Role of Material and its Processing for LIGO-India Ultra High Vacuum System

Abstract

LIGO, which stands for the Laser Interferometer Gravitational-Wave Observatory, is a large-scale physics experiment aiming to directly detect gravitational waves. LIGO-India has been proposed for construction and operation of a large-scale international observatory in India in collaboration with LIGO, USA. The observatory consists of large number of optics components, viz., mirrors, beams splitters and the interferometer. All these components will be installed in Ultra High Vacuum (UHV) systems. The Ultrahigh Vacuum system consists of 8 kilometers of beam tubes of 1.2 meter diameter and large size of UHV chambers i.e. Beam Splitter Chambers (BSC), Horizontal Access Modules (HAM). This UHV system will be on of the largest vacuum systems in the southern hemisphere. To achieve UHV in such a large system, the selection of material and various surface treatments are very important. Special care should be taken during fabrication, cleaning, transportation, installation and commissioning. This paper presents all the aspects required for the LIGO-India vacuum system. The selection of materials its processes for use in vacuum systems are a very important part of the design and should be considered. Different type of material properties should be considered like mechanical, thermal, gas loading etc. The material must be capable of being fabricated into the required components. It should withstand the required conditions of temperature; pressure imposed on it by the material processes, without limiting the attainable vacuum that is required. The material must have adequate strength at maximum and minimum temperatures to be encountered, and must retain its properties over the expected temperature range. The material's vapor pressure must remain low at the expected working temperature. Materials should be free of cracks and crevices which can trap cleaning solvents and become a source of virtual leaks later on. Role of UHV for LIGO-India: LIGO observatory supports UHV chambers and an Lshaped UHV system (Called as Beam Tube), measuring 4 kilometers (2.5 miles) on each side. The laser light wave travels into the beam tube as shown in the figure 1[1]. Figure 1[1]. As it is known that strain sensitivity is directly proportional to the vacuum i.e. better vacuum gives more sensitivity. So to increase the strain sensitivity of the system, ultra high vacuum is the first and preferable choice. Design principles of a UHV system: In order to design the system, the main parameter will be desired vacuum. A high vacuum system may be required baking up to 150oC depending upon the outgassing and type of pumping system. On the other hand, UHV system needs baking to