The ALBA accelerators rely on electromagnets to guide and focus the electrons along the trajectory.

Under the influence of magnetic fields, electrons follow the Lorentz force:

At ALBA four different types of magnets are in use:

• Dipole magnets: these are typical construction electromagnets with an iron yoke, in a C-form or H-from and two coils wound around the yoke. Dipole magnets are used to deflect the electrons in the transfer lines and also to provide orbit correction.
• Quadrupole magnets: as the name indicates, these have four magnetic poles and therefore 4 coils. The magnetic field created by a quadrupole increases linearly with the distance from the center. Quadrupoles are used to focus the electrons and enable the transport of an electron beam over long distances, like for example, an electron beam circulating inside the ALBA storage ring.
• Sextupole magnets: have six poles as its name indicates and are used to provide additional focusing. The magnetic field increases quadratically with the distance from the center.

Main parameters for the magnets used at ALBA. Note the different units on the magnetic field are indicating the different type of field produced.

Construction details

The iron yoke of the magnets is made of steel laminations (0.5 to 1 mm thick), which have been stamped with the right pole profile, glued together and then assembled onto the yoke. The precision of the stamping reaches +/- 15 um. For the combined function magnets of the storage ring an additional machining of the poles has been performed.

The coils have been produced from hollow Cu conductor and are refrigerated with demineralised water. Typical current densities are around 5 A/mm².

The power dissipated by the magnets in the ALBA accelerators during normal operation is around 1 Megawatt.

This injection scheme is one of the most used in 3^rd generation light sources giving its reliability, degrees of freedom in terms of bump tuning, high injection efficiency and transparency for users when optimized for operation.

At ALBA the injection sequence, from the LINAC to the storage ring, is performed every 20 minutes during top up operation to maintain a constant beam current for users, usually with a stable efficiency over 90%. In a single sequence we inject a train of 40 bunches of electrons that will reach the targeted RF buckets present in the storage ring, thus an extremely precise timing and synchronization system is used.

The solution is then to build special magnetic systems that make the electrons bend at a specific curvature radius –depending on the application– in order to produce the required light. These systems are called "insertion devices" because they are installed – in fact, they are inserted – into the straight sections of the Storage Ring.

Insertion devices are made of two magnetic arrays placed in such a way that the trajectory of the electrons follows an oscillation pattern. In general, there is an upper and lower magnetic arrays, which opposite magnetic poles are placed face-to-face. In order to obtain an oscillation of the electrons along the longitudinal direction, each pair of facing magnets is displayed along a longitudinal axis following an alternate pattern. When passing through each magnetic pair – called "semiperiod" – the electrons wiggle and emit light according to the curvature they follow.

There are two main types of insertion devices:

In order to change the energy of the emitted light, the magnetic field produced by insertion devices can be change. When the magnetic array is made of coils, this is achieved by varying the circulating current. When the array is made of permanent magnets, this is done by mechanically separating the upper and lower arrays of magnets.

ALBA has six beamlines fed by insertion devices:

• XALOC and NCD-SWEET are fed by in-vacuum undulators with a period of 21 mm (IVU21)
• CIRCE is fed by an apple-type undulator with a period of 71.36 mm (EU71)
• BOREAS is fed by an apple-type undulator with a period of 62.36 mm (EU62)
• CLÆSS is fed by a multipole wiggler with a period of 80 mm (MPW80)
• MSPD is fed by a multipole superconducting wiggler with a period of 30 mm (SCW30)

The light, produced by the circulating electrons, is emitted tangentially to the curvature they follow. In the case of bending magnets, the light is tangential to the arc described by electrons. In the case of insertion devices, the light is emitted along the axis of oscillation.

In both cases the vacuum chamber in which the electrons are contained has an aperture allowing light extraction. The aperture is attached to a straight vacuum tube that couples the accelerator with the beamline.

This vacuum tube is called "front end", because it is at the "end" of the accelerator and it arrives at the "front" of the beamline hutch. The Front End starts at the accelerator vacuum chamber and ends immediately after passing through a special window in the wall of the bunker tunnel.

Along the length of this vacuum tube, specific pieces of equipment condition the light beam: the slits, the masks and the photon shutter.

• SLITS: sometimes, and depending on each sample in the experiment, there is a need to regulate the intensity of the light beam. This is achieved by using movable slits. They are very special because they have to absorb a significant amount of heat while conserving a precise position and alignment.
• MASKS: depending on the application of the generated synchrotron light, sometimes not all of the emerging fan is used. So, a fixed mask is used to screen the part of the light that will never be used inside the hutch. Masks are placed in the Front End because the screening of X-rays produces radiation that is better kept inside the tunnel.
• PHOTON SHUTTER: The opening or closing of the photon shutter controls the sample illumination.

Apart from optical elements, the Front End is also used to install two important safety systems:

• THE RADIATION SHUTTER is connected to the experimental hutch via an interlocking system designed to avoid radiation exposure to anybody inside the experimental hutches. Only when this device is blocking the light can a person enter and work inside the experimental hutches.
• THE VACUUM TRIGGER UNIT is linked to a fast closing valve. Its function is to isolate the vacuum in the accelerator in the event that it becomes broken in the experimental hutch. This situation happens sometimes in the case of modifications to, or the malfunctioning of, experimental elements.

Finally, the Front Ends are also equipped with an X-Ray Beam Position Monitor (XBPM) in order to measure the position of the emitted light and use this information to align the accelerator with the set up of the experiment.