle acceleration, their design principles differ significantly. A Van de Graff generator uses a moving belt to accumulate charge on a metal dome and does not have a tube structure with cylinders and gaps. Meanwhile, a betatron employs a circular path rather than a linear arrangement, where particles are accelerated in a magnetic field. Therefore, the linear accelerator is uniquely identified by its cylinder-based configuration, making it the correct choice.