Wideband Precision Current Transformer for the Magnet Current of the Beam Extraction Kicker Magnet of the Large Hadron Collider

The LHC beam extraction system is composed of 15 fast kicker magnets per beam to extract the particles in one turn of the collider and to safely dispose them on external absorbers. Each magnet is powered by a separate pulse generator. The generator produces a magnet current pulse with 3 us rise time, 20 kA amplitude and 1.8 ms fall time, of which 90 us are needed to dump the beam. The beam extraction system requires a high level of reliability. To detect any change in the magnet current characteristics, which might indicate a slow degradation of the pulse generator, a high precision wideband current transformer will be installed. For redundancy reasons, the results obtained with this device will be cross-checked with a Rogowski coil, installed adjacent to the transformer. A prototype transformer has been successfully tested at nominal current levels and showed satisfactory results compared with the output of a high frequency resistive coaxial shunt. The annular core of the ring type transformer is composed of a relatively low cost commercially available nano-crystalline strip material on an iron base. The characteristic feature of this material is a structure in which a fine-crystalline grain with an average diameter of 20 nm is embedded in an amorphous residual phase. This structure gives the material a high permeability. In addition, the small strip thickness (approx. 20 um) and the relatively high electrical resistivity, result in extremely low eddy-current losses and excellent frequency behaviour. With a saturation flux density of 1.2 T this material becomes even superior to permalloys, ferrites or amorphous based alloys. In this particular application the transformer core is exposed to a unipolar induction. With normal magnetic materials this type of flux causes a relative high remanent induction. However this material allows controlling the magnetic properties, so-called B-H curve shaping. It is obtained during annealing of the material by an external applied cross field and as a result the remanence ratio is less than 10%, which is excellent for this application. This paper presents the magnetic material, its incorporation in the design of the current transformer, comparative measurements of a prototype with a coaxial shunt precision resistor and explains why this device is an essential part of the LHC beam extraction system.