Colder than space – first helium transfer line modules from Poland arrive at DESY

Many modern particle accelerators run at temperatures close to absolute zero. The accelerator of the European XFEL X-ray laser for example needs to be cooled down to minus 271 centigrade for revving up electrons to the speeds and energies needed. But before assembly, the main accelerator elements have to be tested at operating temperature. For these tests tons of cold helium will flow between the new DESY Accelerator Module Test Facility (AMTF) and the cryogenic plant. Liquid helium will make the test vessels some of the coldest spots in the solar system – 2 degrees colder than outer space! The first modules of the helium transfer line have just arrived at DESY from Wroclaw in Poland. Along with the test vessels (cryostats) and the actual testing, due to start next year, they belong to the in-kind contribution of Poland to the European XFEL.

The transport of cold helium is anything but trivial. The transfer line has to operate reliably in snow, storm, rain and above all sunshine; i.e. the helium in must not warm up. This is ensured by a complex design: four process pipes are running in the core of an external pipe with a diameter of approx. 40 centimetres, one each for the transport and re-transport of cold helium at 4 kelvin (-269 centigrade) and 40 kelvin (-233 centigrade). The “warmer” helium cools a thermal shield which covers the process pipes, thus minimising the temperature gradient in the centre of the pipe. This shield is covered with several layers of thin plastic foil vapour-plated with aluminium (Mylar foil), the so called super insulation, to reflect heat radiation from outside.

In addition, the beam pipe is evacuated, thus creating a high vacuum that prevents thermal conduction via the air. Thermal conduction must also be avoided at the inner process pipe suspensions. However, the process pipes need to move within the outer shell due to thermal deformation of the shell with ambient temperature fluctuations. But any open spots in the insulation would immediately cause cold leaks, as Bernd Petersen, head of the responsible DESY group, explains. Especially the bended pipe sections are a challenge.

The transfer pipe construction is in charge of the Wroclaw University of Technology; the modules are produced by the specialist firm KrioSystems located in the technology park of the Polish university city and are then transported to Hamburg in 12-metre units. The first 60 metres have already arrived. Assembly will start in November. In April 2012 both test vessels – the so called cryostats - are due to arrive from Wroclaw. They will be 3.8 metres high, with a diameter of 1 metre. Each one takes two cubic metres of liquid helium. For the test the helium needs to be two degrees lower than it spurts out of the transfer pipe. This is achieved by exhausting the helium vapour with pumps. After several steps, the remaining cold liquid helium has a temperature of 2 kelvin (-271 centigrade) – this is colder than outer space.

For this effect, the cryostats must be perfectly insulated. “It basically resembles a thermos bottle” Petersen explains. An inner shield and a vacuum insulate the helium vessel against the shell. The cryostats are embedded in the floor of the AMTF hall. In this cold bath, four niobium cavities at a time are simultaneously tested before they are dispatched to the CEA institute in Saclay for mounting. The delicate niobium cavities are transported in special cases, similar to those used for musical instruments. In France, eight cavities at a time are assembled to an accelerator module before they are returned to Hamburg. However, before they are finally in place in the European XFEL accelerator tunnel, each module will be tested again in the AMTF hall. The cavity and module tests are carried out under the responsibility of a team from theHenryk Niewodniczanski Instituteof Nuclear Physics of the Polish Academy of Sciences of Cracow.