This Master thesis has been carried out at the CERN, the European Organization for Nuclear Research. In view of the High Luminosity upgrade of the Large Hadron Collider (LHC), a new family of materials for the collimation system must be characterized. The collimator materials need to satisfy different requirements in order to intercept particles and protect the superconductive magnets: therefore, a compromise has to be found. So far, the most promising materials are Molybdenum Carbide- Graphite (MoGr) and Copper Diamond (CuCD): these composite materials have good thermo-mechanical and electrical properties, but their compatibility with the Ultra High Vacuum (UHV) of the LHC has to be investigated. A high vacuum level in the LHC is essential to guarantee the beam stability and lifetime; therefore, the outgassing of more than 100 collimators must be taken under control. Two parameters must be verified: the total amount of gas particle outgassed by a collimator and the species of gas. Hydrocarbons, in fact, can affect more the beam and air can saturate the Non Evaporable Getters (NEG), whose pumping speed is of paramount importance to reach UHV. In order fully characterize the materials, different experimental set-ups were involved: one of the most important parts of the work was performed in vacuum test benches, where the outgassing rate value and the gas analysis of the materials have been evaluated. This analysis was integrated with thermal desorption spectroscopy (TDS): this technic gives a precise indication of all the gases present in the material and the temperature needed to remove them. The surface state of the samples, as well as the internal porosities dimension and connection, are important to determine the outgassing performance of a material. For this reason, different microscopic analysis were also performed. The secondary electron yield (SEY) is likewise important to assess the UHV compatibility of these new materials in presence of proton beams and it was measured in a dedicated test bench. From a preliminary study, MoGr was found to be out of the acceptance limits, both in term of outgassing and gas analysis; a huge presence of air was detected. One of the aims of the work has been to understand the source of this huge presence of air: in order to explain the problem, different samples have been vacuum tested and microscopically observed. Besides the characterization of the material as received from the supplier, an important part of the work has been focused on possible thermal and surface treatments able to improve the outgassing performance of the material: they were done to understand if the non-compatibility can be solved with post-production process or if a change in the production process must be foreseen. Moreover, the effects of these treatments give also advice on the steps that have to be taken into account for the final production series. Conversely, CuCD was found to have a better behaviour, but a compatible cleaning has to be investigated.
Questa tesi di Laurea Magistrale è stata realizzata al CERN, il Centro Europeo per la Ricerca Nucleare. In vista dell’upgrade del Large Hadron Collider (LHC), si rende necessaria la caratterizzazione di una nuova famiglia di materiali per la collimazione. I materiali utilizzati nei collimatori devono soddisfare diversi requisiti al fine di intercettare in modo efficiente le particelle e proteggere i magneti superconduttori: è, quindi, necessario raggiungere un compromesso. Fino ad ora, i materiali più promettenti individuati sono il Molibdeno Grafite (MoGr) e il Rame Diamante (CuCD). Questi materiali compositi hanno buone proprietà termo-meccaniche ed elettriche, ma la loro compatibilità con l’ambiente di ultra alto vuoto (UHV) di LHC deve ancora essere investigata. Un alto livello di vuoto è fondamentale in LHC, così come in tutti gli acceleratori, per garantire stabilità al fascio; pertanto il degasaggio di oltre 100 collimatori deve essere controllato. I parametri da verificare sono due: bisogna stabilire il numero totale di molecole di gas degasate da un collimatore, ma è importante valutare anche quali siano questi gas. Gli idrocarburi, infatti, influiscono maggiormente sul fascio, mentre l’aria può saturare i getter non evaporabili (NEG), la cui velocità di pompaggio è fondamentale per raggiungere condizioni di UHV. Per una completa caratterizzazione dei materiali, sono stati utilizzati diversi set-up sperimentali: una parte importante del lavoro è stata svolta in un banco prova dove, dopo il raggiungimento di condizioni di UHV, il tasso di degasaggio e l’analisi dei gas sono stati valutati. Questa analisi è stata integrata con misure di spettroscopia di desorbimento termico (TDS): questa tecnica permette di capire quali gas siano contenuti nel materiale e che temperatura è necessaria per rimuoverli. Lo stato superficiale dei campioni, così come le dimensioni e le connessioni tra le porosità interne, sono importanti per capire il comportamento in sistemi di UHV di un materiale. Per questo motivo, sono state effettuate diverse analisi microscopiche. Anche la resa di emissione di elettroni secondari (SEY) è importante per stabilire la compatibilità con un ambiente di UHV di questi nuovi materiali ed è stata misurata in un apposito banco prova. Da studi preliminari, si è trovato che il degasaggio e l’analisi dei gas del MoGr non fossero compatibili con i limiti di accettazione di LHC. In particolare, una grande presenza di aria è stata rilevata nel materiale. Uno degli scopi di questa tesi è stato quello di capire la sorgente di questa aria: per stabilirlo, diversi campioni sono stati testati in UHV e la loro microstruttura osservata. Oltre alla caratterizzazione del materiale così come ricevuto dal fornitore, un’ampia parte di questo lavoro è stata dedicata all’indagine di possibili trattamenti termici e superficiali in grado di migliorare le performance di degasaggio: lo scopo è stato quello di capire se il problema potesse essere risolto con trattamenti post-produzione o se dei cambiamenti nel processo produttivo dovessero essere implementati. Inoltre, l’effetto di questi trattamenti sul degasaggio, fornisce indicazioni sulle fasi di preparazione che devono essere prese in considerazione per una produzione in serie. Al contrario, si è trovato un miglior comportamento del CuCD , ma le tecniche di pulizia dei campioni prima dei test di vuoto devono essere investigate.
Ultra-high vacuum characterization of advanced materials for future particle accelerators
ACCETTURA, CARLOTTA
2016/2017
Abstract
This Master thesis has been carried out at the CERN, the European Organization for Nuclear Research. In view of the High Luminosity upgrade of the Large Hadron Collider (LHC), a new family of materials for the collimation system must be characterized. The collimator materials need to satisfy different requirements in order to intercept particles and protect the superconductive magnets: therefore, a compromise has to be found. So far, the most promising materials are Molybdenum Carbide- Graphite (MoGr) and Copper Diamond (CuCD): these composite materials have good thermo-mechanical and electrical properties, but their compatibility with the Ultra High Vacuum (UHV) of the LHC has to be investigated. A high vacuum level in the LHC is essential to guarantee the beam stability and lifetime; therefore, the outgassing of more than 100 collimators must be taken under control. Two parameters must be verified: the total amount of gas particle outgassed by a collimator and the species of gas. Hydrocarbons, in fact, can affect more the beam and air can saturate the Non Evaporable Getters (NEG), whose pumping speed is of paramount importance to reach UHV. In order fully characterize the materials, different experimental set-ups were involved: one of the most important parts of the work was performed in vacuum test benches, where the outgassing rate value and the gas analysis of the materials have been evaluated. This analysis was integrated with thermal desorption spectroscopy (TDS): this technic gives a precise indication of all the gases present in the material and the temperature needed to remove them. The surface state of the samples, as well as the internal porosities dimension and connection, are important to determine the outgassing performance of a material. For this reason, different microscopic analysis were also performed. The secondary electron yield (SEY) is likewise important to assess the UHV compatibility of these new materials in presence of proton beams and it was measured in a dedicated test bench. From a preliminary study, MoGr was found to be out of the acceptance limits, both in term of outgassing and gas analysis; a huge presence of air was detected. One of the aims of the work has been to understand the source of this huge presence of air: in order to explain the problem, different samples have been vacuum tested and microscopically observed. Besides the characterization of the material as received from the supplier, an important part of the work has been focused on possible thermal and surface treatments able to improve the outgassing performance of the material: they were done to understand if the non-compatibility can be solved with post-production process or if a change in the production process must be foreseen. Moreover, the effects of these treatments give also advice on the steps that have to be taken into account for the final production series. Conversely, CuCD was found to have a better behaviour, but a compatible cleaning has to be investigated.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/135909