2 is equipped with a full set of corrector magnets and the main quadrupole circuits
are separated from the dipoles. This was not the case for String 1. This will
allow the validation of the overall powering configuration and in particular
the bus-bar dimensions and routings as well as the interconnection procedures
during installation. The Power Converters are the same as proposed for the machine
and include the regulation and control philosophy that will be used in the machine.
In order to balance the inductance of the main dipoles between the go and return busbars in the LHC machine, the dipoles are connected in alternate 1/2 cells to either the go or return busbar. This is also be done in String 2 and therefore requires three magnets of each connection type. Likewise, String 2 needs one FD quadrupole and one DF quadrupole to satisfy the full cell requirements.
The electrical curcuit diagram reflects the powering configuration of the machine as far as it is possible with one cell of the machine. The six main dipoles and their correctors (total of 24 connected in four families) are split between the go and return bus-bars so as to balance the inductance. A third bus-bar is used for each family of dipole magnet correctors connected to the centre point of the circuit. This is foreseen to allow the immediate powering of half the corrector circuits in case of a corrector failure and later it can be used to bypass the faulty magnet during a "short intervention". It also provides a convenient measuring point for quench detection purposes.
The two main quadrupoles are powered separately for each aperture to form one focusing and one defocusing circuit. The eight correctors of the short straight sections are connected individually to the electrical feed box that will allow any combination of connection to be made on the warm side.
The main dipoles will be powered in series by the existing 2-quadrant converter of String 1. This converter [20kA, ± 14V] is a conventional 12-pulse thyristor converter, made up of four 5000A modules connected in parallel.
To power the main quadrupoles, two new switch-mode power converters will be installed. These converters will also be made up using a modular concept, where several high-current sources [3.25 kA, 16V] are placed in parallel.
This concept provides operational redundancy, since 5 modules will be installed for each converter. These modules (50 kW) will use soft-switching technology, working at a frequency greater than 20kHz. Each module will employ water-cooled and plug-in subassemblies, thus improving availability and replair time (fast exchange and off-line repair).
A third high-current low voltage switch-mode power converter will be installed to test the definitive topology to be used for the main LHC dipole converters, namely a SCR booster converter [13kA, ±180V] for the ramp up and the ramp down, in parallel (or in series), with a low voltage 1-quadrant switch-mode power converter [13kA, 16V].
All the corrector magnets will be fed by 4-quadrant switch-mode power converters (±600A and ±60A). New designs are under development to improve the efficiency and volume of these converters.
For the power converters and their control, String 2 is an important place to test and validate all the new concepts needed in the LHC machine. The requirements to track the dipole converters and the reference magnet converter currents to 5 ppm, places new constraints on permissible error in many areas. Equally, the techniques for on-line correction need such new methods to be fully evaluated before series production of control electronics can be envisaged.
The main areas which will be evaluated include all the power related aspects mentioned above, the current measurement system including their calibration methods and instrumentation, digital regulation loops, reference waveform generation with on-line corrections and methods for data transmission and machine timing. Many of these areas will be developed using the magnet test benches, but String 2 is full-scale model for test.