NbTi accelerator dipoles are limited to magnetic fields (H) of about 10 T, due to an intrinsic upper critical field (H{sub c2}) limitation of 14 T. To surpass this restriction, prototype Nb{sub 3}Sn magnets are being developed which have reached 16 T. We show that Nb{sub 3}Sn dipole technology is practically limited to 17 to 18 T due to insufficient high field pinning, and intrinsically to 20 to 22 T due to H{sub c2} limitations. Therefore, to obtain magnetic fields approaching 20 T and higher, a material is required with a higher H{sub c2} and sufficient high field pinning capacity. A realistic candidate for this purpose is Bi-2212, which is available in round wires and sufficient lengths for the fabrication of coils based on Rutherford-type cables. We initiated a program to develop the required technology to construct accelerator magnets from 'wind-and-react' (W&amp;R) Bi-2212 coils. We outline the complications that arise through the use of Bi-2212, describe the development paths to address these issues, and conclude with the design of W&amp;R Bi-2212 sub-scale magnets

Cheng, Daniel

Dietderich, Daniel

Ferrracin, Paolo

Godeke, A.

Prestemon, Soren

Sabbi, GianLuca

Scanlan, Ron

NbTi accelerator dipoles are limited to magneticfields (H) of about 10 T, due to an intrinsic upper critical field (Hc2) limitation of 14 T. To surpass this restriction, prototype Nb3Sn magnets are being developed which have reached 16 T. We show that Nb3Sn dipole technology is practically limited to 17 to 18 T due to insufficient high field pinning, and intrinsically to 20 to 22 T due to Hc2 limitations. Therefore, to obtain magnetic fields approaching 20 T and higher, a material is required with a higher Hc2 and sufficient high field pinning capacity. A realistic candidate for this purpose is Bi-2212, which is available in roundwires and sufficient lengths for the fabrication of coils based on Rutherford-type cables. We initiated a program to develop the required technology to construct accelerator magnets from 'windand-react' (W&amp;R) Bi-2212 coils. We outline the complications that arise through the use of Bi-2212, describe the development paths to address these issues, and conclude with the design of W&amp;R Bi-2212 sub-scale magnets

Cheng, D.

Dietderich, D.R.

Ferracin, P.

Prestemon, S.O.

bbi, G. Sa

Scanlan, R.M.

eScholarship - University of California

Lawrence Berkeley National LaboratoryLBL PublicationsTitleLimits of NbTi and Nb3Sn, and Development of W&amp;R Bi-2212 High Field Accelerator MagnetsPermalinkhttps://escholarship.org/uc/item/5v54c6dxAuthorsGodeke, A.Cheng, D.Dietderich, D.R.et al.Publication Date2006-09-01eScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaLimits of NbTi and Nb3Sn, and Development ofW&R Bi–2212 High Field Accelerator MagnetsA. Godeke, D. Cheng, D. R. Dietderich, P. Ferracin, S. O. Prestemon, G. Sabbi, and R. M. ScanlanAbstract— NbTi accelerator dipoles are limited to magneticfields (H ) of about 10 T, due to an intrinsic upper critical field(Hc2) limitation of 14 T. To surpass this restriction, prototypeNb3Sn magnets are being developed which have reached 16 T.We show that Nb3Sn dipole technology is practically limited to 17to 18 T due to insufficient high field pinning, and intrinsically to20 to 22 T due to Hc2 limitations. Therefore, to obtain magneticfields approaching 20 T and higher, a material is required with ahigher Hc2 and sufficient high field pinning capacity. A realisticcandidate for this purpose is Bi–2212, which is available in roundwires and sufficient lengths for the fabrication of coils based onRutherford-type cables. We initiated a program to develop therequired technology to construct accelerator magnets from ‘wind-and-react’ (W&R) Bi–2212 coils. We outline the complicationsthat arise through the use of Bi–2212, describe the developmentpaths to address these issues, and conclude with the design ofW&R Bi–2212 sub-scale magnets.Index Terms— Accelerator magnet, HTS, Bi–2212I. INTRODUCTIONTHE superconducting material of choice for acceleratormagnets has long been NbTi. The world’s largest particleaccelerator, the Large Hadron Collider (LHC) at CERN,utilizes NbTi technology. The record magnetic field withNbTi in a dipole configuration is 10.5 T at 1.8 K [?], andis approaching the intrinsic limitations of NbTi as will bediscussed below. To surpass the intrinsic limitations of NbTi,a number of prototype dipole magnets have been constructedusing Nb3Sn superconductors, since Nb3Sn approximatelydoubles the available field–temperature regime. Prototypedipole magnets that utilize Nb3Sn technology reach a steadilyincreasing magnetic field, with the present record being 16 Tat 4.5 K [?]. This progress resulted, for a significant part, fromthe increasing critical current density (Jc) in strands [?].These successful prototype magnets demonstrate the feasi-bility of Nb3Sn for use in accelerator magnets. This is empha-sized through the U.S. LHC Accelerator Research Program(LARP) [?], which focuses on the development of Nb 3Snmagnets for future LHC upgrades. However, as will be shownbelow, due to these recent successful efforts, Nb3Sn magnetsare also rapidly approaching the material’s limitations anda switch to a new material is inevitable to achieve highermagnetic fields.Manuscript received September 1, 2006. This work was supported by theDirector, Office of Science, High Energy Physics, U.S. Department of Energyunder contract No. DE-AC02-05CH11231.A. Godeke (corresponding author; phone: +1-510-486-4356; e-mail:agodeke@lbl.gov), D. Cheng, D. R. Dietderich, P. Ferracin, S. O. Prestemon,G. Sabbi, and R. M. Scanlan are with Lawrence Berkeley National Laboratory,One Cyclotron Rd, Berkeley CA 94720.Fig. 1. Field-temperature phase boundaries and record magnetic fields indipole magnets for NbTi and Nb3Sn.II. MAGNETIC FIELD LIMITS USING NBTI AND NB3SNTo validate the use of a new material and the ensuingtechnology development, an accurate determination of thelimitations of the present materials is required. A material’scurrent carrying capacity is determined by its effective field-temperature phase boundary (H ∗c2(T )) and its pinning capac-ity. A material’s pinning force (Fp) is maximized when thepinning site density is comparable to the flux-line density inthe operating magnetic field range.For NbTi, H∗c2(T ) is limited by H∗c2(0) ∼= 14.4 T and aneffective critical temperature T ∗c (0) ∼= 9.2 K [?], as depictedin Fig. 1. Pinning sites can be engineered in NbTi in the formof α–Ti precipitates, with a spacing that is comparable to theflux-line spacing in the 5 to 10 T magnetic field range [?](Fig. 2). NbTi is therefore, under present understanding, closeto fully optimized. This pinning capacity optimization yields aparabolic-like Fp(H) that peaks at about 50% of H ∗c2 [?], anda maximum Jc(5 T, 4.2 K) ∼= 3 kA/mm2, or 1150 A/mm2at 8 T and 4.2 K [?]. Fig. 1 shows that an optimized dipolemagnet, using an optimized NbTi wire, achieves about 80%of its intrinsic field-temperature limitation. 80% can thus beregarded as an optimized efficiency for dipole magnets.Recent investigations on the capacities of Nb3Sn supercon-ductors [?], [?], [?] place well-defined practical and intrinsiclimitations on its performance. For Nb3Sn, being stable fromabout 18 to 25 at.%Sn (the A15 phase), H ∗c2(T ) depends onthe Hc2 averaging over the compositions that are present ina wire [?]. In Fig. 1, the maximum detectable Hc2(T ) in0000–0000/00$00.00 submitted to IEEE, 2006Fig. 2. Critical current density as function of magnetic field at 1.9 and 4.2 Kin NbTi and Nb3Sn. Included are the load-lines for record magnets and for ahypothetical Nb3Sn dipole magnet achieving 80% of its intrinsic limitation.The inset shows the normalized pinning force as function of reduced magneticfield.wires is depicted by the dashed line. Also a best estimateis given for H∗c2(T ) for the present record Internal Tin (IT)wire (Jc(12 T, 4.2 K) = 3 kA/mm2) [?], which has anaverage Sn content of 24 at.%Sn [?]. This H ∗c2(T ) is limitedby H∗c2(0) ∼= 28 T and T ∗c (0) ∼= 17 K. Fig. 1 shows thatthe present record dipole magnet, using the present recordwire, achieves about 65% of this H ∗c2(T ). If Nb3Sn, likeNbTi, would achieve 80% of H ∗c2(T ), it would reach a dipolemagnetic field of 20 T at 4.2 K, or 22 T at 1.9 K, whichforms an intrinsic magnetic field limitation for Nb3Sn, whichis closely approached in solenoids.The main reason why a Nb3Sn magnet does not achieve80% of its intrinsic limitation is lack of high field pinningcapacity. Grain boundaries are the main pinning centers inNb3Sn and the average grain size in commercial wires isbetween 100 to 200 nm [?]. The average flux-line spacing,in contrast, is about 10 to 15 nm. The flux-line density istherefore about one order of magnitude larger than the grainboundary density. This results in collective pinning of the flux-line lattice, described by an asymmetric Fp(H) that peaks atonly 20% of H∗c2, i.e. at about 5 T at 4.2 K for the presentrecord IT wires [?] (Fig. 2). This lack of high field pinningtranslates to the concave Jc(H) curves in Fig. 2, and a reducedcurrent carrying capacity towards H ∗c2 [?]. The pinning curvefor optimized NbTi, in contrast, translates to an approximatelylinear Jc(H) reduction up to H ∗c2 [?].It has been demonstrated that the introduction of additionalpinning cites in Nb3Sn causes the maximum in Fp(H) toincrease and shift towards higher magnetic field [?]. Therefore,the only way to retain current carrying capacity in Nb 3Snwhen approaching H ∗c2 (as indicated by the hatched regionin Fig. 2) is by an enhancement of the pinning site den-sity through grain refinement or the inclusion of engineeredpinning centers. Both these options, though demonstrated onlaboratory samples, have not yet been validated in commercialwires. Grading and an unlimited conductor package could, intheory, generate magnetic fields approaching 20 T, but theresulting magnets would be unpractically large and expensivedue to Nb3Sn’s in-efficiency at high magnetic fields. Nb3Sn istherefore exhausted at dipole magnetic fields in the 17 to 18 Tregion. Even if high field pinning capacity in wires could beimproved, Nb3Sn would still be intrinsically limited to 22 Tat 1.9 K as described above.To progress towards magnetic fields of 20 T and higher,it is thus inevitable to switch to a new material with asignificantly higher Hc2 and sufficient high field pinningcapacity. Of the possible candidate materials (YBa2Cu3O7−δ,Bi2Ca2CuSr2O8+x, Bi2Ca2Cu2Sr3O10+y , MgB2), only Bi–2212 is sufficiently developed for use in accelerator magnetapplications. It has an H ∗c2(4.2 K) of about 85 T [?], aNbTi-like pinning curve, and is available in round wireswith sufficient length. The latest generation Bi–2212 wireshave already demonstrated to surpass the overall (engineering)critical current density in Nb3Sn wires at magnetic fields ofabout 18 T and higher [?].III. TECHNOLOGICAL CHALLENGES WITH BI–2212Constructing a magnet using Bi–2212 is significantly morecomplicated than using NbTi or Nb3Sn, as summarized inTable I. Technological challenges are related to the strainsensitivity of Bi–2212, its high formation reaction temperaturein an oxygen-rich environment, and chemical compatibility ofthe insulation and construction materials during the reactionheat treatment. Operational challenges are related to the lownormal zone propagation velocity, hindering energy dissipationand quench detection.A. Stress and strain related issuesThe critical current in Bi–2212 is, like Nb3Sn, sensitiveto strain (ε). A model that has been developed to describethe behavior of Jc(ε) in Bi–2212 in longitudinal strain ex-periments indicates, as with Nb3Sn, that the highest Jc isobtained in the strain free state (Fig. 3 [?]). Compressiveaxial strain during cool-down causes, in contrast to Nb3Sn, anirreversible reduction of Jc (Fig. 3: a). Releasing the thermalpre-compression (Fig. 3: b) does not, as for Nb3Sn, recoverJc, but Jc(ε) shows a plateau up to an axial strain wherecracks occur and Jc collapses (Fig. 3: c). Though Jc(ε) onthis plateau is reversible, any additional pre-compression andrelaxation causes a wider plateau at a reduced Jc value. Sucha wider plateau is often misinterpreted as an indication forreduced strain sensitivity, but is in fact a result of a largeraxial pre-compression in the Bi–2212. The Jc loss is on theorder of 100% per % axial compressive strain [?]. Constructionmaterials that match the thermal contraction of Bi–2212 aretherefore preferred. One publication on the stress sensitivityof Bi–2212 cables indicates a broad-face load limit of about60 MPa [?]. This emphasizes the need for more detailedinvestigations and accurate stress handling in magnets and/ornew conductor reinforcement techniques.B. Formation reaction related issuesBi–2212 is a brittle ceramic material which is formed fromprecursor powders during a partial melt reaction at aboutTABLE ISUMMARY OF TECHNOLOGICAL CHALLENGES IN RELATION TO SUPERCONDUCTOR CHOICE AND ACHIEVABLE DIPOLE MAGNETIC FIELDS.Material Dipole limit Reaction Wire axial Cable Insulation Construction Quenchcompression transverse stress propagationNbTi 10.5 T Ductile: R&W N/A N/A Polyimide Stainless Steel > 20 ms−1Nb3Sn 17–18 T (Fp ↑: 22 T) ∼ 675◦C in Ar/Vacuum Reversible 200 MPa limit S/R–Glass Stainless Steel ∼ 20 ms−1Bi–2212 Stress limited ∼ 890◦C in O2 (± 2◦C) Irreversible 60 MPa limit Ceramic Super alloy ∼ 0.04 ms−1Unstrained Bi-2212Pre-strained Bi-2212Furtherpre-strained Bi-2212Intrinsic axial strain in Bi-22120Critical current densityabc

English

Limits of NbTi and Nb3Sn, and Development of W&amp;R Bi-2212 High Field Accelerator Magnets

UNT Digital Library

Limits of NbTi and Nb3Sn, and development of W& R Bi-2212 High Field Accelerator Magnets

NbTi accelerator dipoles are limited to magnetic fields (H)of about 10 T, due to an intrinsic upper critical field(Hc2) limitationof 14 T. To surpass this restriction, prototype Nb3Sn magnets are beingdeveloped which have reached 16 T. We show that Nb3Sn dipole technologyis practically limited to 17 to 18 T due to insufficient high fieldpinning, and intrinsically to 20 to 22 T due to Hc2 limitations.Therefore, to obtain magnetic fields approaching 20 T and higher, amaterial is required with a higher Hc2 and sufficient high field pinningcapacity. A realistic candidate for this purpose is Bi-2212, which isavailable in roundwires and sufficient lengths for the fabrication ofcoils based on Rutherford-type cables. We initiated a program to developthe required technology to construct accelerator magnets from'windand-react' (W&R) Bi-2212 coils. We outline the complicationsthat arise through the use of Bi-2212, describe the development paths toaddress these issues, and conclude with the design of W&R Bi-2212sub-scale magnets

Dietderich, D. R.

Prestemon, S. O.

Sabbi, G.

Scanlan, R. M.

Limits of NbTi and Nb3Sn, and Development of W&R Bi-2212 HighField Accelerator Magnets

NbTi accelerator dipoles are limited to magneticfields (H)of about 10 T, due to an intrinsic upper critical field (Hc2) limitationof 14 T. To surpass this restriction, prototype Nb3Sn magnets are beingdeveloped which have reached 16 T. We show that Nb3Sn dipole technologyis practically limited to 17 to 18 T due to insufficient high fieldpinning, and intrinsically to 20 to 22 T due to Hc2 limitations.Therefore, to obtain magnetic fields approaching 20 T and higher, amaterial is required with a higher Hc2 and sufficient high field pinningcapacity. A realistic candidate for this purpose is Bi-2212, which isavailable in roundwires and sufficient lengths for the fabrication ofcoils based on Rutherford-type cables. We initiated a program to developthe required technology to construct accelerator magnets from'windand-react' (W&R) Bi-2212 coils. We outline the complicationsthat arise through the use of Bi-2212, describe the development paths toaddress these issues, and conclude with the design of W&R Bi-2212sub-scale magnets

Limits of NbTi and Nb3Sn, and Development ofW&R Bi–2212 High Field Accelerator MagnetsA. Godeke, D. Cheng, D. R. Dietderich, P. Ferracin, S. O. Prestemon, G. Sabbi, and R. M. ScanlanAbstract— NbTi accelerator dipoles are limited to magneticfields (H ) of about 10 T, due to an intrinsic upper critical field(Hc2) limitation of 14 T. To surpass this restriction, prototypeNb3Sn magnets are being developed which have reached 16 T.We show that Nb3Sn dipole technology is practically limited to 17to 18 T due to insufficient high field pinning, and intrinsically to20 to 22 T due to Hc2 limitations. Therefore, to obtain magneticfields approaching 20 T and higher, a material is required with ahigher Hc2 and sufficient high field pinning capacity. A realisticcandidate for this purpose is Bi–2212, which is available in roundwires and sufficient lengths for the fabrication of coils based onRutherford-type cables. We initiated a program to develop therequired technology to construct accelerator magnets from ‘wind-and-react’ (W&R) Bi–2212 coils. We outline the complicationsthat arise through the use of Bi–2212, describe the developmentpaths to address these issues, and conclude with the design ofW&R Bi–2212 sub-scale magnets.Index Terms— Accelerator magnet, HTS, Bi–2212I. INTRODUCTIONTHE superconducting material of choice for acceleratormagnets has long been NbTi. The world’s largest particleaccelerator, the Large Hadron Collider (LHC) at CERN,utilizes NbTi technology. The record magnetic field withNbTi in a dipole configuration is 10.5 T at 1.8 K [?], andis approaching the intrinsic limitations of NbTi as will bediscussed below. To surpass the intrinsic limitations of NbTi,a number of prototype dipole magnets have been constructedusing Nb3Sn superconductors, since Nb3Sn approximatelydoubles the available field–temperature regime. Prototypedipole magnets that utilize Nb3Sn technology reach a steadilyincreasing magnetic field, with the present record being 16 Tat 4.5 K [?]. This progress resulted, for a significant part, fromthe increasing critical current density (Jc) in strands [?].These successful prototype magnets demonstrate the feasi-bility of Nb3Sn for use in accelerator magnets. This is empha-sized through the U.S. LHC Accelerator Research Program(LARP) [?], which focuses on the development of Nb 3Snmagnets for future LHC upgrades. However, as will be shownbelow, due to these recent successful efforts, Nb3Sn magnetsare also rapidly approaching the material’s limitations anda switch to a new material is inevitable to achieve highermagnetic fields.Manuscript received September 1, 2006. This work was supported by theDirector, Office of Science, High Energy Physics, U.S. Department of Energyunder contract No. DE-AC02-05CH11231.A. Godeke (corresponding author; phone: +1-510-486-4356; e-mail:agodeke@lbl.gov), D. Cheng, D. R. Dietderich, P. Ferracin, S. O. Prestemon,G. Sabbi, and R. M. Scanlan are with Lawrence Berkeley National Laboratory,One Cyclotron Rd, Berkeley CA 94720.Fig. 1. Field-temperature phase boundaries and record magnetic fields indipole magnets for NbTi and Nb3Sn.II. MAGNETIC FIELD LIMITS USING NBTI AND NB3SNTo validate the use of a new material and the ensuingtechnology development, an accurate determination of thelimitations of the present materials is required. A material’scurrent carrying capacity is determined by its effective field-temperature phase boundary (H ∗c2(T )) and its pinning capac-ity. A material’s pinning force (Fp) is maximized when thepinning site density is comparable to the flux-line density inthe operating magnetic field range.For NbTi, H∗c2(T ) is limited by H∗c2(0) ∼= 14.4 T and aneffective critical temperature T ∗c (0) ∼= 9.2 K [?], as depictedin Fig. 1. Pinning sites can be engineered in NbTi in the formof α–Ti precipitates, with a spacing that is comparable to theflux-line spacing in the 5 to 10 T magnetic field range [?](Fig. 2). NbTi is therefore, under present understanding, closeto fully optimized. This pinning capacity optimization yields aparabolic-like Fp(H) that peaks at about 50% of H ∗c2 [?], anda maximum Jc(5 T, 4.2 K) ∼= 3 kA/mm2, or 1150 A/mm2at 8 T and 4.2 K [?]. Fig. 1 shows that an optimized dipolemagnet, using an optimized NbTi wire, achieves about 80%of its intrinsic field-temperature limitation. 80% can thus beregarded as an optimized efficiency for dipole magnets.Recent investigations on the capacities of Nb3Sn supercon-ductors [?], [?], [?] place well-defined practical and intrinsiclimitations on its performance. For Nb3Sn, being stable fromabout 18 to 25 at.%Sn (the A15 phase), H ∗c2(T ) depends onthe Hc2 averaging over the compositions that are present ina wire [?]. In Fig. 1, the maximum detectable Hc2(T ) in0000–0000/00$00.00 submitted to IEEE, 2006Fig. 2. Critical current density as function of magnetic field at 1.9 and 4.2 Kin NbTi and Nb3Sn. Included are the load-lines for record magnets and for ahypothetical Nb3Sn dipole magnet achieving 80% of its intrinsic limitation.The inset shows the normalized pinning force as function of reduced magneticfield.wires is depicted by the dashed line. Also a best estimateis given for H∗c2(T ) for the present record Internal Tin (IT)wire (Jc(12 T, 4.2 K) = 3 kA/mm2) [?], which has anaverage Sn content of 24 at.%Sn [?]. This H ∗c2(T ) is limitedby H∗c2(0) ∼= 28 T and T ∗c (0) ∼= 17 K. Fig. 1 shows thatthe present record dipole magnet, using the present recordwire, achieves about 65% of this H ∗c2(T ). If Nb3Sn, likeNbTi, would achieve 80% of H ∗c2(T ), it would reach a dipolemagnetic field of 20 T at 4.2 K, or 22 T at 1.9 K, whichforms an intrinsic magnetic field limitation for Nb3Sn, whichis closely approached in solenoids.The main reason why a Nb3Sn magnet does not achieve80% of its intrinsic limitation is lack of high field pinningcapacity. Grain boundaries are the main pinning centers inNb3Sn and the average grain size in commercial wires isbetween 100 to 200 nm [?]. The average flux-line spacing,in contrast, is about 10 to 15 nm. The flux-line density istherefore about one order of magnitude larger than the grainboundary density. This results in collective pinning of the flux-line lattice, described by an asymmetric Fp(H) that peaks atonly 20% of H∗c2, i.e. at about 5 T at 4.2 K for the presentrecord IT wires [?] (Fig. 2). This lack of high field pinningtranslates to the concave Jc(H) curves in Fig. 2, and a reducedcurrent carrying capacity towards H ∗c2 [?]. The pinning curvefor optimized NbTi, in contrast, translates to an approximatelylinear Jc(H) reduction up to H ∗c2 [?].It has been demonstrated that the introduction of additionalpinning cites in Nb3Sn causes the maximum in Fp(H) toincrease and shift towards higher magnetic field [?]. Therefore,the only way to retain current carrying capacity in Nb 3Snwhen approaching H ∗c2 (as indicated by the hatched regionin Fig. 2) is by an enhancement of the pinning site den-sity through grain refinement or the inclusion of engineeredpinning centers. Both these options, though demonstrated onlaboratory samples, have not yet been validated in commercialwires. Grading and an unlimited conductor package could, intheory, generate magnetic fields approaching 20 T, but theresulting magnets would be unpractically large and expensivedue to Nb3Sn’s in-efficiency at high magnetic fields. Nb3Sn istherefore exhausted at dipole magnetic fields in the 17 to 18 Tregion. Even if high field pinning capacity in wires could beimproved, Nb3Sn would still be intrinsically limited to 22 Tat 1.9 K as described above.To progress towards magnetic fields of 20 T and higher,it is thus inevitable to switch to a new material with asignificantly higher Hc2 and sufficient high field pinningcapacity. Of the possible candidate materials (YBa2Cu3O7−δ,Bi2Ca2CuSr2O8+x, Bi2Ca2Cu2Sr3O10+y , MgB2), only Bi–2212 is sufficiently developed for use in accelerator magnetapplications. It has an H ∗c2(4.2 K) of about 85 T [?], aNbTi-like pinning curve, and is available in round wireswith sufficient length. The latest generation Bi–2212 wireshave already demonstrated to surpass the overall (engineering)critical current density in Nb3Sn wires at magnetic fields ofabout 18 T and higher [?].III. TECHNOLOGICAL CHALLENGES WITH BI–2212Constructing a magnet using Bi–2212 is significantly morecomplicated than using NbTi or Nb3Sn, as summarized inTable I. Technological challenges are related to the strainsensitivity of Bi–2212, its high formation reaction temperaturein an oxygen-rich environment, and chemical compatibility ofthe insulation and construction materials during the reactionheat treatment. Operational challenges are related to the lownormal zone propagation velocity, hindering energy dissipationand quench detection.A. Stress and strain related issuesThe critical current in Bi–2212 is, like Nb3Sn, sensitiveto strain (). A model that has been developed to describethe behavior of Jc() in Bi–2212 in longitudinal strain ex-periments indicates, as with Nb3Sn, that the highest Jc isobtained in the strain free state (Fig. 3 [?]). Compressiveaxial strain during cool-down causes, in contrast to Nb3Sn, anirreversible reduction of Jc (Fig. 3: a). Releasing the thermalpre-compression (Fig. 3: b) does not, as for Nb3Sn, recoverJc, but Jc() shows a plateau up to an axial strain wherecracks occur and Jc collapses (Fig. 3: c). Though Jc() onthis plateau is reversible, any additional pre-compression andrelaxation causes a wider plateau at a reduced Jc value. Sucha wider plateau is often misinterpreted as an indication forreduced strain sensitivity, but is in fact a result of a largeraxial pre-compression in the Bi–2212. The Jc loss is on theorder of 100% per % axial compressive strain [?]. Constructionmaterials that match the thermal contraction of Bi–2212 aretherefore preferred. One publication on the stress sensitivityof Bi–2212 cables indicates a broad-face load limit of about60 MPa [?]. This emphasizes the need for more detailedinvestigations and accurate stress handling in magnets and/ornew conductor reinforcement techniques.B. Formation reaction related issuesBi–2212 is a brittle ceramic material which is formed fromprecursor powders during a partial melt reaction at aboutTABLE ISUMMARY OF TECHNOLOGICAL CHALLENGES IN RELATION TO SUPERCONDUCTOR CHOICE AND ACHIEVABLE DIPOLE MAGNETIC FIELDS.Material Dipole limit Reaction Wire axial Cable Insulation Construction Quenchcompression transverse stress propagationNbTi 10.5 T Ductile: R&W N/A N/A Polyimide Stainless Steel > 20 ms−1Nb3Sn 17–18 T (Fp ↑: 22 T) ∼ 675◦C in Ar/Vacuum Reversible 200 MPa limit S/R–Glass Stainless Steel ∼ 20 ms−1Bi–2212 Stress limited ∼ 890◦C in O2 (± 2◦C) Irreversible 60 MPa limit Ceramic Super alloy ∼ 0.04 ms−1Unstrained Bi-2212Pre-strained Bi-2212Furtherpre-strained Bi-2212Intrinsic axial strain in Bi-22120Critical current densityabc

NbTi accelerator dipoles are limited to magneticfields (H) of about 10 T, due to an  intrinsic upper critical field (Hc2) limitation of 14 T. To surpass this restriction, prototype  Nb3Sn magnets are being developed which have reached 16 T. We show that Nb3Sn dipole technology is  practically limited to 17 to 18 T due to insufficient high field pinning, and intrinsically to 20  to 22 T due to Hc2 limitations. Therefore, to obtain magnetic fields approaching 20 T and higher,  a material is required with a higher Hc2 and sufficient high field pinning capacity. A realistic  candidate for this purpose is Bi-2212, which is available in roundwires and sufficient lengths for  the fabrication of coils based on Rutherford-type cables. We initiated a program to develop the  required technology to construct accelerator magnets from 'windand-react' (W&amp;R) Bi-2212 coils.  We outline the complications that arise through the use of Bi-2212, describe the development paths  to address these issues, and conclude with the design of W&amp;R Bi-2212 sub-scale  magnets

Sa bbi, G.

Lawrence Berkeley National LaboratoryLawrence Berkeley National LaboratoryTitleLimits of NbTi and Nb3Sn, and Development of W&amp;R Bi-2212 High Field Accelerator MagnetsPermalinkhttps://escholarship.org/uc/item/2t34x9m7AuthorsGodeke, A.Cheng, D.Dietderich, D.R.et al.Publication Date2008-05-27eScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaLimits of NbTi and Nb3Sn, and Development ofW&R Bi–2212 High Field Accelerator MagnetsA. Godeke, D. Cheng, D. R. Dietderich, P. Ferracin, S. O. Prestemon, G. Sabbi, and R. M. ScanlanAbstract— NbTi accelerator dipoles are limited to magneticfields (H ) of about 10 T, due to an intrinsic upper critical field(Hc2) limitation of 14 T. To surpass this restriction, prototypeNb3Sn magnets are being developed which have reached 16 T.We show that Nb3Sn dipole technology is practically limited to 17to 18 T due to insufficient high field pinning, and intrinsically to20 to 22 T due to Hc2 limitations. Therefore, to obtain magneticfields approaching 20 T and higher, a material is required with ahigher Hc2 and sufficient high field pinning capacity. A realisticcandidate for this purpose is Bi–2212, which is available in roundwires and sufficient lengths for the fabrication of coils based onRutherford-type cables. We initiated a program to develop therequired technology to construct accelerator magnets from ‘wind-and-react’ (W&R) Bi–2212 coils. We outline the complicationsthat arise through the use of Bi–2212, describe the developmentpaths to address these issues, and conclude with the design ofW&R Bi–2212 sub-scale magnets.Index Terms— Accelerator magnet, HTS, Bi–2212I. INTRODUCTIONTHE superconducting material of choice for acceleratormagnets has long been NbTi. The world’s largest particleaccelerator, the Large Hadron Collider (LHC) at CERN,utilizes NbTi technology. The record magnetic field withNbTi in a dipole configuration is 10.5 T at 1.8 K [?], andis approaching the intrinsic limitations of NbTi as will bediscussed below. To surpass the intrinsic limitations of NbTi,a number of prototype dipole magnets have been constructedusing Nb3Sn superconductors, since Nb3Sn approximatelydoubles the available field–temperature regime. Prototypedipole magnets that utilize Nb3Sn technology reach a steadilyincreasing magnetic field, with the present record being 16 Tat 4.5 K [?]. This progress resulted, for a significant part, fromthe increasing critical current density (Jc) in strands [?].These successful prototype magnets demonstrate the feasi-bility of Nb3Sn for use in accelerator magnets. This is empha-sized through the U.S. LHC Accelerator Research Program(LARP) [?], which focuses on the development of Nb 3Snmagnets for future LHC upgrades. However, as will be shownbelow, due to these recent successful efforts, Nb3Sn magnetsare also rapidly approaching the material’s limitations anda switch to a new material is inevitable to achieve highermagnetic fields.Manuscript received September 1, 2006. This work was supported by theDirector, Office of Science, High Energy Physics, U.S. Department of Energyunder contract No. DE-AC02-05CH11231.A. Godeke (corresponding author; phone: +1-510-486-4356; e-mail:agodeke@lbl.gov), D. Cheng, D. R. Dietderich, P. Ferracin, S. O. Prestemon,G. Sabbi, and R. M. Scanlan are with Lawrence Berkeley National Laboratory,One Cyclotron Rd, Berkeley CA 94720.Fig. 1. Field-temperature phase boundaries and record magnetic fields indipole magnets for NbTi and Nb3Sn.II. MAGNETIC FIELD LIMITS USING NBTI AND NB3SNTo validate the use of a new material and the ensuingtechnology development, an accurate determination of thelimitations of the present materials is required. A material’scurrent carrying capacity is determined by its effective field-temperature phase boundary (H ∗c2(T )) and its pinning capac-ity. A material’s pinning force (Fp) is maximized when thepinning site density is comparable to the flux-line density inthe operating magnetic field range.For NbTi, H∗c2(T ) is limited by H∗c2(0) ∼= 14.4 T and aneffective critical temperature T ∗c (0) ∼= 9.2 K [?], as depictedin Fig. 1. Pinning sites can be engineered in NbTi in the formof α–Ti precipitates, with a spacing that is comparable to theflux-line spacing in the 5 to 10 T magnetic field range [?](Fig. 2). NbTi is therefore, under present understanding, closeto fully optimized. This pinning capacity optimization yields aparabolic-like Fp(H) that peaks at about 50% of H ∗c2 [?], anda maximum Jc(5 T, 4.2 K) ∼= 3 kA/mm2, or 1150 A/mm2at 8 T and 4.2 K [?]. Fig. 1 shows that an optimized dipolemagnet, using an optimized NbTi wire, achieves about 80%of its intrinsic field-temperature limitation. 80% can thus beregarded as an optimized efficiency for dipole magnets.Recent investigations on the capacities of Nb3Sn supercon-ductors [?], [?], [?] place well-defined practical and intrinsiclimitations on its performance. For Nb3Sn, being stable fromabout 18 to 25 at.%Sn (the A15 phase), H ∗c2(T ) depends onthe Hc2 averaging over the compositions that are present ina wire [?]. In Fig. 1, the maximum detectable Hc2(T ) in0000–0000/00$00.00 submitted to IEEE, 2006Fig. 2. Critical current density as function of magnetic field at 1.9 and 4.2 Kin NbTi and Nb3Sn. Included are the load-lines for record magnets and for ahypothetical Nb3Sn dipole magnet achieving 80% of its intrinsic limitation.The inset shows the normalized pinning force as function of reduced magneticfield.wires is depicted by the dashed line. Also a best estimateis given for H∗c2(T ) for the present record Internal Tin (IT)wire (Jc(12 T, 4.2 K) = 3 kA/mm2) [?], which has anaverage Sn content of 24 at.%Sn [?]. This H ∗c2(T ) is limitedby H∗c2(0) ∼= 28 T and T ∗c (0) ∼= 17 K. Fig. 1 shows thatthe present record dipole magnet, using the present recordwire, achieves about 65% of this H ∗c2(T ). If Nb3Sn, likeNbTi, would achieve 80% of H ∗c2(T ), it would reach a dipolemagnetic field of 20 T at 4.2 K, or 22 T at 1.9 K, whichforms an intrinsic magnetic field limitation for Nb3Sn, whichis closely approached in solenoids.The main reason why a Nb3Sn magnet does not achieve80% of its intrinsic limitation is lack of high field pinningcapacity. Grain boundaries are the main pinning centers inNb3Sn and the average grain size in commercial wires isbetween 100 to 200 nm [?]. The average flux-line spacing,in contrast, is about 10 to 15 nm. The flux-line density istherefore about one order of magnitude larger than the grainboundary density. This results in collective pinning of the flux-line lattice, described by an asymmetric Fp(H) that peaks atonly 20% of H∗c2, i.e. at about 5 T at 4.2 K for the presentrecord IT wires [?] (Fig. 2). This lack of high field pinningtranslates to the concave Jc(H) curves in Fig. 2, and a reducedcurrent carrying capacity towards H ∗c2 [?]. The pinning curvefor optimized NbTi, in contrast, translates to an approximatelylinear Jc(H) reduction up to H ∗c2 [?].It has been demonstrated that the introduction of additionalpinning cites in Nb3Sn causes the maximum in Fp(H) toincrease and shift towards higher magnetic field [?]. Therefore,the only way to retain current carrying capacity in Nb 3Snwhen approaching H ∗c2 (as indicated by the hatched regionin Fig. 2) is by an enhancement of the pinning site den-sity through grain refinement or the inclusion of engineeredpinning centers. Both these options, though demonstrated onlaboratory samples, have not yet been validated in commercialwires. Grading and an unlimited conductor package could, intheory, generate magnetic fields approaching 20 T, but theresulting magnets would be unpractically large and expensivedue to Nb3Sn’s in-efficiency at high magnetic fields. Nb3Sn istherefore exhausted at dipole magnetic fields in the 17 to 18 Tregion. Even if high field pinning capacity in wires could beimproved, Nb3Sn would still be intrinsically limited to 22 Tat 1.9 K as described above.To progress towards magnetic fields of 20 T and higher,it is thus inevitable to switch to a new material with asignificantly higher Hc2 and sufficient high field pinningcapacity. Of the possible candidate materials (YBa2Cu3O7−δ,Bi2Ca2CuSr2O8+x, Bi2Ca2Cu2Sr3O10+y , MgB2), only Bi–2212 is sufficiently developed for use in accelerator magnetapplications. It has an H ∗c2(4.2 K) of about 85 T [?], aNbTi-like pinning curve, and is available in round wireswith sufficient length. The latest generation Bi–2212 wireshave already demonstrated to surpass the overall (engineering)critical current density in Nb3Sn wires at magnetic fields ofabout 18 T and higher [?].III. TECHNOLOGICAL CHALLENGES WITH BI–2212Constructing a magnet using Bi–2212 is significantly morecomplicated than using NbTi or Nb3Sn, as summarized inTable I. Technological challenges are related to the strainsensitivity of Bi–2212, its high formation reaction temperaturein an oxygen-rich environment, and chemical compatibility ofthe insulation and construction materials during the reactionheat treatment. Operational challenges are related to the lownormal zone propagation velocity, hindering energy dissipationand quench detection.A. Stress and strain related issuesThe critical current in Bi–2212 is, like Nb3Sn, sensitiveto strain (ε). A model that has been developed to describethe behavior of Jc(ε) in Bi–2212 in longitudinal strain ex-periments indicates, as with Nb3Sn, that the highest Jc isobtained in the strain free state (Fig. 3 [?]). Compressiveaxial strain during cool-down causes, in contrast to Nb3Sn, anirreversible reduction of Jc (Fig. 3: a). Releasing the thermalpre-compression (Fig. 3: b) does not, as for Nb3Sn, recoverJc, but Jc(ε) shows a plateau up to an axial strain wherecracks occur and Jc collapses (Fig. 3: c). Though Jc(ε) onthis plateau is reversible, any additional pre-compression andrelaxation causes a wider plateau at a reduced Jc value. Sucha wider plateau is often misinterpreted as an indication forreduced strain sensitivity, but is in fact a result of a largeraxial pre-compression in the Bi–2212. The Jc loss is on theorder of 100% per % axial compressive strain [?]. Constructionmaterials that match the thermal contraction of Bi–2212 aretherefore preferred. One publication on the stress sensitivityof Bi–2212 cables indicates a broad-face load limit of about60 MPa [?]. This emphasizes the need for more detailedinvestigations and accurate stress handling in magnets and/ornew conductor reinforcement techniques.B. Formation reaction related issuesBi–2212 is a brittle ceramic material which is formed fromprecursor powders during a partial melt reaction at aboutTABLE ISUMMARY OF TECHNOLOGICAL CHALLENGES IN RELATION TO SUPERCONDUCTOR CHOICE AND ACHIEVABLE DIPOLE MAGNETIC FIELDS.Material Dipole limit Reaction Wire axial Cable Insulation Construction Quenchcompression transverse stress propagationNbTi 10.5 T Ductile: R&W N/A N/A Polyimide Stainless Steel > 20 ms−1Nb3Sn 17–18 T (Fp ↑: 22 T) ∼ 675◦C in Ar/Vacuum Reversible 200 MPa limit S/R–Glass Stainless Steel ∼ 20 ms−1Bi–2212 Stress limited ∼ 890◦C in O2 (± 2◦C) Irreversible 60 MPa limit Ceramic Super alloy ∼ 0.04 ms−1Unstrained Bi-2212Pre-strained Bi-2212Furtherpre-strained Bi-2212Intrinsic axial strain in Bi-22120Critical current densityabc

Limits of NbTi and Nb3Sn, and Development of W&amp;R Bi-2212 High Field Accelerator 
Magnets

NbTi accelerator dipoles are limited to magnetic fields (H) of about 10 T, due to an  intrinsic upper critical field(Hc2) limitation of 14 T. To surpass this restriction, prototype  Nb3Sn magnets are being developed which have reached 16 T. We show that Nb3Sn dipole technology is  practically limited to 17 to 18 T due to insufficient high field pinning, and intrinsically to 20  to 22 T due to Hc2 limitations. Therefore, to obtain magnetic fields approaching 20 T and higher,  a material is required with a higher Hc2 and sufficient high field pinning capacity. A realistic  candidate for this purpose is Bi-2212, which is available in roundwires and sufficient lengths for  the fabrication of coils based on Rutherford-type cables. We initiated a program to develop the  required technology to construct accelerator magnets from 'windand-react' (W&amp;R) Bi-2212 coils.  We outline the complications that arise through the use of Bi-2212, describe the development paths  to address these issues, and conclude with the design of W&amp;R Bi-2212 sub-scale  magnets

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Limits of NbTi and Nb3Sn, and development of W& R Bi-2212 High Field Accelerator Magnets

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