High accuracy magnetic field mapping of the LEP spectrometer magnet

The Large Electron Positron accelerator (LEP) is a storage ring which has been operated since 1989 at the European Laboratory for Particle Physics (CERN), located in the Geneva area. It is intended to experimentally verify the Standard Model theory and in particular to detect with high accuracy the mass of the electro-weak force bosons. Electrons and positrons are accelerated inside the LEP ring in opposite directions and forced to collide at four locations, once they reach an energy high enough for the experimental purposes. During head-to-head collisions the leptons loose all their energy and a huge amount of energy is concentrated in a small region. In this condition the energy is quickly converted in other particles which tend to go away from the interaction point. The higher the energy of the leptons before the collisions, the higher the mass of the particles that can escape. At LEP four large experimental detectors are accommodated. All detectors are multi purpose detectors covering a solid angle of almost 4. . The first period of operation of LEP (1989-1995) has been to the scan of the Z boson energy range, and the electrons and positrons beams were accelerated up to about 45 GeVper beam (see section 1.3.1). Since the end of 1995 LEP is devoted to operate at higher energies, up to more than 100 GeVper beam in the 1999-2000 runs. It was thus possible to investigate the W boson properties and the Higg’s particle existence, in case its mass reveals to be at this energy level. The types of event (i.e. the fundamental particles) which can be detected depends on the lepton energy before colliding (see Fig. 1.2). The particle beams energy calibration is thus a focal point for a successful operation of the collider. During normal operation electrons and positrons are bent inside the ideal circular trajectory by mean of a number of dipole magnets. The energy of the leptons depends on the total dipole field along their trajectory. The bending field around LEP is monitored by Nuclear Magnetic Resonance (NMR) probes which sample the field in twenty locations. Such measurements provide part of the data for the energy calibration model (see chapter 2). In 1999 a spectrometer has been installed in a section of the LEP ring as a new tool for the beam energy determination. The spectrometer layout mainly consist in an iron-core bending magnet and six Beam Position Monitor (BPM) stations. The main subject of this thesis work relates to the design, development and application of a special system which is able to measure with high accuracy the total integral field of the spectrometer bending dipole. The dissertation begins with a short introduction to CERN and its accelerator complex. Further on the LEP collider is described in detail and the basics of the beam energy calibration are included. Chapter 3 describes the LEP spectrometer, starting from the concept and layout, outlining the dipole magnet characteristics. The chapter ends with the presentation of some measurements performed on the LEP injection magnets in order to investigate the possible thermal effects to be foreseen on the spectrometer magnet. Chapter 4 introduces basics concepts about magnetic measurements techniques, mainly concentrating on the NMR theory and application. Chapter 5 provides the detailed description of our measurement setup based on a travelling mole equipped with magnetic field detectors and monitored with a laser interferometer. The design, realization and test of the whole system is considered. The following chapter gives the results of the first field mapping with the mole system, performed on an standard LEP iron-concrete core dipole, located in a laboratory at ground level. Chapter 7 finally describes the mapping campaign on the LEP spectrometer dipole magnet with a full analysis of the achieved results.