Prototype system for phase advance measurements of LHC small beam oscillations

Magnet lattice parameters of the Large Hadron Collider (LHC) are measured by exciting beam transverse oscillations that allow measuring their phase advance using the beam position measurement (BPM) system. However, the BPM system requires millimetre oscillation amplitudes, with which nominal high intensity beams would cause large particle loss, dangerous for the LHC superconducting magnets. Therefore, such measurements cannot be done often, as they require special low intensity beams with important set-up time. After its first long shut-down the LHC will be equipped with new collimators with embedded BPMs, for which a new front-end electronics has been developed. Its main processing channels based on compensated diode detectors are designed for beam orbit measurement with sub-micrometre resolution. It is planned to extend this system by adding dedicated channels optimised for phase advance measurement, allowing continuous LHC optics measurement with much smaller beam excitation. This subsystem will be based on diode peak detectors, similarly to the LHC base-band tune (BBQ) measurement systems. The signal acquisition, digital processing and data transmission will therefore need to serve both Diode ORbit (DOR) and Diode OScillation (DOS) sub-systems, resulting in one data stream adequate for Ethernet transmission. The DOR sub-system has already been prototyped for some time with very good results. This paper describes the first prototype of the DOS sub-system, in particular its digital signal processing implemented on a simple 32-bit microcontroller, meant to demonstrate the feasibility of real-time phase advance calculation on such a modest hardware. After introducing the subject, the system architecture and discussing the prototype processing algorithms, first lab measurement results are presented. The achieved phase advance measurement resolution in the order of 0.1º for 0.03 % amplitude modulation and 1 Hz system bandwidth gives perspective of LHC beta-beat measurements with the beam excitation at the 10 μm level.