Outgassing Studies of Some Accelerator Materials

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Title: Outgassing Studies of Some Accelerator Materials
Author: Riihimäki, Ari
Contributor: University of Helsinki, Faculty of Science
Publisher: Helsingin yliopisto
Date: 2019
Language: eng
URI: http://urn.fi/URN:NBN:fi:hulib-201906172962
Thesis level: master's thesis
Discipline: Fysiikka
Abstract: This Master's thesis has been produced at CERN (European Organization for Nuclear Research) in the Vacuum, Surfaces and Coatings group during an interesting time period for accelerator physics. With the High Luminosity Large Hadron Collider (HL-LHC) upgrade in progress and future accelerators under study, we will soon have particle beams with unprecedented luminosity and energy. This provides new challenges to maintain the vacuum conditions at acceptable levels. One of the most critical factors to consider when choosing materials to accelerators and other vacuum systems are their outgassing rates. In this work, outgassing was studied with two systems. One was used to test the outgassing of baked samples with accumulation method at different temperatures. Another system was used to test unbaked samples with throughput method as a function of time and temperature. In addition, a smaller experiment was assembled for testing the effect of different venting gases and venting times to the pumpdown curve of a vacuum fired 316L stainless steel chamber and it is included in the appendix. These measurements are of importance as many of the tested materials or coatings are used or going to be used in the accelerators at CERN. The results can be used for vacuum simulations or in the design of a new component to predict its outgassing, that needs to comply with the strict vacuum acceptance criteria for the accelerator vacuum systems. As a conclusion, the unbaked samples presented high outgassing rates as expected. Water outgassing was prevalent with polymers, but was also significant with Macor and the two graphite samples. A high temperature heat treatment for graphite decreased the outgassing rate considerably and improved the residual gas spectrum. The baked samples were found to have low outgassing rates particularly at room temperature, where the background outgassing of the sample chamber was significant. Upper limits for the outgassing rates were evaluated for Macor and tungsten samples. However, alumina coated SS304L samples were distinct from the background and the outgassing rates were determined at five temperatures to assess the applicability of the coating for the vacuum tanks of the kicker magnets.

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