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What is top-up?

Top-up is a new way of injecting beam into the ESRF accelerators. The new mode sees the accelerators refilled with electrons every 20 minutes rather than every four to 12 hours as previously. This results in a much higher integrated current over a 24-hour period, providing users with better beam stability, low vertical emittance in all filling modes (with correspondingly higher brilliance and resolution) and a nearly constant beam current. Trial periods of top-up in 16-bunch mode and 4*8mA mode were run throughout 2015 and 2016, with the mode being officially launched into operation on 26th April 2016. It should be further improved by the end of 2016 with the implementation of bunch cleaning in the booster.

Why is it being implemented at the ESRF?

Previously the vertical emittance would be artificially increased in some modes in order to increase the lifetime, but the nearly constant beam current provided by top-up mode means that the emittance – and the resulting brilliance and resolution for users – no longer has to be sacrificed. The near-constant beam current will also maintain a constant heat load on the optics and improve the stability.    

Why is it important for ESRF-EBS?

The top-up frequency is a crucial parameter for ESRF-EBS because the lifetime of the beam is dependent on the emittance. As the emittance of the new machine will be much smaller, more frequent injections will compensate for the lower lifetime (expected to be reduced by a factor 3).

What technology is required for top-up?

Preparing the machine for reliable top-up operation has been a huge undertaking, requiring significant modifications to be made, and work has been in progress since 2012.

Major upgrades to the injector have recently been completed, including a new pre-buncher on the linac, two additional RF cavities to provide RF back-up, which also allows a drastic reduction in electrical consumption, and the development of a sequencer that performs injection automatically. New diagnostic tools have also been implemented in the booster, including a feed-forward correction system that limits disturbance to the stored beam caused by the injection itself, and another correction system to minimize disturbance caused by the septum field leakage.

A new storage ring cleaning technique has also been developed for this project and is now routinely used during user service mode (USM) for the 16-bunch and hybrid modes. It reduces the strong vertical blow-up just after injection from 2 nm to only a few pm. A cleaning method in the booster has also been developed and is close to operation. Its implementation should further improve topping up by reducing disturbance to the beam during and after injection.

A new 4 Hz ramped injection power supply system (RIPS) for the booster is in the final stages of commissioning and is foreseen to be in operation by the end of the year. This will also improve reliability.