LHC: from hot to cold

The first cryogenic feedbox designed to supply electricity to the superconducting magnets of one arc has just been installed at Point 8 of the LHC. This latest milestone is the reward for the joint efforts of the AT and TS Departments at CERN, the IHEP Institute in Moscow and CERN's industrial partners who collaborated in its manufacture, and is a precursor to the forthcoming cool down of the first 3.3 km sector of the LHC.

Pictured in front of the first DFBA cryogenic feedbox, some of the team responsible for its manufacture and installation, at the end of a 6-kilometre journey in the tunnel at a speed of one kilometre an hour.

The first of the 8 big electrical feedboxes installed in the LHC tunnel : it is 10 meters long and weighs 12.7 tonnes.

A new piece of the LHC puzzle, a crucial element in the transition from hot to cold, has just been installed at Point 8 on 28 August. Christened DFBA (which stands for Electrical Feedbox for the Arc), this component will provide the electrical supply for many of the superconducting magnets in sector 7-8. Single-handedly, the box will be capable of bringing 150000 amperes from room-temperature down to 269 °C (4.5 K).

The DFBAs, of which there are 16 in total, are positioned at either end of each arc. In addition to the electrical supply, they will also form the ends of the continuous sections of cryostat and will therefore have to withstand a pressure of several tonnes, at the same time as enabling the beam pipes to pass between a cryogenic environment and room-temperature.

The electrical power is brought from room temperature to cryogenic temperature by power leads whose lower extremities are immersed in a bath of liquid helium. They are actively cooled by gaseous helium which, having been injected at their base at 20 K, comes out at the top at room temperature. In order to limit the heat loads, the power leads use ceramic superconductors with high critical temperatures. This application is the first time that these materials, which were discovered only fifteen years or so ago, have been put to use on a large scale. (See Bulletin No. 22/2004). In addition to the DFBAs at the ends of the arcs, the power supply to the superconducting magnets of the machine's straight sections also require electrical feedboxes of a smaller design (referred to as DFBMs and DFBLs), which are positioned around the LHC ring. In total, 44 DFBs of various types, equipped with 1200 current leads carrying between 120 and 13000 amperes, distribute current to several hundreds of different circuits.

'All the DFBs are different as each sector of the LHC has its own specific electrical supply requirements', explains Antonio Perin who is responsible for their design and manufacture. Moreover, given that the local gradient in the tunnel varies according to the position in the LHC, the various DFBs have to be adjusted to ensure that they are horizontal so that all the current leads are immersed in their liquid helium baths, at the same time as guaranteeing the alignment of the beam pipe.

At the crossroads between cryogenics, power supply and mechanical engineering for the LHC, the design and manufacture of the DFBs is a task that requires a multi-disciplinary approach. 'The manufacture of these feedboxes is the fruit of a collaboration between CERN's AT and TS Departments, the Protvino IHEP Institute in Russia and several of CERN's industrial partners', explains Rob van Weelderen, the engineer responsible for the project.

The very first installation of a DFBA in the tunnel is therefore an important landmark for all these teams. It also heralds the forthcoming cool down of the very first cryogenic sector of the LHC, an event which is also eagerly anticipated.