Interstrand Contact Resistance in ${\rm Nb}_{3}{\rm Sn}$ Cables Under LARP-Type Preparation Conditions

Abstract

The Nb 3 Sn cables being developed under the LARP program will need to have suitable, reproducible, and uniform interstrand contact resistances (ICR) in the interests of acceleratorfield quality (low coupling magnetization) and stability (currentsharing between strands). In the first of a pair of studies ICR was measured in response to variation of strand surface condition. As a sequel to this the present set of measurements explores the influence of cable preparation conditions on ICR, determined as before using calorimetric and magnetic AC-loss measurement techniques in face-on and edge-on applied AC fields. Uncored Rutherford cables were wound at Fermilab from OST MJR-type strand and wrapped with S-glass tape impregnated in most cases with a ceramic binder (applied before reaction HT). The HT/pressurization sequence of the cables was made to mimic as closely as possible the expected LARP magnet-fabrication schedules. Derived from maxima in the loss-vs-frequency curves was an ICR "inhomogeneity"-two ICRs for each cable pack each associated with a particular fraction of the strands. Furthermore, both of the ICRs, at 0.4 and 4 , would in a magnet be too small to satisfy the generally accepted magnetization requirements.

Coupling Loss, Interstrand Contact Resistance, and Magnetization of Nb$_{3}$Sn Rutherford Cables With Cores of MgO Tape and S-Glass Ribbon

“…The uncored value of 0.27 conforms well to several previous results obtained under collaborations with LBNL and FNAL: 0.15, 0.16, 0.30, 0.33, and 0.36 [4], [5], 0.24 [3], and 0.4 [6]. Such unacceptably low values stemming from the small crossover contact resistances, , characteristic of wind-and-react Nb Sn have called for the introduction of a core to separate the "upper and lower" parts of the cable winding.…”

“…Hence, as detailed in previous papers on the subject (e.g. [6] and [8]), values of are obtainable using (i) the slope of the linear versus , (ii) the "raw" initial slope of a nonlinear , (iii) the initial slope after fitting the data to (3) even if the maximum frequency of the experiment is less than .…”

“…Both MgO-and s-glass tapes provided comparable values although MgO (9.6 ) was slightly better than s-glass (8. 6 ). Although these values are significantly less than the stated 20 goal an increase is possible by increasing the core width.…”

Magnetic measurement of interstrand contact resistance and persistent-current magnetization of Nb3Sn Rutherford cables with cores of MgO tape and woven S-glass ribbon

“…For the present cables the uncored <R eff > AV. of 0.2 µΩ agrees well with previously measured results for uncored Nb 3 Sn cables [7][8] [9] but would lead to an unacceptable M coup,LARP,0.2µΩ = 338 kA/m. The presence of the insulating cores with <R eff>AV.…”

“…For the NbTi-wound LHC-inner cable (with <w/t> = 7.94, L p = 55 mm, N = 28, R c = 20 µΩ) ramping at 6.5 mT/s Equation (1) provides our "bench-mark" coupling magnetization of M coup = 1.85 kA/m. The same cable wound with Nb 3 Sn strand, under the pressurized windand-react conditions that it would experience in a magnet, with R c s such as 0.15 µΩ, 0.16 µΩ [7], 0.24 µΩ [8], 0.3-0.36 µΩ [7] and 0.4 µΩ [9], would exhibit coupling magnetizations some two orders of magnitude larger than this. Thus as frequently pointed out a core is needed to increase the R c of Nb 3 Sn Rutherford cables.…”