LHC beam stability and feedback control

Autor: Steinhagen, Ralph
Přispěvatelé: Böhm, Albrecht
Jazyk: angličtina
Rok vydání: 2007
Předmět:
Zdroj: Aachen : Publikationsserver der RWTH Aachen University VII, 210 S. : Ill., graph. Darst. (2007). = Aachen, Techn. Hochsch., Diss., 2007
Popis: This thesis presents the stability and the control of the Large Hadron Collider's (LHC) two beam orbits and their particle momenta using beam-based feedback systems. The LHC, presently being built at CERN, will store, accelerate and provide particle collisions with a maximum particle momentum of 7 TeV/c and a nominal luminosity of L =10^34 cm^-2 s^-1. The presence of two beams, with both high intensity as well as high particle energies, requires excellent control of particle losses inside a superconducting environment, which will be provided by the LHC Cleaning and Machine Protection System. The performance and function of these systems depends critically on the stability of the beam and may eventually limit the LHC performance. The aim of this thesis is to contribute to a safe and reliable LHC commissioning and machine operation. Environmental and accelerator-inherent sources as well as failure of magnets and their power converters may perturb and reduce beam stability and may consequently lead to an increase of particle loss inside the cryogenic mass. In order to counteract these disturbances, control of the key beam parameters -- orbit, tune, energy, coupling and chromaticity -- will be an integral part of LHC operation. Since manual correction of these parameters may reach its limit with respect to required precision and expected time-scales, the LHC is the first proton collider that requires feedback control systems for safe and reliable machine operation. The first part of the analysis gives an estimate of the expected sources of orbit and energy perturbations that can be grouped into environmental sources, machine-inherent sources and machine element failures: the slowest perturbation due to ground motion, tides, temperature fluctuations of the tunnel and other environmental influences are described in this thesis by a propagation model that is both qualitatively and quantitatively supported by geophone and beam motion measurements at LEP and other CERN accelerators. These confirm that ground motion contribution to the orbit can be negligible in the LHC for frequencies above 1 Hz. For frequencies below 1 Hz, orbit drifts are dominated by random ground motion and can reach 200-300 um within 10 hours. Solar and lunar tides primarily vary the beam momentum between $-0.9\ldots0.5\cdot10^{-4}$ on a time scale of about 6 hours and, as a secondary effect, the orbit shifts by $\unit[\stackrel{+100}{-170}]{\mu m}$ in regions with dispersion function values of 2 m. The fastest perturbations are expected during the energy ramp and during the change of the final focus optics (beta-squeeze) in the experimental insertions. During the ramp, these drifts can reach up to 600-700 um with drift velocities of about 15 um/s and, dependent on the magnet alignment in the experimental insertions, the beta-squeeze induced orbit drifts can reach up to 30 mm with drift velocities of up to 25 um/s. These orbit perturbations exceed the required beam stability by one order of magnitude. The LHC Cleaning System, imposing one of the tightest constraints on beam stability, requires a beam stability in the range of about 15-25 um at the location of the collimator jaws. LHC Machine Protection and other systems have similar orbit stability requirements. Due to the large number of requirements and their spacial distribution, it is advisable to stabilise the orbit not only in selected local regions but in the entire accelerator. The second part of this analysis deals with the control of the two LHC beams' orbit and energy through automated feedback systems. Based on the reading of the more than 1056 beam position monitors (BPMs) that are distributed over the machine, a central global feedback controller calculates new deflection strengths for the more than 1060 orbit corrector magnets (CODs) that are suitable to correct the orbit and momentum around their references. The achievable stability is essentially limited by the quality and performance of the individual feedback components. Hence, this thesis provides an analysis of the BPMs and CODs involved in the orbit and energy feedback. The BPMs are based on a 'wide-band time normaliser' circuit that converts the transverse beam position reading of each individual particle bunch into two laser pulses that are separated by a time delay and transmitted through optical fibres to an acquisition card that converts the delay signals into a digital position. The calculation of the orbit is accordingly computed by averaging the individual bunch position measurements for a given number of bunches and turns inside the machine. The position measurement resolution is dependent on the bunch length as well as the number of particles per bunch. Based on tests with nominal beam performed in the SPS, the BPMs show a bunch-by-bunch position resolution of about 60 um, which after averaging, corresponds to an orbit resolution of about 5 um. A simple error model has been tested and compared to the measurement accuracy of LHC type BPMs, obtained through beam-based measurements in the SPS. The measurement accuracy that is normalised to the BPM half-aperture was shown to be in the order of 1% and, within the specified beam parameters, is in accordance to the specification.
Databáze: OpenAIRE
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