In order to attenuate the fundamental resonant vibration of the ESRF machine girder around 7 Hz, and to improve beam stability, damping links (a damping device) were completely implemented in the storage ring after the 2001 March shutdown.

The damping link design adds a viscoelastic link between the girder and the floor. It consists of three parts (Figure 186):

- a sandwich structure with Aluminum plates and VEM (Al + VEM + Al)
- a girder mounting fixture (GMF) links the sandwich structure to the girder
- a floor mounting fixture (FMF) links the sandwich structure to the floor.


Fig. 186: Damping link and installation on a G20 magnet girder assembly.

Our aim was to use the sandwich structure with VEM to absorb the dynamic strain energy of the girder assembly related to the rocking motion. The damping links were installed on the two extremities of the girder and floor (as shown in Figure 186) in parallel to the existing jacks.

Vibration tests were performed on quadrupoles before and after the damping link installation. The peak value in the frequency response function at the fundamental resonant frequency is the 'so-called' Q-value. The average Q-value of all the quadrupoles was, respectively, 43.4 and 7.6 before and after the installation of the damping links. The reduction factor is 5.8.

The motion of the electron beam was permanently monitored during the installation of the damping links. The r.m.s amplitude of the horizontal motion in the frequency range 4-12 Hz, where the damping links are efficient, is shown in Figure 187. The amplitude was reduced from 10 µm initially to 2.7 µm (factor 4) after completion of the installation. In broadband (4-200 Hz), the rms. amplitude is reduced from 12 µm to 4 µm, which still significant. Note that the installation of damping links was started during the July 2000 shutdown.


Fig. 187: Rms horizontal amplitude (4-12 Hz) of the electron beam motion along the installation of the damping links in the storage ring.

The power spectral density (PSD) of the horizontal displacement of the electron beam, before and after installation, is shown in Figure 188. Initially, there was a main peak at 6.8 Hz in the horizontal displacement PSD. Once the storage ring had been equipped completely with damping links, the peak at 6.8 Hz in the PSD was dramatically attenuated by a factor of 49. A broad peak around 30 Hz was also observed on the PSD. The damping links are inefficient there, because this peak is due to the lateral rocking motion of the quadrupole QF2 (or QF7) relative to the girder. The resonant motion of the quadrupoles QF2 and QF7 at 30 Hz is excited by the water flow in the cooling circuits. As the girder does not move for this vibration mode, the damping links are therefore not effective for the vibration of the quadrupoles, as well as for the motion of the electron beam around 30 Hz. Some countermeasures to reduce the vibrations of quadrupoles QF2 and QF7 have been studied by finite element simulation, and could be very effective.


Fig. 188: Horizontal displacement PSD of the electron beam before and after the installation of the damping links in the storage ring.

The significant enhancement of the electron beam stability was also observed on the X-ray beam. As an example, Figure 189 shows the spectra of the X-ray beam intensity variation measured with the ID14-EH1 beamline in January 2000 and in April 2001. Damping links for the machine girders were installed between these two dates. The spectra are expressed as a percentage of the DC value. The fluctuation of intensity should be as small as possible, therefore the spectral value should be significantly smaller than 1. The peak at 6.8 Hz in the X-ray bean intensity spectra was removed completely after the installation of the damping links in the storage ring. Note that a local feedback on the electron beam was able to significantly reduce the intensity variation around the peak frequency 6.8 Hz, but the peak was still visible.


Fig. 189: Spectra of the X-ray beam intensity variation measured with the ID14-EH1 beamline.