Cryogenic vacuum links

The current Virgo vacuum level needs to be improved by a factor of about 100 in order to be compliant with the required Advanced Virgo sensitivity of 3 x 10-24/vHz at 200–400 Hz. Such an improvement requires baking out the interferometer arms. To separate these arms from the towers that hold the mirrors and allow the bakeout, four cryogenic vacuum links will be installed.

Links will consist of `hollow cylinder’ cryostats, cooled at 77 K with a bath of liquid nitrogen. The
selected length is 2.0 m and the inner diameter is 1.0 m, a convenient size to be installed inside the 1.2 m diameter link tubes between the relevant tower and the existing vacuum infrastructure (e.g. large valve, isolating the 3 km tube). The setup will be completed by baffles of aperture 600 mm (diameter) to minimize effects of diffused light. The link-induced thermal radiation effects on the mirrors have been modeled by finite element thermo-mechanical simulations. The two main effects are (1) a change of radius of curvature of the test mass surface of about 2 m, negligible with respect to its original value of about 1500 m, and (2) a change in optical path length inside the test mass. Both changes are smaller and of opposite sign with respect to those due to the YAG beam absorption.

Cryostat mechanical design (eventueel een aparte pagina)

In the current conceptual design, the cryotraps have a cold surface with a length of 2023 mm and a diameter of 1000 mm. The overall length is 3490 mm and the outer diameter is 1208 mm. It will be constructed from stainless steel 316L. The vessel is equipped with pump-out and service ports. Stainless steel hydro-formed bellows are used to accommodate expansion of the structure. The inner surface of the trap is cooled with liquid nitrogen. The volume of the bath is about 200 l. To minimize boiling and LN2 consumption, this bath is thermally shielded from the outer surface of the vessel by using vacuum and multilayer kapton-based superinsulation. Since the inner cold surface will move due to thermal expansion (about 3.2 mm/m) with respect to the outer vacuum vessel, expansion bellows are used. These bellows also act as heat bridges that minimize thermal losses due to heat conduction. The LN2 inlet will be designed such that LN2 will flow smoothly into the bath, minimizing any noise induced by bubbling. The liquid nitrogen level in the bath can easily be controlled within ±10 mm. Note that the bath has a sizable width of more than 300 mm. Again this guarantees that
bubbles have an easy escape path to the surface over the entire length of the cryotrap. A separate inlet is provided in order to admit hot nitrogen gas in case rapid heat-up of the structure is needed. The cryotrap can be operated for more than one year before regeneration, assuming a load of about 10-4 mbar l/s from the mirror vessel. During this time a water layer of about 1 micron will be deposited on the inner surface. This causes the initial emissivity of about 0.1 to increase to 0.2. This relative low value for the emissivity leads to an average heat load of about 300 W, and results in an estimated LN2 consumption of about 3.5 l per hour. In a `transient’ phase of commissioning operations, when towers are frequently vented , the growing rate of the water layer will increase.
Regenerations shall be maintained at the level of once per year, allowing an higher LN2 consumption.

Figuren:
1. cryo.gif
2. West input tower assy1.pdf (het rechter deel)