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Left: third harmonic cavity installed in the BESSY II storage ring. Right: the cavity prototype at the ALBA lab with part of the RF team (2021).
Cerdanyola del Vallès, 29th June 2023 The radiofrequency (RF) systems are responsible of the electrons acceleration in the Booster and the Storage Ring. In the Booster, the energy of the electrons is increased from 100 MeV to 3 GeV. In the Storage Ring, the RF systems just restore the energy that the electrons lose due to synchrotron radiation.
Electric fields are created in resonant cavities which are fed by RF amplifiers. RF cavities are made of copper and have a specific size and shape that causes the electromagnetic field to resonate inside. The acceleration is accomplished creating these high electric fields in the path of the electrons.
ALBA's RF cavities currently oscillate at a frequency of 500 MHz. It means that the direction of the electric field they generate changes 500 million times per second. Now, the goal is to add a new 3rd harmonic system where the oscillation frequency will be three times higher (hence the name 3rd harmonic). So, the cavities will oscillate at 1.5 GHz: 1.5 billion times per second!
Although the two systems are similar, the purpose of each is very different. While the first cavities will continue to serve to recover the energy that the electrons lose when emitting synchrotron light; the new ones will be used to enlarge the bunch length. In other words, with this higher frequency it is possible to increase the elongation of the electron bunch. By having more space, the electrons density decreases, so the number of collisions among them is also reduced, which translates into a longer electrons’ lifetime in the accelerator and a higher beam stability.
To do so, ALBA's RF group has designed and built a prototype of a 3rd harmonic cavity. A collaboration agreement between the German synchrotrons BESSY II (HZB) and DESY and ALBA was signed on 2021 in order to test the 3rd harmonic system with real beam.
During the 2021 year, the prototype of the RF cavity and of the Low Level Radio Frequency (LLRF) system were finished, and passed successfully the acceptance tests. And so, at the end of the year, both systems were sent to Berlin, to the HZB institute. On May 2022, the cavity was installed in the BESSY II Storage Ring in order to be tested in real conditions with beam after the summer shutdown. The first results have proved a great performance of the cavity and have managed to increase the beam lifetime up to a factor of 3.
The interest of the three parties of the collaboration on harmonic cavities is motivated mainly by the 4th generation upgrade projects that are under development by each one: Petra IV at DESY, MLS II and BESSY III at HZB and ALBA II at ALBA.
The planning for ALBA is to install the 3rd harmonic system in the storage ring in 2026. In this way, tests can begin before the long shutdown necessary to install all ALBA II components. The ultimate goal is to have 10 RF cavities installed at the storage ring: 6 fundamental (the current 500 MHz ones) and the 4 new 3rd harmonic ones.
The beam lifetime is a key parameter for the performance of a synchrotron light source, a large lifetime reduces losses and the frequency of injections. For ALBA II the goal is to have a lifetime of several hours for 300 mA. Since the beam lifetime is proportional to the bunch length, enlarging the bunch length with a 3rd harmonic system becomes the proper solution.
The prototype construction was co-funded by the European Regional Development Fund (ERDF) within the Framework of the Operative Programme 2014-2020.
With the collaboration of Fundación Española para la Ciencia y la Tecnología. The ALBA Synchrotron is part of the of the Unidades de Cultura Científica y de la Innovación (UCC+i) of the FECYT and has received support through the FCT-21-17088 project.
