A three-car LiCAS RTRS system in the first stages of installation in DESY's 'red-green' tunnel.

The guys from collider alignment and survey have to get it straight. If they don't the beam will go pear-shaped and it certainly won't "fly right". Emittance will go up and luminosity down – and data-taking physicists at the ILC experiments won't be happy about that. So the LiCAS team – Linear Collider Alignment and Survey – rolled up their sleeves after rolling up their technical drawings and are setting up a test system in an unused tunnel section between DESY's Doris and DESY rings. They will be testing their equipment there for a year and then hope to use an improved version on the XFEL that will start running in 2013 and also on Cornell's planned CESR-TF damping ring test facility. Ultimately the lessons learned from all this will make sure that the ILC will be as straight as can be – with the notable exception of the damping rings of course - with no more than 200 microns vertical offsets over 600 m of machine.

"I think this is a good deal better than any large scale survey to date," says Oxford university's Armin Reichold. The team from Oxford and DESY's surveyors therefore joined forces and found a solution that looks very promising. "We measure the position of the modules with the help of a large network of reference coordinates. Every 4.5 metres in the tunnel wall opposite to the machine there are retro-reflectors that form this coordinate network," explains Reichold. "What you see on the wall here at DESY is an R&D prototype of our ILC survey system. Its named RTRS for Rapid Tunnel Reference Surveyor, and travels automatically along a rail on that wall."

Each one of its finally six measurement cars will be stopped closely in front of one such marker and measure the maker's co-ordinates by multilateration, using six distance measurements – each performed by one Frequency Scanning Interferometer (FSI). The cars measure their position relative to each other using a laser straightness monitor (LSM). The LSM sends its laser beam through a vacuum tube which connects all cars together. This avoids the refraction effects that would make the measurement in open air impossible. At the end of the tube the beam is reflected back onto itself to pass once more through all cars. The vacuum same tube also houses five other sets of six FSIs which measure the distance between cars. The trio of instruments in each car is completed by a high-precision tilt sensor which levels the car in front of each wall marker.

The prototype currently in the DESY tunnel has three measurement cars; the XFEL will need six, and the ILC will most likely need two RTRS – one for each ILC "arm", and each with six cars. It could be more though – the alignment of the damping rings will also need careful survey, and together with a team from Cornell university the Europeans are working on a system for surveying in a ring, something that will have to become everyday business at Cornell's test facility if it is to reach its ultra-low emittance goals. When the ILC is built, RTRS systems can make sure that shut-down times are as short as possible. This kind of surveying work would normally be done manually, and for a 50-kilometre machine (that's the total length of beam lines) it would take the best part of a year even with a large number of survey teams. A set of LiCAS-style RTRS might do it in a month.

In the course of the coming year, all measurement units of the RTRS will be calibrated and tested to the core to finally choose the essential and most efficient parts. Train and vacuum connectors, couplers, the vacuum tube and safety system will be extremely well understood. After all, the ILC shouldn't only be linear – it should also be straight!

-- Barbara Warmbein