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Ultimo aggiornamento 3 nov 2017
Anna Pugliese
Sesso F
Esperimento MICA
Tipo Laurea Magistrale
Destinazione dopo il cons. del titolo Altro
Università Universita' Di Napoli
Abstract In modern particle accelerators, as the LHC at CERN, the collider experiment aim is to maximize the luminosity, for this purpose high current intensity are required. Current intensity increase is linked to instability problems, which restricts the current increase itself. One of the main cause of instabilities is the coupling between the beam and the accelerator with its components, each of which contributes in a specific way to the total instability. The Beam Coupling Impedance concept was born to describe, in the frequency domain, this interaction; this parameter can be evaluated for each accelerator component. One of the main contribution to the LHC impedance comes from the collimation system. In this thesis work, the impedance contribution of LHC tertiary collimators (TCT) is taken into account. For these structures, the impedance behaviour as function of frequency shows many resonant peaks, due to trapped Higher Order Modes (HOM) excited by the presence of discontinuities. It is possible to distinguish localized modes with high quality factor Q for lower frequencies (Narrow-band impedance) and a smooth dependence above the cut-off frequency, due to the resonances overlap (Broad-band impedance). The bunched particles, moving inside the accelerator vacuum chamber, has its own frequencies spectrum. If one of these frequencies correspond to one of the narrow band impedance peak of the collimator, the beam acts as source of these modes inside the structure. To understand this concept just think to a resonant cavity that can express its own resonance modes only if an external field is feeding them at the same frequency. For high current intensity beams and short bunches, this mechanism can produce strong instabilities, and thus beam loss. The reduction of the impedance contribution of these structures is mandatory. The remedy can be: 1. to shift the resonance coupled with the beam spectrum frequencies of a sufficient amount in order to reach a safety margin of distance; 2. to damp trapped modes by enlarging and reducing the resonance. Until recently, small pieces of ferrite were added in the resonant components in order to damp those modes by enlarging and reducing the resonance. One should bear in mind that these phenomena develop take place in a hostile environment from the point of view of heat removal. First, it is impossible to rely convection heat removal. As matter of fact, the ferrite has very low heat conductivity and for electromagnetic reasons not always they can be placed on the metallic walls of the vacuum tank. Moreover, these materials are subject to degassing when exposed to high temperatures and therefore, can degrade the accelerator chamber vacuum. In addition to this, radiation heat transfer is very low unless the item reaches very high temperature: in case of ferrite this material can even pulverise, poisoning all the surrounding. The present work, inserted in the INFN project MICA, was born at CERN, where the first studies have been carried out, with the idea to find a valid alternative to ferrite, avoiding its problems. The choice has fallen on engineered absorbing materials, called “metamaterials”. Two kinds of metamaterials have been investigated as solution to reduce both narrow-band and broad-band impedance. In particular, 2D Split Ring Resonators (SRRs) and 3D pyramidal structures have been chosen as respectively single mode damper and broad-band absorbers. For the first time, the possibility to use metamaterials as absorbers and HOM dampers in order to reduce impedance contributions in the LHC collimation system has been studied. This study has been performed by means of measurements and simulations in a well-known resonant structure, a Pill-Box cavity. In the cavity, the impedance study translates in the electromagnetic characterization of the cavity with and without the metamaterial structures.
Anno iscrizione 2014
Data conseguimento 19 lug 2017
Luogo conseguimento Napoli
Maria Rosaria Masullo Rita Massa Vittorio Giorgio Vaccaro 
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