High Power, High Brightness Proton Accelerators*

Proton accelerators were originally developed for nuclear physics experiments as an alternative to radioactive source or cosmic rays. From the earliest days, the energy of the proton has been the most important design parameter of the accelerator builders. As early proton accelerators one can name the Van de Graaff[2] and Cockcroft-Walton[3] devices. They are electrostatic machines, of which their energy is limited to less than 1 MeV for the CockcroftWalton and to the order of ten MeV for the Van de Graaff. However, the Van de Graaff is unable to provide as much proton current as a Cockcroft-Walton limited by the capability of the high voltage generator. The output energy of these devices is limited to the voltage that could be generated, which is too low for some of the experiments of interest. A new method of acceleration was needed. Acceleration of charged particle using a radio frequency source was first proposed by Ising[4] and known as resonant acceleration. Using Ising’s idea, Wideroe built the first demonstration linear accelerator[5]. With the technology of the time, the linear accelerator or linac was rather difficult to build and there was no further development for some time. Inspired by Wideroe’s written account and the fact that time required to make complete circle in constant magnetic field being independent of energy, the fixed frequency cyclotron was invented by Lawrence[6] and the first demonstration cyclotron was built by Livingston in 1931. The cyclotron, however, has also an energy limitation due to the relativistic time dilation effect. The cyclotron works in the energy range where the momentum of the proton is approximately proportional to the velocity. Even with the development of the synchrocyclotron, it was clear that a new idea was needed to accelerate to higher energy and satisfy the experimental requirements. At this point, we must mention the story of the betatron. A betatron accelerates the beam by changing the magnetic flux enclosed by the beam path or orbit. If the ratio of the guide field for the beam and the magnetic field enclosed by the orbit is limited to a two to one ratio, the beam particles can be accelerated by simply increasing the magnetic field. The Norwegian physicist Wideroe suggested this “betatron acceleration” mechanism[7] and called it a “Strahlung transformator” or “ray transformer” because the particle orbit acts like a secondary winding of a transformer while the magnetic coil acts as primary. This simple device is independent of the relativistic effect and only dependent on the geometry of the magnet, and ideal for accelerating electrons as relativistic effect prevents electron cyclotron of any significant energy. Wideroe wrote his idea in his notebook, but his attempt to build a demonstration accelerator failed. Years later, when Kerst[8] independently invent and built the first betatron, Wideroe mentioned his notebook to Kerst. The limit of the betatron top energy is The development of accelerator science and technology has been accommodating ever increasing demand from scientific community of the beam energy and intensity of proton beams. The use of high-powered proton beams has extended from the traditional application of nuclear and high-energy physics to other applications, including spallation neutron source replacing nuclear reactor, nuclear actinide transmutation, energy amplification reactors. This article attempts to review development of proton accelerator, both linear and circular, and issues related to the proton beam energy, intensity as well as its output power. For related accelerator physics and technical review, one should refer to the recent article in the Reviews of Modern Physics [1]