Steel tape-wound cut cores as magnet yokes for the beam-dump kickers of the Large Hadron Collider

Fast-pulsed magnets, also called kickers, are used in particle accelerators for beam injection, extraction and similar applications. To excite these magnets, typically current pulses with rise and fall times in the range of 100 ns to 10 μs are used, with a pulse duration of up to 100 μs and amplitudes in the order of kilo amperes. The short rise time imposes low-inductance circuits and high-voltage operation. The yokes are usually made out of ferrite, with field saturation being reached at about 0.5 T. To remove the 7-TeV proton beams safely from the large hadron collider (LHC), 15 beam-dump kickers are used for each of the two rings which must provide an overall bending strength of 6.2 T·m with a rise time of about 3 μs and a flat top duration of 90 μs. For the first time steel with saturation above 1.5 T has been used as yoke material for such a short rise time. Due to eddy current losses the yokes must be laminated. The lamination thickness of 50 μm is determined by the frequency spectrum of the current pulse and by the steel quality. The insulation layer of the laminations shall withstand a voltage of 1 V/layer, which is difficult to achieve at the cut edges. As stamping, handling and assembly of 1.2-m-long magnets with such thin sheets would be extremely difficult, the concept of tape-wound C-shaped cut cores was developed. Two cores are moulded together in charged epoxy resin with a thin intermediate insulator. The yoke is finally assembled from a series of such "twin packs," using threaded inserts for precision mounting. This paper discusses the choice of the steel quality and lamination thickness, and addresses technical challenges related to the manufacturing of the C-cores, arising from the tight mechanical, electrical and magnetic requirements.