Thirteen state-of-the-art new radiofrequency (RF) cavities have been installed in the storage ring as part of the ESRF’s Extremely Brilliant Source (EBS) project to increase the brilliance and coherence of its X-ray beams by a factor of 100.
RF cavities are fed with hundreds of kW of power from radiofrequency transmitters, which they convert into millions of volts of accelerating voltage needed to replace the energy lost by electrons in the storage ring as they emit X-rays. The new cavities are innovative because they can also dampen Higher Order Modes (HOMs) – electromagnetic resonances at higher frequencies (like a harmonic on a plucked guitar string) that can destabilise and even destroy the electron beam. The enhanced sensitivity of the EBS machine to HOMs means the new cavities will play a vital role in ensuring a stable electron beam – thus producing stable X-ray beams.
The 13 cavities have been installed in three different zones of the storage ring, which posed a logistical challenge to the teams installing them. Due to the presence of a road bridge rendering part of the storage ring tunnel roof inaccessible, the cavities, which weigh 1.3 tonnes each, had to be lifted from the outside of the tunnel wall to the technical zone on the inside, before being transported upstream, lifted back over the tunnel wall and installed on supporting girders inside the storage ring.
The installation required a lot of preparation by the teams of RF, vacuum, mechanical engineering and metrology experts. Each section of five cavities was put under vacuum and fitted with valves, meaning that only two of the five cavities had to be vented to room pressure during installation to connect them to adjoining cavities. The two were then put back under operating pressures of 10-10 mbar in situ. Metrology engineers pre-aligned the cavity feet with the lower girders on which the cavities sit in the tunnel, making it possible to lower the cavities directly into place.
“Thanks to careful preparation by the teams, the installation was very successful,” said head of the RF group Jörn Jacob. “By anticipating each point in the process we’ve managed to save ourselves a lot of time.”
That time that will now be spent on cabling and connecting the cavities to air- and water-cooling systems, as well as connecting the waveguide system that feeds them with RF power and setting up their control, so that RF power commissioning can start in November.