The superconducting radio frequency quadrupole structures review

Since 1985 the idea of using the fast developing technology of superconducting cavities for the construction of Superconducting Radio Frequency Quadrupole (SRF Q) has been a challenge proposed by D. Swenson to the accelerator community and the first prototype was proposed by I. Ben-Zvi in 1988. The combination of the effectiveness of the RFQ structures for the focussing and the acceleration of very low ion beams with the possibility of the CW operation of the superconducting accelerators commonly in use in the Nuclear Physics Laboratories around the world was very promising in particular after the development of the high cha rge state positive ion source capable of high intensity beams, such as the ECR. The paper presents the state of the art of the SRFQ structures starting with the proof of principle, describing the quest for the high surface field, following the evolution of the structures dictated by the beam dynamics and giving an overview of the present projects in the di fferent laboratories. The description of the different technological solutions is also included presenting the mechanical solution to cope with the acoustical noise.