Vacuum system and EXPO 2000
Spring 1998: Progress Report
Mid of January the heating of the eastern tube started, but the phase cutting current control only survived the first half hour. The thyristors were underrated. Preparation is being made for leak testing the second arm.
The blown-up thyristors have been replaced by appropriate ones and the phase cutter works again. The eastern tube has been air baked at 200°C for about one and a half day. The vacuum bake lasted for three days. The mass 2 (hydrogen) readings of the quad in the central station showed too high values. In addition, as the quad was not baked simultaneously with the rest of the tube a lot of dirt accumulated in the quad. Another air bake and a subsequent vacuum bake (in combination with changing the gauges for clean ones) solved this problem. After switching off the heating of the tube, two leaks developed at the 600 mm COF flanges between the tube and the 600 mm gate valves. Tightening the bolts solved this problem.
The pressure in the eastern tube is now (27.3.98) 2•10-8 mbar and the quad reading shows a clear air signature. We hope it is one of the flanges in the trench and not the tube itself. As this pressure is clearly higher than the nitrogen partial pressure was before the bake, it is a new leak we are facing.
The northern tube is pumped down and has reached a pressure in the 10-5 mbar range. Leak testing is under way. The stacks for the Mode Cleaners are well on their way.
Summer 1998: Progress Report
The experiments on dual recycling at the Garching prototype worked very well. The dual recycling lock was extremely stable. The trickiest part of the experiment was manual alignment of signal recycling even when aided by auto-alignment. The signal enhancement was improved by optimising the demodulation phases and the modulation frequency. The signal recycling led to a significantly larger contrast of the interferometer. The agreement between our model and the measured signal sizes and modulation frequency is good. These experiments show that dual recycling can be used to improve the performance of GEO 600.
Both tubes have been resuspended using stainless steel wires. The pressure in the eastern tube is already in a region that is low enough. At the ends of the tube it is in the mid 10-9 mbar region with a clear air signature.
The mode-cleaner vacuum system is ready for a bake-out (70 to 80 °C for four days).
No fairy tale: A mouse built a nest in the glass-fibre cable tray and the inmates munched through the whole bundle. We try to repair it by installing plugs at the free ends. (We are in search of a pest controller...)
The northern tube has been wrapped with rockwool. Even when wrapped possible leaks can be detected "easily" enough by means of a helium leak test.
Mike Plissi and Calum Torrie came to Hannover for a final design review of the modecleaner double pendulums. For the first time three complete stacks have been assembled, including motors and flex-pivots. The top-plates have been manufactured by Skodock; they were damped by partly filling them with graphite-doped RTV. A dummy mirror served to test the suspension. The result of this dry exercise was that the mechanical design is ok, up to a few little changes to be made by our workshop.
Stacks and Top Plate
After being equipped with all the intermediate masses and mirrors the top plates have to be inserted into the tank and placed onto the stacks. This sequence shows the device for the soft lowering of the top plates. The test run proved the feasibility of this concept.
Assembly of the Double Pendulum
Stacks, Top Plate and Pendulum
These pictures show various stages of a test run trying the assembly of the modecleaner suspension. After being equipped with an intermediate mass, coils and a dummy mirror, the top plate is lowered onto the stacks.
In the first week of December we have tested the local-control electronics on a modecleaner double pendulum. After some minor problems it worked excellent. Once the local-control loop was closed no tilt or rotation of the dummy mirror could be seen. At the site, the cleaning of the central building and the fabrication of a clean tent took place.
GEO600 has been registered as one of the Projects of the World Exposition EXPO 2000 in Hannover ("Projects all over the World in Germany").
Installation of the Modecleaner Double Pendulums
After the EXPO Registration, the installation of four modecleaner double pendulums at the GEO site has been started. Cleaning (stacks, top plates, double-pendulum components) and wiring (local-control modules) are the main activities at this stage. After the cleaning of the central building, cleanroom clothes are needed to enter the central area.
Mike, Calum, Jim, Geppo, Paul, and Dave from Glasgow and Andreas from Garching were a valuable and appreciated help in these days.