Light emitted by bending magnets is determined by the magnetic field and the energy of the electrons circulating in the Storage Ring. Given a fixed energy, which in the case of ALBA Storage Ring is 3 GeV, the energy of the emerging light depends on the magnetic field: a high magnetic field generates high energy light (hard X-rays), and a small magnetic field generates low energy light (visible, ultraviolet or soft X-rays).

Magnetic fields determine the curvatures induced on the circulating particles, and therefore its path. Because this path should be confined to be within the vacuum chamber, it cannot be changed once the accelerator is built. Therefore the magnetic field of the bending magnets is fixed, and consequently the characteristics of the light they emit.

However, for certain experiments, scientists need light at energy levels, or with very specific characteristics (circular polarization, small divergence, high intensity, etc), that cannot be provided by bending magnets.

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.

 Image of insertion device

In the image above, you can see the upper and lower magnetic arrays, and also the vacuum chamber in the middle.

There are two main types of insertion devices:

  • WIGGLERS. With wigglers, the objective is to apply an intense magnetic field locally –in order to obtain energetic X-rays– and repeat the oscillation several times in the longitudinal direction. At each wiggler, light is produced, and at the end of the device, we have a high energy and very intense light beam.
  • UNDULATORS. In this case, the light emerging at each wiggle interferes with the light which emerged in the other wiggles, and we have an interference pattern, both in the space and in the energy planes. This means that the light is spatially very concentrated into a narrow cone, and also in several specific energies that we call harmonics. Undulators are used when extremely brilliant light is required.

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.

    Insertion Devices graphic showing wiggler and undulator

ALBA has six beamlines fed by insertion devices:

  • XALOC and NCD 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)
  • CLAESS 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)