962 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 7, NO. zyx 2, JUNE 1997 zy Suppression and Control of Coupling Currents in Stabrite-Coated Rutherford Cable with Cores of Various Materials and Thicknesses E.W. Collings and M.D. Sumption The Ohio State University, Columbus, OH 43210, U.S.A. S.W. Kim, M. Wake, and T. Shintomi National Laboratory for High Energy Physics (KEK), Tsukuba, Ibaraki 305, Japan A. Nijhuis, and H.H.J ten Kate University of Twente, Enschede, The Netherlands R.M. Scanlan Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A. Abstract-Calorimetric and magnetic measurements of AC zyxwvu loss have been performed on stabrite-coated Rutherford cable conforming to the Large Hadron Collider (LHC) inner- winding (28 strand) specification, with the applied field directed either perpendicular to the broad face (the face-on, FO, orientation) or parallel to it (edge-on, EO). It was found that the insertion of a thin metallic or insulating core into the cable suppressed the FO AC loss under typical conditions by a factor of 10 and rendered it practically insensitive to the application of cold uniaxial stress (of up to about 78 MPa). The FO loss having been suppressed, it could also be controlled (i.e. finely adjusted) by changing the level of internal compaction (by varying core thickness at fixed overall thickness) or external compaction (by changing the overall thickness of the cable). Of course the EO loss was much less sensitive to the presence of a core, the use of which therefore enabled the FO and EO losses to be independently adjusted. I. INTRODUCTION zyxwvuts A. Coupling Currents zyxwvutsrqp in D C Magnets Critical problems which seem to reoccur with each new generation of magnets and machines stem from the interstrand coupling (eddy) currents that accompany every change of magnet excitation. Associated with these currents are an eddy-current magnetization and a dissipation of energy, "AC loss". In extreme cases, AC loss imposes excessive load on the cryogenic system, as with the Fermilab Tevatron [l], and even quenching, as with the SSC's high-energy-booster (HEB) magnets [2]. But even when AC loss in the above sense is regarded as absent, the magnetization associated with the inevitable presence of coupling currents may have several deleterious consequences including: (i) a degradation of field quality, (ii) a contribution to the long decay time of dipole magnetization which makes field-correction more difficult. Examples of (i) and (ii) are to be found in the literature [3,4,5]. It is suggested that the LHC program [6] should also be concerned, if not about AC loss per se, then certainly about coupling-induced field- quality degradation, especially if stabrite-coated cables are to be used. Manuscript received August 26, 1996 Research supported in part by the US. Department of Energy, Division of High Energy Physics, under Grants No. DE-FG02-95ER40900 (OSU) and DE-AC03-76F00098 (LBNL). B. Classification zyxw of Interstrand Coupling Currents The main coupling-current loops that are induced in a Rutherford cable by a time-varying magnetic field in the face-on or FO orientation are: (i) diamond-shaped loops consisting of "upper" and "lower" segments of strand and their crossover contacts of resistance Rc.L; (ii) "parallel- strand" loops (the minimal diamond) in which the current path is completed by side-by-side contact, RC,,, or crossover contact at the cable edges, Rce. The large diamond paths provide the greater loss. For this reason, the insertion of a core drastically suppresses the total loss. Sytnikov's expressions for AC loss [7] which we have made use of in previous publications [8,9], and find convenient for our present purposes, are given below. 2 W LpBmax N2 1 (1) Qc,m zyxw = - 3 t where Qc is the coupling loss per cycle per m3 of cable outside dimensions, B, is the field sweep amplitude, t, w, and L, are the thickness, width and transposition pitch of the cable, and N is the number of strands. The units are tesla, meter and second. The equations include Rc,L and R , , in the FO orientation, just R , , in the EO orientation, but do not recognize an ke. However, work by Takacs et al. [lo], as well as that of Carr and Kovachev [l 11 have included Rc, explicitly. Also, cable-length effects have recently been revisited by Takacs et al. [12], as well as Akhmetov et al. [13,14]. However, the most important recent developments in cable theory are: (i) the extension by Carr of his anisotropic continuum model, originally constructed for multifilamentary strands, to cable [l 11; (ii) further calculations of the Boundary Induced Coupling Current (BICC) [15,16]. 1051-8223/97$10.00 zyxwvut 0 1997 IEEE