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Geneva – 4 th November2008

Geneva – 4 th November2008. Guidelines on Magneto-Thermal Stability. Bernardo Bordini. OUTLINE. INTRODUCTION SELF-FIELD INSTABILITY IN STRAND MEASUREMENTS CONDUCTOR REQUIREMENTS TO AVOID INSTABILITIES IN MAGNETS CONCLUSIONS. HIGH - J c Nb 3 Sn STRAND AND MAGNETO-THERMAL INSTABILITIES.

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Geneva – 4 th November2008

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  1. Geneva – 4th November2008 Guidelines on Magneto-Thermal Stability Bernardo Bordini

  2. OUTLINE • INTRODUCTION • SELF-FIELD INSTABILITY IN STRAND MEASUREMENTS • CONDUCTOR REQUIREMENTS TO AVOID INSTABILITIES IN MAGNETS • CONCLUSIONS Bernardo Bordini

  3. HIGH -JcNb3Sn STRAND AND MAGNETO-THERMAL INSTABILITIES • High-Jc Nb3Sn wires is the best candidate for next generation High Field (>10 T) accelerator magnets • Although very promising, state of the art high-Jc Nb3Sn wires suffer flux jumps • Flux jumps can quench the superconductor and severely limit the strand performance • Flux jumps are caused by magneto-thermal instabilities: • ‘Magnetization’ instability depending on Jc , Deffand Cu RRR 2) ‘Self field’ instability depending on Jc and strand diameter Bernardo Bordini

  4. MAGNETO-THERMAL INSTABILITIES AND MAGNET PERFORMANCE • Magnetization instability has been the primary cause of the limited quench performance (40-70 % of the short sample limit) at 4.4 K of some Nb3Sn high field magnets built at FNAL [1] and LBNL [2] in the early 2000s • At present the problem of magnetization instability at 4.4 K is contained through optimized heat treatments and cabling processes that guarantee a high RRR [1] A. V. Zlobin et al , “R&D of Nb3Sn Accelerator Magnets at Fermilab”, IEEE Trans. Appl. Supercond., vol. 15, no. 2, Jun. 2005 [2] D. R. Dieterich et al , “Correlation between strand stability and magnet performance”, IEEE Trans. Appl. Supercond., vol. 15, no. 2, Jun. 2005 RRR 120 RRR 8 • Strand measurements at 4.3 K performed at CERN B. Bordini, L. Rossi, presented at Applied Superconductivity Conference, Chicago, USA, Aug. 2008 B. Bordini, E. Barzi, S. Feher, L.Rossi, A.V. Zlobin, IEEE Trans. Appl. Supercond., vol. 18, no. 2, pp. 1309 - 1312, Jun. 2008 Bernardo Bordini

  5. MAGNETO-THERMAL INSTABILITIES AND MAGNET PERFORMANCE • Recently, it has been shown that at 1.9 K the self field instability is the dominating mechanism that limits the performance of high-JcNb3Sn strands [1]; this instability might be the primary cause of premature quenches of HF magnets at 1.9 K Strand Measurements 0.8 mm RRP – RRR 80 Test of the TQS02c Magnet at CERN [2] 4.3 K - Iq/Iexpected=1 1.9 K - Iq/Iexpected=0.7 [1] B. Bordini, E. Barzi, S. Feher, L.Rossi, A.V. Zlobin, “Self-Field Effects in Magneto-Thermal Instabilities for Nb-Sn Strands”,IEEE Trans. Appl. Supercond., vol. 18, no. 2, pp. 1309 - 1312, Jun. 2008 [2] B. Bordini, M. Bajko, H. Felice, C. Giloux, L. Rossi, “A Test Procedure to Study the Magneto-Thermal Stability around 1.9 K of TQS02c, a 1 m long Nb3Sn Quadrupole Magnet”, CERN Internal Note Bernardo Bordini

  6. WHAT IS THE SELF-FIELD INSTABILITY ? The self field instability is caused by the uneven distribution of transport current (I) within the wire. 0.8 mm RRP® Nb3Sn strand • While ramping up I at a fixed Ba , the multifilamentary strand acts as a large monofilament with a radius equal to the composite radius: The current only flows in the outermost sub-elements at the critical current density. Simulation of the transport current distribution while increasing the current from 0 to 1200 A in a fixed applied field, Ba=6 T Bernardo Bordini

  7. SELF-FIELD INSTABILITY :Simulation of Premature Quench The color represents the Transport Current distribution The color represents the Temperature distribution Ba=6 T -- I=1200 A -- Ti=4.2 K Bernardo Bordini

  8. OUTLINE • INTRODUCTION • SELF-FIELD INSTABILITY IN STRAND MEASUREMENTS • CONDUCTOR REQUIREMENTS TO AVOID INSTABILITIES IN MAGNETS • CONCLUSIONS Bernardo Bordini

  9. V-I MEASUREMENTS • Several samples of 0.8 mm 54/61 RRP® strands with similar Jc and significantly different RRR were test at 4.3 K and 1.9 K • Increasing the RRR up to 150 improves the stability of the strand; further increasing the RRR does not produce significant changes • Is-SF = Self Field Stability Current ; Is-SF (4.2 K)>Is-SF (1.9K)<Ic (12T,1.9K) • In one case: Is-SF(1.9K)=0.68 Ic (12T,1.9K)<Ic (12T,4.2K) • High RRR is not sufficient to prevent Self Field Instability in 12 T magnets • Strand measurements show that the self field stability of the strand can be improved : • Reducing the strand diameter • Decreasing the Jc Bernardo Bordini

  10. ARE STRAND MEASUREMENTS REPRESENTATIVE OF THE CONDUCTOR BEHAVIOUR IN THE MAGNET? Self-field instability is sensitive to the perturbation energy that initiates the instability* Strand measurements cannot perfectly reproduce the conductor behavior in the magnet; during strand measurements the conductor is generally more stable because the perturbations are smaller Simulations at 1.9 K using a new Finite Element Model*: 54/61 RRP 0.8 mm RRR 8 - Pert. 100%  0.73 μJ/mm Strand measurements at 1.9 K: 54/61 RRP 0.8 mm RRR 80 (S.3) and 8 (S.4) *B. Bordini, L. Rossi, “Self field instability in high-Jc Nb3Sn strands with high Copper Residual Resistivity Ratio”, presented at Applied Superconductivity Conference, Chicago, USA, August, 2008 Bernardo Bordini

  11. OUTLINE • INTRODUCTION • SELF-FIELD INSTABILITY IN STRAND MEASUREMENTS • CONDUCTOR REQUIREMENTS TO AVOID INSTABILITIES IN MAGNETS • CONCLUSIONS Bernardo Bordini

  12. EFFECTS OF LOCAL STRAND’S DAMAGES • A small local damage of the copper stabilizer can completely jeopardize the dynamic stabilization of a high Jc Nb3Sn strand Sample holder diameter ~ 32 mm LARP 54 /61 RRP 0.7 mm RRR > 250 4.2 K 1.9 K Bernardo Bordini

  13. Magnet Designed to Operate at 4.3 K – Jc 2000 A/mm2 • Under adiabatic assumptions, the Magneto-Thermal Stability of a Magnet that has to operate at its critical current depends on: the expected peak field at a certain temperature, the strand diameter, the effective filament sizes and the conductor critical current density • Calculations based on a semi-analytical model* using the layout of RRP strands (Cu/nonCu~1; composite radius ~ 80% strand radius) • The colored (black) lines represent the ratio between the stability current due to magnetization instability (self field) and the conductor critical current *B. Bordini, E. Barzi, S. Feher, L.Rossi, A.V. Zlobin, “Self-Field Effects in Magneto-Thermal Instabilities for Nb-Sn Strands”, IEEE Trans. Appl. Supercond., vol. 18, no. 2, pp. 1309 - 1312, Jun. 2008 Bernardo Bordini

  14. Magnet Designed to Operate at 4.3 K – Jc 2000 A/mm2 ADIABATIC SIMULATIONS Bernardo Bordini

  15. Magnet Designed to Operate at 4.3 K – Jc 2600 A/mm2 ADIABATIC SIMULATIONS Bernardo Bordini

  16. Magnet Designed to Operate at 4.3 K – Jc 3000 A/mm2 ADIABATIC SIMULATIONS Bernardo Bordini

  17. Magnet Designed to Operate at 1.9 K • Different colors different strand  (assumption composite radius ~ 80% strand radius – RRP strand) • A magnet that has to reach its critical current when the peak field is 15 T and the temperature is 1.9 K have to use strands with a composite diameter ≤ 0.8*0.8 mm (adiabatic approximation) ADIABATIC SIMULATIONS Bernardo Bordini

  18. CONCLUSIONS • Increasing the RRR above 150 does not improve the conductor magneto-thermal stability • A local damage of the copper stabilizer can completely Jeopardized the dynamic stabilization of the conductor  we can not relay on the dynamic stabilization is better to design a magnet assuming a low RRR value • High-Jc Nb3Sn conductor (Jc>2600 A/mm2 at 4.2 K and 12 T)is not perfectly suitable for 12 T magnets because of magneto-thermal instabilities; this problem disappears if we move towards higher field magnets • Magneto-thermal instability is not a problem for 15 T magnets that have to operate at 4.3 K (as Fresca2) if the strand diameter () and the effective filament size (Deff ) are sufficiently small ( ≤ 1 mm, Deff ≤ 70 μm) • A 15 T magnet designed to work at 1.9 K could still have problems if the strand diameter and the Jcare not sufficiently small (for an RRP strand Jc=2600 A/mm2 at 4.2 K and 12 T   ≤ 0.7 mm); for larger strands and Jc we have to improve the strand layout. Bernardo Bordini

  19. CONCLUSIONS • Increasing the RRR above 150 does not improve the conductor magneto-thermal stability • A local damage of the copper stabilizer can completely Jeopardized the dynamic stabilization of the conductor  we can not relay on the dynamic stabilization is better to design a magnet assuming a low RRR value • High-Jc Nb3Sn conductor (Jc>2600 A/mm2 at 4.2 K and 12 T)is not perfectly suitable for 12 T magnets because of magneto-thermal instabilities; this problem disappears if we move towards higher field magnets • Magneto-thermal instability is not a problem for 15 T magnets that have to operate at 4.3 K (as Fresca2) if the strand diameter () and the effective filament size (Deff ) are sufficiently small ( ≤ 1 mm, Deff ≤ 70 μm) • A 15 T magnet designed to work at 1.9 K could still have problems if the strand diameter and the Jcare not sufficiently small (for an RRP strand Jc=2600 A/mm2 at 4.2 K and 12 T   ≤ 0.7 mm); for larger strands and Jc we have to improve the strand layout. Bernardo Bordini

  20. NO EFFECT OF EFFECTIVE FILAMENT SIZE AT ~1.9 K MEASUREMENTS BY A. Ghosh (BNL) • COURTESY OF D. Dietderich (LBNL) Bernardo Bordini

  21. EFFECT OF STRAND LAYOUT ON SELF FIELD INSTABILITY AT 1.9 K • Preliminary results based on the more accurate new finite element model* (Dynamic Calculation) show that increasing the Cu/non-Cu ratio, the Cu volume between the sub-elements and reducing the sub-element size improve the strand stability but it not increase significantly the quench current 0.8 mm wire – Jc (4.3 K, 12 T) ~ 2600 A/mm2 54/61 RRP Cu/non-Cu 0.93 288/295 PIT Cu/non-Cu 1.22 *B. Bordini, L. Rossi, “Self field instability in high-Jc Nb3Sn strands with high Copper Residual Resistivity Ratio”, presented at Applied Superconductivity Conference, Chicago, USA, August, 2008 Bernardo Bordini

  22. V-I MEASUREMENTS: RRR ranging from 8 to 120 Samples: four 0.8 mm 54/61 RRP® strands reacted in a way to have almost the same Jc with a significantly different RRR Is-SF (4.2 K)>Is-SF (1.9K)<Ic (12T,1.9K) Sample 3: Is-SF (1.9K)=0.68 Ic (12T,1.9K)<Ic (12T,4.2K) ! 1.9 K 4.2 K Bernardo Bordini

  23. V-I MEASUREMENTS: RRR ranging from 140 to 290 • Samples: three 0.8 mm 54/61 RRP® strands reacted in a way to have almost the same Jc with a significantly different RRR larger than 120 • Is-SF (4.2 K)>Is-SF (1.9K) • Is-SF (1.9K)<Ic (12T,1.9K) 4.2 K 1.9 K Bernardo Bordini

  24. V-I MEASUREMENTS: Improving Self-Field Stability Samples 8 and 9, drawn at the University of Geneva, are significantly more self-field stable than sample 5, which is the most stable of the previous sample set (S. 1-7), thanks to: 1) the smaller strand diameter (S. 8); 2) the lower Jc and probably the different strand layout (S. 9). The PIT strand (S. 9) adopted solutions that theoretically helps the dynamic stabilization of the strand 1.9 K In=Iq/Ic(12T,4.2K) Bernardo Bordini

  25. EFFECTS OF COOLING CONDITIONS ON INSTABILITIES A thick layer of stycast did not change the critical current and did not significantly change the premature quench current values 54/61 RRP 0.8 mm RRR 120 4.2 K 1.9 K Bernardo Bordini

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