Elettra operates in top-up mode

Elettra, the 2/2.4 GeV third generation Italian light source, has successfully joined the synchrotron facilities that operate fully in top-up mode. Located on the outskirts of Trieste, Elettra has operated for users since 1994, but during the past few years a large upgrade programme has taken place. This has included the construction and start-up operation of a full-energy injector. The new injector chain and the other machine and beam-line upgrades, together with the demands for intensity and thermal stability, naturally led to the change to top-up mode, in which frequent beam injections maintain a constant beam current in the storage ring during user operations. This is in contrast with the decay mode, where the stored beam is allowed to decay to some level before refilling occurs.

Elettra was not originally designed for this type of operation (and indeed even operated for many years without a full-energy injector). However, in May, only a year after establishing the stable operations of the new injector, the storage ring began to work successfully with top-up at the two user energies of 2 GeV and 2.4 GeV. Elettra has thus become another example showing how a third-generation synchrotron that previously operated in decay mode can advance to full top-up operation, in this case at multiple energies.

With top-up operation the photon intensity produced at Elettra is stable and the integrated intensity is 60% higher over a time period equal to the beam lifetime. Thus while keeping the optical components of the beam lines in thermal equilibrium, the integrated number of photons is also higher, so providing an additional gain in beam time for the experiments. At the same time the intensity-dependent electronics also remain stable, allowing submicron accuracy in the position of the electron beam and hence a higher stability of the photon beam.

Elettra’s upgrade to top-up started in 2009 and included the addition of various diagnostic and radiation-safety instruments, modification of the control and interlock software, fine tuning of the timing of the kicker and septa, as well as a revised operation strategy. A great deal of effort in collaboration with the radiation-protection team resulted in a high-level application with a “top-up controller” handling and controlling all aspects of the procedure. Careful radiation measurements at each beam line under various conditions of the injected beam, together with the high injection efficiencies achieved at both energies, meant that no additional shielding was required for the beam lines. Radiation levels in all beam lines remain below 1 μSv/h for efficiencies higher than 90%.

The project for the full-energy injector started in 2005 and finished by providing beam in March 2008 on time and within budget. The new injection chain consists of a 100 MeV linear accelerator and a 2.5 GeV booster that sends the beam into the storage ring at a rate of up to 3 Hz. The storage ring beam current at 2 GeV is set by the users to 300 mA and top-up occurs every 6 minutes by injecting 1 mA in 4 s, thus keeping the current level constant to 3‰. At 2.4 GeV the stored beam current is set to 140 mA and top-up occurs every 20 minutes, injecting 1 mA in 4 s to maintain the current level constant to 7‰.

The users have chosen fixed current-interval top-up (1 mA) instead of a fixed time interval. The injection system is perfectly tuned and for the majority of the beam lines does not produce interference with data-acquisition processes. A gating signal is also provided, but up to now only a few, very sensitive beam lines see some interference and therefore are gated.

The change to top-up mode required no transition period and once it began all went exceptionally smoothly, thanks to the very good preparation and the high level of expertise of the personnel involved. Although at the beginning, the operation in top-up was programmed for 20% of users’ beam time, it became immediately clear that the users strongly preferred this mode and so Elettra has operated in top-up for 100% of the beam time dedicated to users right from the start in May.