A wiggler that wiggles well

05 Dec 2022

In September 2022, the X-ray source for BEATS, a three-pole wiggler, was successfully installed and aligned inside the SESAME storage ring. After installation, the next important step in the construction and commissioning of the beamline consists of testing the wiggler with beam to ensure the device does not have an undesired effect on the storage ring.

In a synchrotron, wigglers, or undulators, play an essential role in the production of X-rays for a beamline. These devices consist of a series of magnets, either permanent or superconducting, set-up with alternate magnetic fields. Despite their name, they do not move themselves but instead force the electrons that pass through them to undulate or wiggle. These oscillations lead to the emission of intense and highly collimated electromagnetic radiation with wavelengths over a broad spectrum, from the visible light to hard X-rays.

If well-designed, the magnetic field of the wiggler forces the electrons to follow a wiggly path without affecting the parameters that determine the electron orbit, especially position and angle, when the electrons exit the device. If focussing effects are created inside the wiggler, they can lead to a rupture in the symmetry of the machine and a change in chromaticity. To guarantee the wiggler does not adversely affect the electrons, the magnetic field integrals along the full trajectory of the device must be as close as possible to zero.

The integrals were measured before installation in the storage ring tunnel (see BEATS Deliverable D3.6), however final verification is only possible by measuring the real effect of the wiggler on the machine with beam. Experts from the SESAME machine group carried out these measurements during the machine dedicated time in November.

Samira Kasaei (Accelerator physics) and Maher Attal (Accelerator physics and Technical Director) in the SESAME control room during the tests of the wiggler for BEATS.

These measurements returned the following results. The BEATS X-ray source:

  • creates (uncorrected) orbit distortion of 500 µm horizontally and negligible values vertically,
  • has a negligible effect on the optical symmetry of the machine, and
  • causes only a small change in chromaticity of 4%.

Making use of the 3PW’s corrector coils, the SESAME machine group was able to reduce these small distortions to reach completely acceptable values.

We can conclude that the SESAME storage ring tolerates the BEATS X-ray source with only minor corrections.

The next step now is to carry out radiation tests on the hutches on the beamline side, and then install the optical and experimental equipment. #BEATSisReal!