Applications of Permanent Magnets in Accelerators and Electron Storage Rings

After an explanation of the general circumstances in which the use of permanent magnets in accelerators are desirable, a number of specific magnets are discussed. This discussion includes magnets needed for the opera tion of accelerators as well as magnets that are employed for the utilization of charged particle beams, such as the production of synchrotron radiation. Generic Advantages of Permanent Magnets Before describing some specific devices, it is useful to discuss the general circumstances under which the use of permanent magnets is indicated, and what the prefer able permanent magnets materials are. When one scales an electromagnet in all dimensions while keeping the magnetic field at equivalent locations fixed, it is easy to see that the current density in the coils is inversely proportional to the linear dimensions L of the magnet. Since superconductors have an upper limit for the current density j that can be carried, and dissipative coils have an upper limit for j due to the need to remove the dissipated power, j needs to be reduced below that prescribed by simple scaling when L reaches a certain small value that depends, of course, on many details of the magnet design. When j is reduced, the field in a magnet that does not use iron obviously is also reduced, even if the total ampere turns are maintained by increasing the coil size. The same is also true for a magnet using iron, since an increase of the coil size in variably leads to a field reduction due to increased satu ration of the iron. Permanent magnets, on the other hand, can be scaled to any size without any loss in field strength. From this follows that when it is necessary that a magnetically significant dimension of a magnet is very small, a permanent magnet will always produce higher fields than an electromagnet. This means that with permanent magnets one can reach regions of pa rameter space that are not accessible with any other technology. The critical size below which the permanent magnet outperforms the electromagnet depends of course on a great many details of both the desired field strength and configuration as well as the properties of the readily available permanent magnet materials. In the region of the parameter space that is accessible to both technologies, the choice of one technology over the other will be made on the grounds of cost or conven ience main specifics: power supplies, power needed to run the system, equipment associated with cooling , and in this arena permanent magnet systems are often also preferable, but less so when the smallest magnetically relevant dimension becomes larger. Magnetic Properties of Some Permanent Magnet Materials All permanent magnets described below use anisotropic material whose magnetic properties are adequately de scribed by B = Br + μ0μ H (B > 0) 1