Efficiency of the transfer of electron bunches between the various components of the acceleration system from the linac to their eventual capture in the storage ring needs to be optimised to maximise the rate of current storage during refill and to reduce losses that lead to environmental radiation and vacuum chamber activation. Efficiency is closely monitored during the preparation of each new beam delivery mode and then subsequently at each refill during operation. A system for monitoring the transfer efficiency has been in use since the start of operations at the ESRF using torroidal current transformers that monitor the passage of beam charge in the TL1 and TL2 transfer electron beamlines as well as in the booster and storage rings. The proximity of these charge measuring elements to transient noise sources (kicker magnets) and their broadband acceptance, makes them susceptible to error at low beam current. The current injection efficiency system is no longer capable of keeping up with the increasingly more stringent regulations on permitted radiation dose exposure. Furthermore, optimisation of the accelerator complex now needs to be done at even lower beam current.

A new system has been developed using narrow band high signal level detectors (striplines) placed in the vicinity of the electron beam passage at different locations in the ESRF accelerator complex. The induced RF signal at the ESRF characteristic frequency (352.2 MHz) is amplitude demodulated using digital processing techniques. The fast digitalisation of analog signals (analog to digital converters) coupled with very high speed digital circuits based on commercially produced FPGAs, have allowed a significant improvement in the precision of such RF measurements. A device developed by Instrumentation Technologies called 'Libera' incorporates these new developments. This device has been developed for high precision beam position monitoring for the latest generation of electron storage rings. Its embedded firmware has been optimised to perform relative and absolute measurements of beam charge at 4 locations at the ESRF: linac to booster transfer line TL1, booster, booster to storage ring transfer line TL2 and the storage ring (see Figure 146). Digital conversion and digital filtering is performed within the device, which then reports values of the beam charge and transfer efficiency between the various accelerator components to the control system.

Fig. 146: Schematic diagram of the new injection efficiency measurement system. FPGA based electronics monitor the current in the various parts of the accelerator complex by means of high sensitivity stripline chambers. The signals are demodulated at 352 MHz and processed in real time to provide current, charge and transfer efficiency measurements to the operator in the control room.