Introduction

Modern radiation therapy traces its origins back to the invention of the ''klystron'' by brothers Russell and Sigurd Varian in 1937. The Varian brothers first used their invention in radar systems. However, after World War II, either the klystron or the magnetron, another invention of the time, was used to propel charged particles through a vacuum tunnel, resulting in a device called a linear accelerator or linac. The linac was initially used for research in high energy physics, but in the early 1950s, Dr. Henry Kaplan, head of Stanford University's Department of Radiology, met with Edward Ginzton, a Stanford physicist and a Varian co-founder. Kaplan proposed that a linac be specifically designed to generate high energy X rays for the treatment of cancer. The idea was that klystrons would accelerate electrons to near the speed of light. The electrons would then be made to strike a tungsten target causing an emission of X rays of comparable energies. These high-energy X-ray beams would then be used to bombard a cancerous tumor [8]

Resonating cavities and waveguides as elements of a linac are the object of interest of this brief introduction. These instruments are the basics elements in the linac. Due to its property of relatively low cost and power consumption, linac is now the gold standard for the cancer therapy. Every resonant accelerators, linac included, are based on the cavities and waveguides physics so we should introduce this topic before.

Resonant accelerators have the following features in common:

1. Applied electric fields are harmonic. The continuous wave (CW) approximation is valid; a frequency-domain analysis is the most convenient to use. In some accelerators, the frequency of the accelerating field changes over the acceleration cycle; these changes are always slow compared to the oscillation period.
2. The longitudinal motion of accelerated particles is closely coupled to accelerating field variations.
3. The frequency of electromagnetic oscillations is often in the microwave regime. This implies that the wavelength of field variations is comparable to the scale length of accelerator structures. The full set of the Maxwell equations must be used.

We are interested to theoretical aspects of this topic but we can't neglect other pratical aspects like the creation and manteinance of vacuum in this practise, aspect which are treated by my friends.

Electromagnetic fields in the presence of metallic boundaries form a practical aspect of the subject of considerable importance. At high frequencies where the wavelengths are of the order of meters or less the only practical way of generating and transmitting electromagnetic radiation involves metallic structures with dimensions comparable to the wavelengths involved. The problems of waves guided in hollow metal pipes and of resonant cavities are treated from a fairly general viewpoint.