A novel, elegant method for passive compensation of magnetization-current-induced field errors in the CERN LHC 10 T superconducting dipole magnets by permanent multipole magnets

Abstract

CERN is at present studying a Large Hadron Collider --LHC- with some 1'600 up to 12 m long 10 T-, twin bore, high field uperconducting dipoles to be eventually installed in the 27 km tunnel of the e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> collider, the LEP machine, actually under construction. In these dipoles the largest magnetic field error perturbing the particle closed orbit at 0.5 T injection field into the LHC, is caused by magnetization currents in the NbTi <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.8K</inf> or Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> sn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4.5K</inf> filaments of expected 10 µm diameter. To compensate this negative 1.9 O/oo relative sextupole error at 3 cm bore diameter, a novel, elegant and inexpensive method is proposed ; one could either place a continuous, thin and concentric permanent sextupole layer within each 12 m long S.C. dipole or referably insert two short lumped correctors at its ends. To determine the compensation efficiency, systematic and random errors of the magnetization current effect in the S.C. windings and error sources in the permanent sextupole correctors due to geometrical, magnetic and temperature inlluences as well as to the interplay with the main dipole field are considered, taking notably into account that the correctors will have to operate at liquid helium temperature. Low temperature measurements on samples of permanent SmCo and NdBFe magnets indicate consistant and interesting improvement in the remanent field B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</inf> of about 7 %, excellent reproducibility of B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</inf> to a few 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> at LN <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> , and LHe temperatures as well as very good mechanical behaviour. Low temperature sample measurements in external and opposed dipole fields up to 1.9 T resulted in a remanent field reduction of -AB <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</inf> /B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</inf> = - 5.7 %. Reducing the opposed dipole field to zero, a permanent demagnetization effect of - 2.5 % remained. Based on computations, mechanical properties and low temperature permanent magnet sample measurements, the proposed compensation of magnetization current induced error in windings of superconducting magnets is entirely feasible ; an error reduction factor of3..5 can be expected. A correcting permanent sextupole prototype magnet has been ordered and will be completely tested.