Detector magnet measurements

Abstract

The many differences between detector magnets and accelerator magnets are emphasized, especially the large magnetic volumes and stored energies. External measuring machines are described, as used for mapping the rather open magnets of the 1960s and 1970s. For the almost closed magnets of modern detectors an internal machine is described. The required precision is discussed. Methods for checking the field map are presented, with actual results from one detector. 1. DETECTOR MAGNETS AND ACCELERATOR MAGNETS The magnetic measurements of detector magnets and accelerator magnets have a few similarities and many differences. The similarities are that the order of magnitude of the fields have up to now been similar, usually between 1 and 2 Tesla, so that the traditional techniques of search coils, Hall probes, NMR probes are suitable for both. Iron yokes have usually been involved, with consequent saturation effects at full field, and the well-known hysteresis effects. The differences stem from the use of the magnets; in accelerator magnets particles of well-defined momentum pass all the way through, but in detector magnets particles of unknown momentum ranging over a factor of 1000 (e.g. from 0.1 GeV/c to 100 GeV/c, and either sign of charge) may have any trajectory, as shown in Fig. 1. Fig. 1 Tracks of various momenta in a typical detector The trajectory is detected at sampling points, or continuously in a bubble chamber, and the momentum calculated. If the field were uniform and the trajectory a helix, this would be simple. Over large volumes the attainable fields are not uniform, hence the need for a detailed field map. Then the measured trajectory, even though not a helix, allows the momentum to be determined. The procedure for doing so is far from obvious, and much effort has been devoted to various software techniques[1]. Each detector magnet is unique; the degree of non-uniformity has no precedent. Twenty or thirty years ago this could bring some nasty surprises, but nowadays computer design codes for magnets are very good. Perhaps most important, there is only one opportunity to make the magnetic measurements. Between the engineers assembling and commissioning the magnet, and the physicists filling it with detectors, there will be a few days allowed for the magnetic measurements. Clearly a suitable measuring machine must be ready for this opportunity and must work without breakdown throughout a carefully prepared schedule. 2. STORED ENERGIES AND POLE-TIP FORCES It is worth drawing attention here to some of the consequences of the very large volumes of magnetic field, in essentially air, of these detector magnets. This contrasts sharply with accelerator magnets. It also shows a spectacular, perhaps frightening, trend over the past few decades and towards the future. The stored energy density is