Design and Optimization of the Beam Orbit and Oscillation Measurement System for the Large Hadron Collider

The Large Hadron Collider (LHC) accommodates some 100 collimators whose role is to perform beam cleaning and protect the machine from dangerous particle losses. The collimators are mechanical devices consisting of moveable jaws. Precise positioning and control of the jaws is critical for the cleaning efficiency. Therefore, after the first long shut-down, the LHC was equipped with 18 collimators of a new type. Movable jaws of the new collimators have embedded beam position monitors (BPM) which allow their precise positioning with respect to the circulating beam. However, the existing electronic systems for BPM signal processing could not achieve the required resolution and precision of the position measurements. In addition to the new collimator BPMs, the LHC accommodates more than 1000 BPMs for measuring the transverse positions of the two counter-rotating beams. The standard LHC BPM system uses these BPMs to measure the orbits and oscillations of the beam. The most important BPMs are located next to the LHC experiments. The beam measurements in such locations are the most challenging as the two beams have to be controlled at a fine precision in order to achieve their efficient colliding. Improving the resolution and precision of the position measurements can contribute to the improvement of the machine performance. Performance of the LHC also depends on the magnet optics. Important machine parameters like betatron coupling, beta-beating and phase advance are obtained by exciting the transverse beam oscillations and measuring the amplitudes and phases of the beam response using BPMs. The standard BPM system requires millimetre-order beam excitation to obtain the measurements of a sufficient quality. For machine protection reasons these measurements can be performed only with special beams and dedicated machine set-up. The main task of this doctoral work was to design, prototype, build and optimize a new electronics system for beam position and oscillation measurements in LHC. The system called DOROS (Diode Orbit and Oscillation was primarily designed for the new LHC collimators. The same system was also used to provide high-resolution orbit and oscillation measurements in the selected LHC BPMs. The DOROS systems consist of front-ends, each processing signals from up to two BPM sensors composed of horizontal and vertical transverse plains or to two collimators consisting of upstream and downstream BPMs. The RF signals in a front-end are first filtered, amplified and then split into two diode detector sub-systems which work in parallel. A so called Diode Orbit (DOR) subsystem, based on novel compensated diode detector technique, was designed to perform beam orbit measurements. This thesis describes the analogue processing channels followed by the digital signal processing of the turn-by-turn data and real-time algorithms. The algorithms provide beam based calibration of the channel asymmetries as well as autonomous gain control of the front-end amplifiers. The DOR subsystem was characterized with the laboratory measurements and its performance was demonstrated on a number of beam measurements, showing the achieved sub-micrometre resolution, precision, and long-term stability. The position readings from selected front-ends are also used by the LHC interlock system which terminates operation with beams if the beam positions exceed safe limits. A so called Diode Oscillations (DOS) subsystem, which is based on direct diode detection technique, was designed and optimised to measure small beam oscillations. This thesis describes both the analogue and digital signal processing in a front-end as well as its synchronization and timing circuits. The sampling of the ADCs can be synchronized to the beam allowing to perform precise measurements of the beam coupling and phase advance. The front-end units continuously transmit the measurement readings over Ethernet at 25 Hz rate to the system servers synchronously to the LHC timing. Together with the measurement readings the front- ends transmit also statuses and other data important for diagnostics and reliability of the system. At the same time the acquisition data is stored in parallel to the front-end buffers for detailed turn-by-turn and post-mortem signal analysis.