Superconducting Cavities for Particle Accelerators

The use of superconducting cavities has enhanced the performance of particle gas pedals. This technology is used in fundamental and applied research, as well as in the medical and industrial fields: large gas pedals for nuclear and particle physics, synchrotron light sources or free-electron lasers, proton and neutron sources. Major societal applications (e.g. hadrontherapy, transmutation of nuclear waste, etc.) are currently under development.

The main components of a gas pedal are :

• an injector. This is a source of charged particles (electrons, protons, ions) and beam shaping;

• elements capable of generating a magnetic field to deflect and/or focus the trajectory of particles;

• elements capable of generating an electric field to accelerate particles.

Superconductivity is increasingly used both in the manufacture of the electromagnets used to deflect beams [D 590] and in the manufacture of the radiofrequency (RF) cavities that generate the electric fields required for particle acceleration. The use of superconducting materials considerably reduces heat dissipation due to the Joule effect. In many applications, the gain in efficiency and/or machine size considerably outweighs the additional investment required to manufacture cryogenic equipment. Unlike copper cavities, superconducting cavities can operate in continuous radio-frequency fields and at high accelerating fields.

The design of a gas pedal depends on the desired application; there are two main categories of gas pedal. In circular machines, the beam passes several times through the gas pedal elements. In this case, the critical points are essentially the intense magnetic fields required to deflect the beam, and the dissipation in the walls of the radio-frequency cavities. In linear gas pedals, on the other hand, where the beam passes only once, the critical point is the gas pedal field, which must be at a maximum.

We'll be focusing here on radio-frequency cavities and the contribution of superconducting materials to this technology.