Overview of magnet measurement methods

Abstract

Electromagnets used as beam guiding elements in particle accelerators and storage rings require very tight tolerances on their magnetic fields along the particle path. Also large volumes of magnetic field in modern spectrometer magnets must be mapped in order to allow for the precise tracking of secondary particles. The article describes the methods and equipment used for these types of magnetic measurements. Short descriptions are given of the magnetic resonance techniques, the fluxmeter method, the Hall generator and the fluxgate magnetometer. A few of the more exotic methods are aslo briefly mentioned. References of historical nature as well as citations of recent work are indicated. It is mentioned when sensors and associated equipment are commercially available. 1 . INTRODUCTION Before computers became common tools, electromagnets were designed using analytical calculations or by measuring representative voltage maps in electrolytical tanks and resistive sheets. Magnetic measurements on the final magnets and even on intermediate magnet models were imperative at that time. Nowadays it has become possible to calculate strength and quality of magnetic fields with an impressive accuracy. However, the best and most direct way to verify that the expected field quality has been reached is to perform magnetic measurements on the finished magnet. It is also the most efficient way of verifying the quality of series produced electromagnets in order to monitor wear of tooling during production. It is curious to note that while most measurement methods have remained virtually unchanged for a very long period, the equipment has been subject to continual development. In the following only the more commonly used methods will be discussed. It is noticeable that these methods are complementary and that a wide variety of the equipment is readily available from industry. For the many other existing measurement methods, a more complete discussion can be found in the two classical bibliographical reviews [1, 2]. An interesting description of early measurement methods can be found in [3]. 2 . MEASUREMENT METHODS 2 . 1 Choice of measurement method The choice of measurement method depends on several factors. The field strength, homogeneity and variation in time, as well as the required accuracy all need to be considered. Also the number of magnets to be measured can determine the method and equipment to be deployed. As a guide, Fig. 1 shows the accuracy which can be obtained in an absolute measurement as a function of the field level, using commercially available equipment. An order of magnitude may be gained by improving the methods in the laboratory. 2 . 2 Magnetic resonance techniques The nuclear magnetic resonance technique is considered as the primary standard for calibration. It is frequently used, not only for calibration purposes, but also for high precision field mapping. The method was first used in 1938 [4,5] for measurements of the nuclear magnetic moment in molecular beams. A few years later the phenomenon was observed in 10 10 10 10 10 10 10 -6 -5 -4 -3 -2 -1 INDUCTION METHOD