Wideband current transformers for the surveillance of the beam extraction kicker system of the Large Hadron Collider

The LHC beam dumping system must protect the LHC machine from damage by reliably and safely extracting and absorbing the circulating beams when requested. Two sets of 15 extraction kicker magnets form the main active part of this system. A separate high voltage pulse generator powers each magnet. Because of the high beam energy and the consequences which could result from significant beam loss due to a malfunctioning of the dump system the magnets and generators are continuously surveyed in order to generate a beam abort as soon as an internal fault is detected. Amongst these surveillance systems, wideband current transformers have been designed to detect any erratic start in one of the generators. Output power should be enough to directly re-trigger all the power trigger units of the remaining 14 generators. The current transformers were developed in collaboration with industry. To minimize losses, high-resistivity cobalt alloy was chosen for the cores. The annealing techniques originally developed for LEP beam current measurement in collaboration between CERN and industry allowed to extend the frequency response beyond that of traditional core materials. The paper shows the results obtained, exposes the problems encountered with shielding, conductor position sensitivity, load resistor technology and their solutions. The know-how acquired during the collaboration was further applied by the industrial partner to cover a wider range of sensitivity, size and frequency.