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Differences between electron and ion linacs

The fundamental difference between electron and ion masses is at the origin of the different technologies used for electron and ion linear accelerators. In this paper accelerating structure design, beam dynamics principles and construction technologies for both types of accelerators are reviewed, underlining the main differences and the main common features. 1 Historical background While the origin of linear accelerators dates back to the pioneering work of Ising, Wideroe, Sloan and Lawrence in the decade between 1924 and 1934 [1], the development of modern linear accelerator technology starts in earnest in the years following the end of World War II, profiting of the powerful generators and of the competences in high frequencies developed for the radars during the war. Starting from such a common technological ground two parallel developments went on around the San Francisco Bay in the years between 1945 and 1955. At Berkeley, the team of L. Alvarez designed and built the first Drift Tube Linac for protons [2], while at Stanford the group of E. Ginzton, W. Hansen and W. Panofsky developed the first disc-loaded linac for electrons [3]. Following these first developments ion and electron linac technologies progressed rather separately, and only in recent years more ambitious requirements for particle acceleration as well as the widening use of superconductivity paved the way for a convergence between the two technologies. The goal of this paper is to underline the basic common principles of ion and electron RF linacs, to point out their main differences, and finally to give an overview of the design of modern linacs. 2 Special relativity, particle velocity and synchronicity The basic difference between electrons and ions is their rest mass. The mass of an electron at rest is 9.1 × 10 kg (corresponding to 511 keV/c), while a proton has a mass of 1.6 × 10 kg (938 MeV/c), i.e. a factor about 2’000 (exactly, 1’836) difference. Heavy ions used in linear accelerators have even greater masses. Because of their difference in mass, electrons and ions will react differently to an externally applied electric field. An RF linear accelerator is basically a linear array of small space intervals (‘gaps’) where a time varying electric field has been generated. The field can be made synchronous with a particle beam in such a way that the particles see in each gap a field in the same longitudinal direction. The change in momentum of a particle inside the beam is determined by the well-known dynamics equation: [ ( )] ( ) d q E v B mv dt + × = In absence of a magnetic field, the force produced on the particle by the electric field increases its momentum mv. The velocity however can not increase indefinitely, being limited to the speed of