Determination of Beam Intensity and Position in a Particle Accelerator

A subject of the thesis is conception, design, implementation, tests and deployment of new position measurement system of particle bunch in the CERN PS circular accelerator. The system is based on novel algorithms of particle position determination. The Proton Synchrotron accelerator (PS), installed at CERN†, although commissioned in 1959, still plays a central role in the production of beams for the Antiproton Decelerator, Super Proton Synchrotron, various experimental areas and for the Large Hadron Collider (LHC)‡. The PS produces beams of different types of particles, mainly protons, but also various species of ions. Almost all these particle beams pass through the PS. The quality of the beams delivered to the LHC has a direct impact on the effective luminosity, and therefore the performance of the instrumentation of the PS is of great importance. The old trajectory and orbit measurement system of the PS is dated back to 1988 and no longer fulfilled present day requirements. It used 40 beam position monitors (BPMs) and an analogue signal processing chain to acquire the trajectory of one single particle bunch out of many, over two consecutive turns at a maximum rate of once every 5ms. The BPMs were in good condition, however the electronics was aging and increasingly difficult to maintain. The new measurement system digitizes the BPM signals using 125MS/s, 12 bit ADCs. The digitized sample stream are processed on the fly into individual bunch positions, using numerical algorithms implemented on fast programmable logic (FPGA). The system stores the lateral positions of all bunches in the machine over the full duration of an acceleration cycle, requiring large memories. Post processing can be applied to the data in order to extract orbits (averaged positions over many turns), mean radial position,phase space images or machine tune data. Client orbit display programs running on operator consoles can then concurrently request measurements from any interesting part of the cycle. The ADCs digitize their input signals at a constant rate, whereas the revolution frequency of the particle bunches varies along the acceleration cycle. The increase of this frequency depends on the increase of particle velocity, and varies over more than an octave for heavy ions. The system is able to keep track of each individual bunch from injection all through to ejection using tracking and synchronization algorithm. In essence, its task is to decide which of the ADC samples belong to each particle bunch. The new system uses an entirely numerical synchronization algorithm, implemented in the FPGA and running at the ADC sampling rate. Synchronization is made more complicated by the possibility of the PS accelerator to change the harmonic number of the machine (the number of possible bunches in the machine) during the acceleration. Harmonic change requires fast re-synchronization. These operations are used, among others, to split the bunches into two or three bunchlets, in order to better match the beam to the properties of the subsequent accelerators. The new measurement system is able to keep track of the beam throughout the accelerator. Development of the effective algorithm, which enables precise trajectory tracking of individual particle bunches with sub-mm precision was the aim of this thesis. Moreover, using a similar, already verified algorithm, a new absolute beam intensity measurement system was developed as well. These two measurement systems are complementary and for the first time give the user full information about the beam position and intensity in the accelerator. The scope of the thesis includes measurement system conception, hardware design and development, algorithm development and tests.