In any synchrotron light source, electrons are moving along a fixed path through the bending magnets; they are confined into a narrow beam, able to produce intense and concentrated light through the quadrupoles and sextupoles; finally, they produce light with specific properties through the insertion devices. Therefore, in all accelerators, the role of magnetic fields is very important, and to have a precise characterization of them is a crucial factor.

At ALBA, the magnetic characterization of magnets and insertion devices is done at the Magnetic Measurements Laboratory. It is provided with five different measurement benches, each one suited to each magnet type.

In order to characterize the bending magnets, one is used to measure the so-called "field maps", i.e., a map in which the intensity of the magnetic field along the trajectory of the electrons is represented. This allows high-accuracy calculations to be made regarding the trajectory of the electrons inside the accelerator before it is assembled. The measurement bench includes a 3D robotic arm which has a tip in which a magnetic sensor –a "Hall probe" sensor– is placed. The bench is so accurate that is able to measure with a precision of 10-4 of magnetic fields and with a positional accuracy to within 50 microns.

To characterize the multipole magnets –quadrupoles and sextupoles– another method of characterization is used: in this case, the field has to be repetitive along a circumference which is not only centred on the electron trajectory but also perpendicular to it. Any discrepancy will influence the extent of the confinement of the electrons into a narrow beam. So, the measurement of this kind of magnet is done using a so-called “rotating coil”. The core of this bench is a coil that is rotated inside the multipole magnet. The relative movement inside the magnetic field induces a potential in the coil that is recorded with respect to the angle. A perfect magnet should yield a sinusoidal curve. By extracting the harmonics of this curve, by means of Fourier analysis, one can deduce the accuracy of the original magnetic field under evaluation.

Regarding the insertion devices, the Laboratory is equipped with three specific benches: (1) a Helmholtz coil bench, (2) a fixed stretched wire bench and, (3) a flipping coil bench.

(1) is dedicated to measuring the magnetization of each one of single blocks that are used to build a magnetic array, (2) is used to measure field integrals of undulator array subsets, and (3) is used to measure the field integrals of complete devices. Because these devices are placed in straight sections of the accelerators, it is very important that their net contribution to the deviation of electrons is null. This is characterized through the so-called magnetic field integral. So, these benches are specifically designed and used to ensure that the field integrals of these insertion devices approaches zero.

Magnetic Measurements Lab - General view  Magnetic Measurements Lab - showing detail in a bench