String 2 reaches the home straight

The LHC already works ! The team responsible for String 2, the test facility of CERN's new accelerator, can confirm it. On 17 June they cracked open a bottle of bubbly when the dipole circuit reached the nominal current of 11860 A, producing a magnetic field of 8.335 Tesla.
This is the second time that the powering up of String 2 has been celebrated (see Bulletin No.39/2001). Last September, the first phase of this new facility was inaugurated. At that stage it consisted of three prototype dipole magnets flanked by two straight sections, each consisting of a quadrupole and corrector magnets, plus a prototype cryogenic distribution line and the electrical feedbox. The assembly was tested between September and December 2001.
Since then three dipoles have been added. However, unlike the first magnets which were prototypes, these magnets are part of the pre-series supply from industry and will go down into the tunnel. The full cell is almost 120 metres long and is curved like the future accelerator. «We now have a complete LHC cell», explained Roberto Saban, who is in charge of String 2, «The amount of instrumentation and the complexity of the String 2 processes are close to those of a string in the future accelerator».
The first tests have proved very satisfactory. The team first tested the mechanical assembly to check that there were no leaks and that it could withstand the pressures that occur during a quench. A quench is a transition from the superconducting to the normal state, which means that resistance occurs as a result of overheating. In a quench the superfluid helium cooling the magnets begins to boil and becomes gaseous, thereby increasing the pressure inside the helium tank. A quench can be provoked for instance by the loss of a tiny part of the beam in the coils.
The assembly was then cooled down to the nominal temperature (1.9 Kelvin, or -271.1°C) over a period of just under ten days. This temperature close to absolute zero is needed to obtain superconductivity (circulation of the current without quenching) which is essential to achieve the requisite high magnetic field. After checking the electrical insulation and the protection systems (quench detection, interlock and energy extraction systems, power converters, etc.) the team then powered up the main circuits.
The experimental programme has now started. One of the many experiments involves observing the propagation of a magnet quench to the neighbouring magnets. Several weeks will be devoted to the study and optimisation of the magnet cooling and temperature regulation process and also of the beam screen inside the tubes in which the protons will circulate. Finally, various aspects of the fifteen electrical circuits (bending dipoles, focussing quadrupoles, corrector magnets, etc.) will be studied such as the quenches in the cables, current regulation and simultaneous excitement of various circuits to simulate the conditions of LHC operation.
The experimental programme will continue until the end of the year. The experience gained on Strings 1 and 2 will validate the LHC operating procedures and the teams will be better prepared for the operation of the future machine.