The first link in the great chain

The very first interconnection has now been made between two cryomagnets for the LHC. The 1,700 interconnections for the whole collider will require 123,000 separate welding and assembly operations.

Until now it was merely an image on a computer screen, but today the great blue ring is actually starting to take shape in the LHC tunnel. With the magnets now being lowered one by one, it's the turn of the team responsible for the interconnections to take centre stage in the great project. On 3 May, the first connection was made between a cryodipole magnet and a short straight section (SSS).

The task of connecting up all the machine components is a challenging one - 1,700 links between superconducting magnets comprising a total of 250,000 components. Vacuum systems, superconducting cables, beam screens, cryogenic pipes, thermal and electrical insulants - all have to be interconnected. And each interconnection takes sixty operations.

Certain ground-breaking technologies are being implemented, such as the three different techniques to connect the superconducting cables - inductive brazing, resistive brazing and ultrasonic welding. "The level of current in the LHC conductors required special developments to minimise the electrical contact resistance," points out Alain Poncet, Cryostats and Interconnections Group Leader.

The principal difficulty with the electrical connections relates to superconductivity. So intense are the currents transported, over 12,000 amps in the case of the main magnets, that the slightest resistance at the connection points leads to the release of heat. But the LHC needs to be cooled to a temperature close to absolute zero (1.9 K, or -271°C). "Every watt of heat released means extra energy to be expended by the refrigerators. When you're operating so close to absolute zero, this is a critical factor," explains Alain Poncet. The resistance at the welding points must be as low as possible in order to optimise the operating costs of the cryogenic system. Indeed the technique of inductive brazing, which uses a tin/silver filler metal, produces a weld thickness of only 100 micrometres and a resistance of less than one billionth of an ohm.

Furthermore, superconducting cables comprise thousands of superconducting niobium-titanium filaments, each with a diameter of about 6 micrometres, inside a copper matrix. In each of these filaments the current intensity can vary. Jean-Philippe Tock, Head of the Interconnections section explains that "At the connection point it is possible to re-distribute the current inside the various strands".

In addition to these electrical interconnections, 50,000 stainless steel welds have to be made to connect up the beam tubes, the cryogenic pipework and the thermal shields. Finally, all kinds of assembly operations have to be undertaken for the thermal and electrical insulations.

The interconnection techniques were developed at CERN and tested using prototype machines on the LHC test string. They were then sent out for tender and technology transfer. The successful bidder, Franco-Dutch consortium IEG (Ineo-Endel-GTI), took responsibility for executing all the interconnection work and for supplying the welding and brazing machines. "Many of the operations are automated," says André Jacquemod, a member of the section responsible for the interconnections. "The machines are really state-of-the-art and can even check the connection parameters on-line."

Several teams on several fronts

When the LHC installation schedule had to be revised due to the problems encountered during the installation of the cryogenic line last year, the concept of parallel working was introduced. This also applies to the interconnection work. After an initial setting-up phase lasting a few weeks, two teams from the Ineo-Endel-GTI consortium will get to work from 1 July onwards. "The aim by the end of the year is to have six teams working on six fronts in two shifts," says Jean-Philippe Tock, Head of the Interconnections section.

These large-scale operations are being conducted with Polish assistance. In 2003, a Cooperation Agreement was signed with the Krakov Institute of Nuclear Physics (HNINP) under which a twenty-strong team of scientists from that institute was entrusted with the task of checking the LHC interconnections. Eight Polish scientists have already been trained and are starting this difficult yet crucial job. Although many checks are made automatically on-line, some have to be made visually. For all the teams responsible for the interconnections, the great challenge is to work in parallel with other contractors. "The operators have to keep their concentration despite the constant to-and-fro of people and equipment by the interconnection sites," André Jacquemod explains.