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 six different measurement benches, each one suited to each magnet type.

In order to characterize the bending magnets, two of the benches are 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. These measurement benches do so by scanning a magnetic sensor –a 3D “Hall probe” sensor– through the region where the magnetic field has to be determined. In the most conventional bench the magnetic sensor is placed at the tip of a 3D robotic arm, allowing to perform measurements on magnets with lateral access (like “C-shape” bending magnets). The other bench has a more innovative design, with the magnetic sensor placed on top of a carbon fiber tape stretched between the ends of a support structure, in a configuration similar to a violin bow. This second bench allows carrying out measurements inside magnets without lateral access (like “H-shape” bending magnets or solenoids) by passing the flexible tape through the open ends of the structure to be characterized. Both benches are so accurate that they are able to measure the magnetic field with a precision of 10^-4 and with an absolute 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 must have a well-defined sinusoidal variation along a circumference that is centred on the electron trajectory and perpendicular to it. Any deviation from this ideal behaviour 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 an electric voltage in the coil that is recorded with respect to the angle. By extracting the harmonics of the recorded voltage curve by means of Fourier analysis, one can determine the deviation of the magnetic field under evaluation from the ideal case.

Regarding insertion devices, these devices are placed in the straight sections of the accelerator, and hence it is crucial that their net contribution to the deviation of the electron beam is as small as possible. This contribution is characterized through the so-called magnetic field integral. Therefore, the Laboratory is equipped with three benches which are specifically designed and used to ensure that the field integrals of insertion devices are close to zero: (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 the 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.