Construction and Testing of Superconducting Magnets for the BEPC-II Interaction Region

1. Construction and Testing of Superconducting Magnets for the BEPC-II Interaction Region Animesh Jainon behalf ofSuperconducting Magnet DivisionBrookhaven National Laboratory, Upton, NY 11973, USA 4th BEPC-II IMAC Meeting, Beijing, April 26-28, 2006

2. Introduction • Brookhaven National Laboratory has designed, built and tested two superconducting magnets for theBEPC-II interaction region. • Each of these magnets contains several coils to produce normal and skew quadrupole, normal and skew dipole, and solenoidal fields. • All coils in both the magnets have performed satisfactorily with ample margin. • This talk briefly describes the construction and testing of these magnets, with particular emphasis on field quality. Animesh Jain: BNL

3. Nominal Design Parameters Not listed here: Anti-Solenoids (AS1, AS2 and AS3) Animesh Jain: BNL

4. BEPC-II Coil Design • All coils consist of one or more double-layers of a “Serpentine” winding pattern. • This type of winding pattern was recently developed at BNL, and has several advantages over conventional “Spiral wound” coils.(B. Parker and J. Escallier, Proc. PAC’05, pp.737-9.) • The patterns are wound directly on a cylindrical surface using an automatic winding machine. • Except for a “Serpentine” pattern, all other construction features of the BEPC-II coils were similar to magnets built by BNL in the past for the HERA upgrade. Animesh Jain: BNL

5. No. of Layers and Turns Animesh Jain: BNL

6. Winding Different Conductor Types SCQ VDC AS2 Animesh Jain: BNL

7. Ensuring Good Field Quality • For a short length magnet, the ends contribute significantly to both the allowed and the unallowed harmonics. • The harmonics from the ends were compensated in the design by modulating the angular positions of the conductor in the entire pattern. • Warm field quality was measured after each double layer was wound. • In the case of the main quadrupole (SCQ), the results of the warm measurements were used to modulate the subsequent double-layers to progressively improve the field quality. Animesh Jain: BNL

8. Coil Section at the Magnet Center Animesh Jain: BNL

9. Warm and Cold Tests of Coils • Warm measurements were carried out after each double layer was wound using a 0.92 m long, 68.5 mm radius rotating coil system. • The completed coil assemblies were cold tested in a vertical dewar for satisfactory performance beyond the nominal operating currents. • Field quality measurements were also made in the superconducting state using the same rotating coil system that was used for the warm measurements. • Field quality was measured in all the coils individually, and also in the SKQ, VDC and HDC (SCB) coils with the SCQ powered in the background at 477A. • The solenoids were measured warm using a Hall probe. Animesh Jain: BNL

10. Animesh Jain: BNL

11. AS2 Solenoid Axial Field Profiles Animesh Jain: BNL

12. Ensuring Good Quench Performance • All coils are designed to have ample margin above the nominal operating current. • It is necessary to have enough precompression in the coils to prevent any conductor motion due to Lorentz forces, which could cause a quench. • Large gaps in the pattern (e.g., at the poles) were filled with G-10 spacers (Nomex for VDC and SKQ). • All gaps were filled with expansion-matched epoxy. • Each double-layer was compression wrapped withS-glass to provide the prestress, and then cured. Animesh Jain: BNL

13. Quench Tests of Various Coils • All coils were ramped to a maximum test limit, at first individually, and then in combination (SR & Collider modes). • Only the AS1 in #1 and AS3 in #2 had one training quench. All other coils were ramped without any quench. • All coils were forced to quench using spot heaters at 50% and 100% of the operating current. Animesh Jain: BNL

14. Spot Heater Quench Results Animesh Jain: BNL

15. Magnet Work after Cold Test • Coil assembly is inserted into a double-walled helium containment vessel to form the cold mass. • The cold mass is covered with superinsulation and is surrounded by an inner and an outer heat shield. • The cold mass is inserted into the cryostat. • The cold mass orientation is aligned to the level surfaces on the cryostat by doing warm magnetic measurements in the main quadrupole (SCQ). The orientation is maintained by welding in place. • Electrical and mechanical work in the lead end. • Final warm measurements with survey of fiducials. Animesh Jain: BNL

16. Some Assembly Components Helium Containment Outer Heat Shield This manifold is no longer in the design. Inner Heat Shield Complete Magnet Animesh Jain: BNL

17. Cold Mass Angle Alignment Precision Level AngleAdjustment Animesh Jain: BNL

18. Status as of IMAC in May, 2005 • Magnet #1 cold tests were completed and the first set of cold field quality data were available. • Magnet #2 coil winding was completed and was waiting to be cold tested. • As per the Committee report: • The quench test results were quite satisfactory. • There were concerns about the delay in delivery. • There were concerns about unexplained sextupole in the quadrupole (SCQ) cold measurements. • It was suggested that the possibility of eddy currents in the idle coils should be excluded based on measurements. Animesh Jain: BNL

19. Present Status • Both magnets have now been delivered to IHEP: • Magnet #1 was shipped from BNL in October, 2005. • Magnet #2 was shipped from BNL in December 2005. • There was a long delay in the completion of the first magnet assembly due to leaks in the heat shield assembly that were very difficult to locate. • The aluminum tubes originally used in the heat shield were eventually replaced by stainless steel tubes. • Extensive measurements were carried out in magnet #2 to pinpoint the source of the unexpected field harmonics seen during the cold test of magnet #1. Animesh Jain: BNL

20. Continued Support from BNL • BNL continues to provide support after shipping: • Andrew Marone and John Escallier from BNLvisited IHEP in January, 2006 to provide supportfor valve box assembly and magnet installation. • George Ganetis and Wing Louie from BNL are scheduled to visit IHEP to help with the initial powering of the magnets after cool down, and set up quench detection and other electrical systems.(will need at least one month’s notice for travel) • There will be provisions in the system for remote monitoring, which could be used when necessary. Animesh Jain: BNL

21. Understanding the Unexpected Sextupole in theCold SCQ Magnets Animesh Jain: BNL

22. Low Sextupole Content Warm Measurements had shown good field quality in the SCQ Animesh Jain: BNL

23. Cold Field Quality Measurements • Cold field quality measurements were carried out in a vertical dewar. • The quadrupoles (SCQ) were measured at currents ranging from 20 A to 550 A. • The variation of sextupole terms (in Tesla.m at50 mm) was linear with current, as expected. • The “geometric” sextupole terms were derived from the slope of a straight line fit. • Sextupole in “units” is calculated by comparing this slope with a similar slope for the quadrupole term. Animesh Jain: BNL

24. Quadrupole Slope is 0.7935 T.m/kA Magnet #1 Animesh Jain: BNL

25. Quadrupole Slope is 0.7935 T.m/kA Magnet #1 Animesh Jain: BNL

26. Quadrupole Slope is 0.7945 T.m/kA Magnet #2 Animesh Jain: BNL

27. Quadrupole Slope is 0.7945 T.m/kA Magnet #2 Animesh Jain: BNL

28. Warm to Cold Discrepancy • The geometric values of sextupole derived from the cold data were much larger than the warm values measured before cold test. (For comparison, such changes in the BNL-built HERA magnets were below 1 unit.)Magnet #1: b3 = 5.3 units cold (was 0.35 warm)a3 = 4.5 units cold (was –1.28 warm)Magnet #2: b3 = 1.5 units cold (was 1.80 warm)a3 = –5.5 units cold (was –1.63 warm) • Possible sources: Distortion under cool down; persistent current effects from other layers; measurement errors due to a tilt of the measuring coil with respect to the magnet axis, iron in and aroundthe dewar, ..... Animesh Jain: BNL

29. Warm to Cold Discrepancy • Out of the possible sources, distortion under cool down, persistent current effects, and iron around the dewar seemed to be very unlikely causes.Distortion: Unlikely that only one harmonic will be affected. Also, no such effects were seen in earlier magnet productions. (Distortions also ruled out by measurements at 35 K in magnet #2: to be discussed later)Persistent Currents: Should produce a hysteresis (Up Ramp to Down Ramp difference), which is not seen.Iron around the Dewar: Should affect both magnets in a similar way, since they were tested in the same dewar, and were mounted similarly. Animesh Jain: BNL

30. Example of Persistent Currents Octupole Term in the BNL-built HERA Quad No hysteresis in unallowed terms is seen in BEPC quadrupoles. Animesh Jain: BNL

31. Warm-Cold Difference: Tilt of Coil • For long magnets, and magnets with negligible end harmonics, a tilt of the measuring coil with respect to the magnet axis does not affect the measurement of harmonics in a dipole or a quadrupole magnet. • The BEPC magnets are short, with serpentine coil design, and have large end harmonics. • The skew octupole harmonic is large, and of opposite sign, in the lead end and non-lead end of the magnet. • A tilt of the measuring coil will cause a sextupole term by feed down, and the contributions from the two ends will add up. Animesh Jain: BNL

32. in “Units” at 50 mm A tilt of the measuring coil implies offsets of opposite sign at the two ends. This, coupled with the opposite signs of the skew octupole, will cause a spurious sextupole due to feed down. Animesh Jain: BNL

33. Sextupole term with no tilt. Integral ~ 0 unit Animesh Jain: BNL

34. Sextupole with tilt:Integ. b3 = 3 unitInteg. a3 = 1.7 unit Axis used for computations: Animesh Jain: BNL

35. Is Tilt Really the Cause? • If the measured warm to cold difference is indeed a result of the tilt of the measuring coil, then even a warm measurement in the vertical dewar should show similarly large sextupole. • Measurements were carried out in the vertical dewar inmagnet #1 after the cold tests were completed. • Warm sextupole in dewar was much smaller than the cold value, although not as low as the initial warm measurements. • Magnet #1 was also warm measured horizontally after the cold test, and was found to have low sextupole, matching the initial warm values before cold test. • An estimate of maximum effect from tilt was also obtained by measuring with the coil deliberately tilted. • A comparison of final warm measurements horizontally and vertically gives an estimate of the actual effect of tilt. Animesh Jain: BNL

36. Animesh Jain: BNL

37. Estimates of Tilt-Corrected Sextupole (Cold) • A tilt of the measuring coil perhaps contributed to about +1.74 unit of b3 and about +0.2 unit of a3 in magnet #1 (see the table in the previous slide). • Subtracting this contribution from the cold values, the best estimates of cold sextupole harmonics in the magnet #1 are: b3 = +3.6 unit, and a3 = +4.3 unit. • A similar exercise for magnet #2 gives estimates of cold sextupole harmonics as:b3 = 1.2 unit, and a3 = –5.2 unit. • Although the tilt correction improves sextupole a little, it is still mostly larger than the nominal 3 unit limit. Animesh Jain: BNL

38. Distortions due to Cool Down? • In view of the surprising results in magnet #1, we carried out extensive studies during cool down of magnet #2 to investigate any effect of cool down itself. • Measurements were carried out at ±1 A in the vertical dewar before cool down, and then at various stages of cool down at 35 K and 80 K. • The temperatures were chosen to be high enough such that no superconductor magnetization effects are present, but low enough that nearly all the mechanical contraction had already taken place. • No significant differences between the warm and the cold harmonics (at ±1A) were seen, thus ruling out any distortions as a possible cause of the sextupole change. Animesh Jain: BNL

39. Animesh Jain: BNL

40. Effect of Support Tube Magnetic Properties? • The support tube material was chosen to be stainless steel 316L, and is certified to be seamless by the vendor. • We measured the ferrite content around the circumference of the support tube in magnet #2 before it was cooled down. • The ferrite number varied azimuthally from 0.02 to 0.9 near the non-lead end, and from 0.05 to 0.6 at the lead end. (A ferrite no. of 1 is m-1 ~ 0.3) • These ferrite numbers are quite large, and represent significant azimuthal asymmetry in the magnetic properties, affecting mostly the low field measurements. Animesh Jain: BNL

41. A Closer Look at the Cold Data • It is expected that the ferrite content in the support tube will affect mostly the low field measurements. • At higher fields, the small ferrite particles saturate, and the permeability becomes essentially ~1. • If this is true, significant non-linearity should be seen at the low field region of the cold data. • A departure from the high field slope was indeed found for currents below ~25 A in both the magnets. • The very low field slopes match very well with the warm measurements. Animesh Jain: BNL

42. Low Field Sextupole in Magnet #2 High Field slope: a3 = –5.4 unit Low Field slope: a3 = –2.3 unit Additional cold data taken inmagnet #2 in 5 A to 40A range Animesh Jain: BNL

43. Summary • The superconducting IR magnets for BEPC-II are some of the most complex magnets that we have built. • Considerable care was exercised to obtain good field quality in the SCQ quadrupoles, resulting in very good warm field quality. • All magnet coils performed well above operating current without any quench, except for one training quench in AS1. • Assembly delays were caused by vacuum leaks that were difficult to detect, eventually leading to rework of the heat shield using stainless steel tubes. • Large sextupole in the cold data was thoroughly investigated, and is most likely caused by magnetic properties of the stainless steel coil support tube which may have affected the warm measurements. Animesh Jain: BNL