Full intensity for the LHC

The power supply to one eighth of the LHC was successfully tested over a period of 24 hours.

The LHC is gradually taking shape. While the major operation of lowering the magnets into the tunnel continues, many of the machine's other components are gradually being installed and commissioned. On 13 October, the power supply to one sector of the accelerator was successfully tested over a period of 24 hours. This was the first time that all the power converters for the supply of electricity to one eighth of the machine had been operated together in situ.

Once installed the accelerator will have a power rating of around 200 megawatts (MW), which is equivalent to that of LEP. By way of comparison, the peak consumption for the Canton of Geneva is 400 MW. However, what makes LHC different is its use of superconducting technology. As a result of this, the LHC magnets will not lose energy by heating up when strong currents pass through them. However, in order to produce the magnetic fields of up to 9 tesla that they will need to control the trajectories of the protons they will have to be supplied with high-intensity current.

Thus, although their power consumption is relatively low (with peak rates of 50 MW during the acceleration phase and 25 MW during standard operations for physics runs, compared to the 200 MW rating for the whole of the machine), the magnets' special characteristics have called for a design and installation procedure that is completely different to that of LEP.

'The need for very strong currents with moderate voltages required the power converters to be placed as close to the machine as possible and new topologies had to be developed to reduce their size and increase their performance', explains Frédérick Bordry, Leader of the AB Department's Power Converter Group. Given that space underground is limited, the power converters selected are as compact as possible.

The LHC is equipped with more than 1600 power converters, which is more than 200 for each sector. They are mainly located in secondary tunnels, either side of the even points of the machine. However, a number are also located at the odd points, while smaller power converters are installed directly below the dipole magnets.

Their job is to convert the alternating mains current (18 kilovolts or 400 volts) into a high-intensity direct current, of up to 13000 amperes, that is as stable as possible.

'We are able to achieve a precision of several millionths for the intensity of the current delivered to the magnets', explains Frédérick Bordry. Large numbers of sensors and safety systems will be used to monitor that the machine is operating properly, triggering energy dumping if necessary. If any quenches occur on a magnet, enormous resistors will absorb the energy. The energy stored in the LHC's 1232 dipoles will be in excess of 10 Gigajoules, which is equivalent to that of an Airbus A380 travelling at 700 km/h.

Obviously, these high-intensity currents induce a lot of heating in non-superconducting cables, since, in contrast to the accelerator, this part of the electrical circuits is resistant. A substantial water and ventilation cooling system provides the cooling for the power converters and the enormous electric cables. 'As part of this first commissioning phase, we are particularly interested in checking the proper functioning of the power converters' peripheral infrastructures', underlines Roberto Saban who is responsible for co-ordinating LHC commissioning.

In addition to the Power Converter Group (AB/PO), which is responsible for designing and operating the power converters, a number of Groups from the AB, AT and TS Departments have also participated in the operation. The Cooling and Ventilation Group (TS/CV) checked the performance levels of the ventilation and cooling installations. The Electrical Engineering Group (TS/EL) monitored the heating of the cables, the electricity consumption and any disruptions to the power supply. The Magnets and Electrical Systems Group (AT/MEL) monitored the thermal performance of the energy extraction system. The Controls Group (AB/CO) provided the computing infrastructures as well as the control software and monitored the operation together with the Operation Group (AB/OP).

After this first period of operation at full power, the next stage will be the commissioning of the power converters next year, this time after they have been connected to the magnets. At that stage, the operation of other key elements, such as the electrical distribution box which provides the interface between the power-supply system and the accelerator, will be tested. In transferring the power to the magnets, the box has to ensure the transition from room temperature to that of the machine at close to absolute zero (around -270°C).